From bea54cc72b9411b89b99777f605b282f5d4b5bed Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Fri, 19 Jun 2020 14:06:38 +0200
Subject: [PATCH 01/30] add BLAS.get_num_threads

---
 stdlib/LinearAlgebra/src/blas.jl  | 24 ++++++++++++++++++++----
 stdlib/LinearAlgebra/test/blas.jl | 11 +++++++++++
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index a2367d7ee4b90..6385b24f90dac 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -113,10 +113,8 @@ Set the number of threads the BLAS library should use.
 """
 function set_num_threads(n::Integer)
     blas = vendor()
-    if blas === :openblas
-        return ccall((:openblas_set_num_threads, libblas), Cvoid, (Int32,), n)
-    elseif blas === :openblas64
-        return ccall((:openblas_set_num_threads64_, libblas), Cvoid, (Int32,), n)
+    if blas === :openblas || blas == :openblas64
+        return ccall((@blasfunc(openblas_set_num_threads), libblas), Cvoid, (Int32,), n)
     elseif blas === :mkl
         # MKL may let us set the number of threads in several ways
         return ccall((:MKL_Set_Num_Threads, libblas), Cvoid, (Cint,), n)
@@ -130,6 +128,24 @@ function set_num_threads(n::Integer)
     return nothing
 end
 
+"""
+    get_num_threads()
+
+Get the number of threads the BLAS library is using.
+"""
+function get_num_threads()
+    blas = LinearAlgebra.BLAS.vendor()
+    if blas === :openblas || blas === :openblas64
+        return ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ())
+    elseif blas == :mkl
+        return ccall((:MKL_Get_Max_Num_Threads, libblas), Cint, ())
+    elseif Sys.isapple()
+        return Base.parse(Cint, ENV["VECLIB_MAXIMUM_THREADS"])
+    else
+        error("Unknown BLAS") # better error? return nothing
+    end
+end
+
 const _testmat = [1.0 0.0; 0.0 -1.0]
 function check()
     blas = vendor()
diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index 23c6d68cdc997..e6a4a4d758781 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -553,4 +553,15 @@ Base.stride(A::WrappedArray, i::Int) = stride(A.A, i)
     end
 end
 
+@testset "get_set_num_threads" begin
+    default = BLAS.get_num_threads()
+    @test default isa Integer
+    @test default > 0
+    new = rand(1:10)
+    BLAS.set_num_threads(net)
+    @test BLAS.get_num_threads() == new
+    BLAS.set_num_threads(default)
+    @test BLAS.get_num_threads() == default
+end
+
 end # module TestBLAS

From 2de23cdcce75635cc6fb45b150bc509e1f53c656 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Fri, 19 Jun 2020 14:11:39 +0200
Subject: [PATCH 02/30] fix

---
 stdlib/LinearAlgebra/src/blas.jl  | 2 +-
 stdlib/LinearAlgebra/test/blas.jl | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 6385b24f90dac..85d42251b3321 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -138,7 +138,7 @@ function get_num_threads()
     if blas === :openblas || blas === :openblas64
         return ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ())
     elseif blas == :mkl
-        return ccall((:MKL_Get_Max_Num_Threads, libblas), Cint, ())
+        return ccall((:mkl_get_max_threads, libblas), Cint, ())
     elseif Sys.isapple()
         return Base.parse(Cint, ENV["VECLIB_MAXIMUM_THREADS"])
     else
diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index e6a4a4d758781..c609ef17f1a97 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -558,7 +558,7 @@ end
     @test default isa Integer
     @test default > 0
     new = rand(1:10)
-    BLAS.set_num_threads(net)
+    BLAS.set_num_threads(new)
     @test BLAS.get_num_threads() == new
     BLAS.set_num_threads(default)
     @test BLAS.get_num_threads() == default

From 0e6f8fba0da1445ce2defcd4f4dfe3be91221b7e Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Fri, 19 Jun 2020 15:42:00 +0200
Subject: [PATCH 03/30] fix

---
 stdlib/LinearAlgebra/test/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index c609ef17f1a97..14f5f89c8e923 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -557,7 +557,7 @@ end
     default = BLAS.get_num_threads()
     @test default isa Integer
     @test default > 0
-    new = rand(1:10)
+    new=rand(1:Sys.CPU_THREADS)
     BLAS.set_num_threads(new)
     @test BLAS.get_num_threads() == new
     BLAS.set_num_threads(default)

From 9f396c2b485bd4bc67744659cf6dfef633f4aacd Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Fri, 19 Jun 2020 16:12:32 +0200
Subject: [PATCH 04/30] fix

---
 stdlib/LinearAlgebra/src/blas.jl | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 85d42251b3321..4d2b9b22f4e48 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -139,11 +139,11 @@ function get_num_threads()
         return ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ())
     elseif blas == :mkl
         return ccall((:mkl_get_max_threads, libblas), Cint, ())
-    elseif Sys.isapple()
+    end
+    @static if Sys.isapple()
         return Base.parse(Cint, ENV["VECLIB_MAXIMUM_THREADS"])
-    else
-        error("Unknown BLAS") # better error? return nothing
     end
+    error("Unknown BLAS") # better error? return nothing?
 end
 
 const _testmat = [1.0 0.0; 0.0 -1.0]

From e84c2581705cbb7037aba2e6cda88bcdacce7b72 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Fri, 19 Jun 2020 18:06:43 +0200
Subject: [PATCH 05/30] fix

---
 stdlib/LinearAlgebra/test/blas.jl | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index 14f5f89c8e923..bc51f812f5ef9 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -557,9 +557,8 @@ end
     default = BLAS.get_num_threads()
     @test default isa Integer
     @test default > 0
-    new=rand(1:Sys.CPU_THREADS)
-    BLAS.set_num_threads(new)
-    @test BLAS.get_num_threads() == new
+    BLAS.set_num_threads(1)
+    @test BLAS.get_num_threads() == 1
     BLAS.set_num_threads(default)
     @test BLAS.get_num_threads() == default
 end

From 9a0edcb1bee51ac69f7a0e3df31a3113908a9bfb Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Fri, 19 Jun 2020 21:53:00 +0200
Subject: [PATCH 06/30] warn if get/set of num_bals_threads fails

---
 stdlib/LinearAlgebra/src/blas.jl | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 4d2b9b22f4e48..d4eb988bf98a0 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -118,11 +118,11 @@ function set_num_threads(n::Integer)
     elseif blas === :mkl
         # MKL may let us set the number of threads in several ways
         return ccall((:MKL_Set_Num_Threads, libblas), Cvoid, (Cint,), n)
-    end
-
-    # OSX BLAS looks at an environment variable
-    @static if Sys.isapple()
+    elseif Sys.isapple()
+        # OSX BLAS looks at an environment variable
         ENV["VECLIB_MAXIMUM_THREADS"] = n
+    else
+        @warn "Failed to set number of BLAS threads."
     end
 
     return nothing
@@ -139,11 +139,18 @@ function get_num_threads()
         return ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ())
     elseif blas == :mkl
         return ccall((:mkl_get_max_threads, libblas), Cint, ())
+    elseif Sys.isapple()
+        key = "VECLIB_MAXIMUM_THREADS"
+        s = get(ENV, key, "")
+        nt = Base.tryparse(Cint, s)
+        if nt === nothing
+            @warn "Failed to read environment variable $key"
+        else
+            return nt
+        end
     end
-    @static if Sys.isapple()
-        return Base.parse(Cint, ENV["VECLIB_MAXIMUM_THREADS"])
-    end
-    error("Unknown BLAS") # better error? return nothing?
+    @warn "Could not get number of BLAS threads. Returning Sys.CPU_THREADS instead."
+    return Sys.CPU_THREADS
 end
 
 const _testmat = [1.0 0.0; 0.0 -1.0]

From f6daa7905fda8aea111feea0f7d27448a2fb1f10 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 07:44:38 +0200
Subject: [PATCH 07/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index d4eb988bf98a0..6787933007235 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -149,7 +149,7 @@ function get_num_threads()
             return nt
         end
     end
-    @warn "Could not get number of BLAS threads. Returning Sys.CPU_THREADS instead."
+    @warn "Could not get number of BLAS threads. Returning Sys.CPU_THREADS instead." maxlog=1
     return Sys.CPU_THREADS
 end
 

From 8b2c8c47a20c942c590d368a63a82403e9ce5b8c Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 07:44:50 +0200
Subject: [PATCH 08/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 6787933007235..b17605f876b50 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -144,7 +144,7 @@ function get_num_threads()
         s = get(ENV, key, "")
         nt = Base.tryparse(Cint, s)
         if nt === nothing
-            @warn "Failed to read environment variable $key"
+            @warn "Failed to read environment variable $key" maxlog=1
         else
             return nt
         end

From a15f85194c0f27ad0af5900f1570e6f8a8eb94fc Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 07:45:07 +0200
Subject: [PATCH 09/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index b17605f876b50..3c921111cf103 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -122,7 +122,7 @@ function set_num_threads(n::Integer)
         # OSX BLAS looks at an environment variable
         ENV["VECLIB_MAXIMUM_THREADS"] = n
     else
-        @warn "Failed to set number of BLAS threads."
+        @warn "Failed to set number of BLAS threads." maxlog=1
     end
 
     return nothing

From 826d8ff3b08f7be0bf1f4d511d41595e92850f1e Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 08:15:19 +0200
Subject: [PATCH 10/30] fix

---
 stdlib/LinearAlgebra/src/blas.jl | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 3c921111cf103..2743c5f8ad663 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -107,9 +107,14 @@ end
 openblas_get_config() = strip(unsafe_string(ccall((@blasfunc(openblas_get_config), libblas), Ptr{UInt8}, () )))
 
 """
-    set_num_threads(n)
+    set_num_threads(n::Integer)
+    set_num_threads(::Nothing)
+
+Set the number of threads the BLAS library should use equal to `n::Integer`.
+If `nothing` is passed to this function, julia tries to figure out the optimial number of threads.
+The exact heuristic is an implementation detail.
 
-Set the number of threads the BLAS library should use.
+On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
 function set_num_threads(n::Integer)
     blas = vendor()
@@ -128,10 +133,17 @@ function set_num_threads(n::Integer)
     return nothing
 end
 
+function set_num_threads(::Nothing)
+    n = max(1, Sys.CPU_THREADS ÷ 2)
+    set_num_threads(n)
+end
+
 """
     get_num_threads()
 
 Get the number of threads the BLAS library is using.
+
+On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
 function get_num_threads()
     blas = LinearAlgebra.BLAS.vendor()
@@ -149,8 +161,9 @@ function get_num_threads()
             return nt
         end
     end
-    @warn "Could not get number of BLAS threads. Returning Sys.CPU_THREADS instead." maxlog=1
-    return Sys.CPU_THREADS
+    ret = nothing
+    @warn "Could not get number of BLAS threads. Returning $ret instead." maxlog=1
+    return ret
 end
 
 const _testmat = [1.0 0.0; 0.0 -1.0]

From ce6163635e2d200359cc5f688cd84d473627faf2 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 08:35:18 +0200
Subject: [PATCH 11/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 2743c5f8ad663..860a33ffee690 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -162,7 +162,7 @@ function get_num_threads()
         end
     end
     ret = nothing
-    @warn "Could not get number of BLAS threads. Returning $ret instead." maxlog=1
+    @warn "Could not get number of BLAS threads. Returning `$ret` instead." maxlog=1
     return ret
 end
 

From b38afaa9681f148f8a5691c4e519c2c8fea7374f Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 09:14:44 +0200
Subject: [PATCH 12/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 860a33ffee690..7b12058d07998 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -133,8 +133,14 @@ function set_num_threads(n::Integer)
     return nothing
 end
 
+_tryparse_env_cint(key) = tryparse(Cint, get(ENV, key, ""))
+
 function set_num_threads(::Nothing)
-    n = max(1, Sys.CPU_THREADS ÷ 2)
+    n = something(
+        _tryparse_env_cint("OPENBLAS_NUM_THREADS"),
+        _tryparse_env_cint("OMP_NUM_THREADS"),
+        max(1, Sys.CPU_THREADS ÷ 2),
+    )
     set_num_threads(n)
 end
 

From 0ee0efaef576934b1cbac2b20bf38b52c4e73ae4 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 09:16:06 +0200
Subject: [PATCH 13/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 7b12058d07998..aa415aeb406fb 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -167,9 +167,8 @@ function get_num_threads()
             return nt
         end
     end
-    ret = nothing
-    @warn "Could not get number of BLAS threads. Returning `$ret` instead." maxlog=1
-    return ret
+    @warn "Could not get number of BLAS threads. Returning `nothing` instead." maxlog=1
+    return nothing
 end
 
 const _testmat = [1.0 0.0; 0.0 -1.0]

From 8e4fedd2785011a02d2d82f2d94f5ef36d419def Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 09:30:18 +0200
Subject: [PATCH 14/30] fix

---
 stdlib/LinearAlgebra/src/blas.jl | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index aa415aeb406fb..3310cd5c77290 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -114,12 +114,12 @@ Set the number of threads the BLAS library should use equal to `n::Integer`.
 If `nothing` is passed to this function, julia tries to figure out the optimial number of threads.
 The exact heuristic is an implementation detail.
 
-On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
+On exotic variants of `BLAS` this function can fail.
 """
-function set_num_threads(n::Integer)
+function set_num_threads(n::Integer)::Nothing
     blas = vendor()
     if blas === :openblas || blas == :openblas64
-        return ccall((@blasfunc(openblas_set_num_threads), libblas), Cvoid, (Int32,), n)
+        return ccall((@blasfunc(openblas_set_num_threads), libblas), Cvoid, (Cint,), n)
     elseif blas === :mkl
         # MKL may let us set the number of threads in several ways
         return ccall((:MKL_Set_Num_Threads, libblas), Cvoid, (Cint,), n)
@@ -129,7 +129,6 @@ function set_num_threads(n::Integer)
     else
         @warn "Failed to set number of BLAS threads." maxlog=1
     end
-
     return nothing
 end
 
@@ -151,16 +150,16 @@ Get the number of threads the BLAS library is using.
 
 On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
-function get_num_threads()
+function get_num_threads()::Union{Int, Nothing}
     blas = LinearAlgebra.BLAS.vendor()
     if blas === :openblas || blas === :openblas64
-        return ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ())
+        return Int(ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ()))
     elseif blas == :mkl
-        return ccall((:mkl_get_max_threads, libblas), Cint, ())
+        return Int(ccall((:mkl_get_max_threads, libblas), Cint, ()))
     elseif Sys.isapple()
         key = "VECLIB_MAXIMUM_THREADS"
         s = get(ENV, key, "")
-        nt = Base.tryparse(Cint, s)
+        nt = Base.tryparse(Int, s)
         if nt === nothing
             @warn "Failed to read environment variable $key" maxlog=1
         else

From 33a95d58d1b31844591f51c9357651c30bd4338c Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 09:43:13 +0200
Subject: [PATCH 15/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 3310cd5c77290..68484a94fc324 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -132,7 +132,7 @@ function set_num_threads(n::Integer)::Nothing
     return nothing
 end
 
-_tryparse_env_cint(key) = tryparse(Cint, get(ENV, key, ""))
+_tryparse_env_int(key) = tryparse(Int, get(ENV, key, ""))
 
 function set_num_threads(::Nothing)
     n = something(

From 35cf5a65974381387a150a64ede82b833019c8ae Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 09:43:24 +0200
Subject: [PATCH 16/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 68484a94fc324..3b50a083c4f8f 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -136,8 +136,8 @@ _tryparse_env_int(key) = tryparse(Int, get(ENV, key, ""))
 
 function set_num_threads(::Nothing)
     n = something(
-        _tryparse_env_cint("OPENBLAS_NUM_THREADS"),
-        _tryparse_env_cint("OMP_NUM_THREADS"),
+        _tryparse_env_int("OPENBLAS_NUM_THREADS"),
+        _tryparse_env_int("OMP_NUM_THREADS"),
         max(1, Sys.CPU_THREADS ÷ 2),
     )
     set_num_threads(n)

From bc370b4de4e7d1864758ef9b30c121354f61edb1 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 09:43:38 +0200
Subject: [PATCH 17/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 3b50a083c4f8f..ba91cb30f0bdd 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -158,8 +158,7 @@ function get_num_threads()::Union{Int, Nothing}
         return Int(ccall((:mkl_get_max_threads, libblas), Cint, ()))
     elseif Sys.isapple()
         key = "VECLIB_MAXIMUM_THREADS"
-        s = get(ENV, key, "")
-        nt = Base.tryparse(Int, s)
+        nt = _tryparse_env_int(key)
         if nt === nothing
             @warn "Failed to read environment variable $key" maxlog=1
         else

From dd455b534d0c2e8dba1cab49eacd30d26a2c3f79 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Sat, 20 Jun 2020 15:57:37 +0200
Subject: [PATCH 18/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Okon Samuel <39421418+OkonSamuel@users.noreply.github.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index ba91cb30f0bdd..9185bdcafceb1 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -151,7 +151,7 @@ Get the number of threads the BLAS library is using.
 On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
 function get_num_threads()::Union{Int, Nothing}
-    blas = LinearAlgebra.BLAS.vendor()
+    blas = vendor()
     if blas === :openblas || blas === :openblas64
         return Int(ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ()))
     elseif blas == :mkl

From abd2084457972504b23a0015f6c0412ff7c585a6 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Mon, 22 Jun 2020 12:34:19 +0200
Subject: [PATCH 19/30] fix

---
 stdlib/LinearAlgebra/src/blas.jl  | 36 ++++++++++++++++++++-----------
 stdlib/LinearAlgebra/test/blas.jl |  8 +++++++
 2 files changed, 32 insertions(+), 12 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 9185bdcafceb1..454e1e517ffa0 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -106,6 +106,17 @@ end
 
 openblas_get_config() = strip(unsafe_string(ccall((@blasfunc(openblas_get_config), libblas), Ptr{UInt8}, () )))
 
+function guess_vendor()
+    # like determine_vendor, but guesses blas in some cases
+    # where determine_vendor returns :unknown
+    ret = vendor()
+    if Sys.isapple() && (ret == :unknown )
+        ret = :osxblas
+    end
+    ret
+end
+
+
 """
     set_num_threads(n::Integer)
     set_num_threads(::Nothing)
@@ -116,17 +127,17 @@ The exact heuristic is an implementation detail.
 
 On exotic variants of `BLAS` this function can fail.
 """
-function set_num_threads(n::Integer)::Nothing
-    blas = vendor()
-    if blas === :openblas || blas == :openblas64
+function set_num_threads(n::Integer, _blas=guess_vendor())::Nothing
+    if _blas === :openblas || _blas == :openblas64
         return ccall((@blasfunc(openblas_set_num_threads), libblas), Cvoid, (Cint,), n)
-    elseif blas === :mkl
+    elseif _blas === :mkl
         # MKL may let us set the number of threads in several ways
         return ccall((:MKL_Set_Num_Threads, libblas), Cvoid, (Cint,), n)
-    elseif Sys.isapple()
+    elseif _blas === :osxblas
         # OSX BLAS looks at an environment variable
         ENV["VECLIB_MAXIMUM_THREADS"] = n
     else
+        @assert _blas === :unknown
         @warn "Failed to set number of BLAS threads." maxlog=1
     end
     return nothing
@@ -134,13 +145,13 @@ end
 
 _tryparse_env_int(key) = tryparse(Int, get(ENV, key, ""))
 
-function set_num_threads(::Nothing)
+function set_num_threads(::Nothing, _blas=guess_vendor())
     n = something(
         _tryparse_env_int("OPENBLAS_NUM_THREADS"),
         _tryparse_env_int("OMP_NUM_THREADS"),
         max(1, Sys.CPU_THREADS ÷ 2),
     )
-    set_num_threads(n)
+    set_num_threads(n, _blas)
 end
 
 """
@@ -150,13 +161,12 @@ Get the number of threads the BLAS library is using.
 
 On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
-function get_num_threads()::Union{Int, Nothing}
-    blas = vendor()
-    if blas === :openblas || blas === :openblas64
+function get_num_threads(_blas=guess_vendor())::Union{Int, Nothing}
+    if _blas === :openblas || _blas === :openblas64
         return Int(ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ()))
-    elseif blas == :mkl
+    elseif _blas === :mkl
         return Int(ccall((:mkl_get_max_threads, libblas), Cint, ()))
-    elseif Sys.isapple()
+    elseif _blas === :osxblas
         key = "VECLIB_MAXIMUM_THREADS"
         nt = _tryparse_env_int(key)
         if nt === nothing
@@ -164,6 +174,8 @@ function get_num_threads()::Union{Int, Nothing}
         else
             return nt
         end
+    else
+        @assert _blas === :unknown
     end
     @warn "Could not get number of BLAS threads. Returning `nothing` instead." maxlog=1
     return nothing
diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index bc51f812f5ef9..0899c5af362ff 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -561,6 +561,14 @@ end
     @test BLAS.get_num_threads() == 1
     BLAS.set_num_threads(default)
     @test BLAS.get_num_threads() == default
+
+    if BLAS.guess_vendor() !== :osxblas
+        # test osxblas which is not covered by CI
+        BLAS.set_num_threads(1, :osxblas)
+        @test BLAS.get_num_threads(:osxblas) === 1
+        BLAS.set_num_threads(2, :osxblas)
+        @test BLAS.get_num_threads(:osxblas) === 2
+    end
 end
 
 end # module TestBLAS

From b8e9055123e27a3a97600009640ae0476ce9293d Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Mon, 22 Jun 2020 20:59:40 +0200
Subject: [PATCH 20/30] fix

---
 stdlib/LinearAlgebra/src/blas.jl  |  8 ++++----
 stdlib/LinearAlgebra/test/blas.jl | 14 +++++++-------
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 454e1e517ffa0..ca96069628898 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -127,7 +127,7 @@ The exact heuristic is an implementation detail.
 
 On exotic variants of `BLAS` this function can fail.
 """
-function set_num_threads(n::Integer, _blas=guess_vendor())::Nothing
+function set_num_threads(n::Integer; _blas=guess_vendor())::Nothing
     if _blas === :openblas || _blas == :openblas64
         return ccall((@blasfunc(openblas_set_num_threads), libblas), Cvoid, (Cint,), n)
     elseif _blas === :mkl
@@ -145,13 +145,13 @@ end
 
 _tryparse_env_int(key) = tryparse(Int, get(ENV, key, ""))
 
-function set_num_threads(::Nothing, _blas=guess_vendor())
+function set_num_threads(::Nothing; _blas=guess_vendor())
     n = something(
         _tryparse_env_int("OPENBLAS_NUM_THREADS"),
         _tryparse_env_int("OMP_NUM_THREADS"),
         max(1, Sys.CPU_THREADS ÷ 2),
     )
-    set_num_threads(n, _blas)
+    set_num_threads(n; _blas)
 end
 
 """
@@ -161,7 +161,7 @@ Get the number of threads the BLAS library is using.
 
 On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
-function get_num_threads(_blas=guess_vendor())::Union{Int, Nothing}
+function get_num_threads(;_blas=guess_vendor())::Union{Int, Nothing}
     if _blas === :openblas || _blas === :openblas64
         return Int(ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ()))
     elseif _blas === :mkl
diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index 0899c5af362ff..26867e0928315 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -555,19 +555,19 @@ end
 
 @testset "get_set_num_threads" begin
     default = BLAS.get_num_threads()
-    @test default isa Integer
+    @test default isa Int
     @test default > 0
     BLAS.set_num_threads(1)
-    @test BLAS.get_num_threads() == 1
+    @test BLAS.get_num_threads() === 1
     BLAS.set_num_threads(default)
-    @test BLAS.get_num_threads() == default
+    @test BLAS.get_num_threads() === default
 
     if BLAS.guess_vendor() !== :osxblas
         # test osxblas which is not covered by CI
-        BLAS.set_num_threads(1, :osxblas)
-        @test BLAS.get_num_threads(:osxblas) === 1
-        BLAS.set_num_threads(2, :osxblas)
-        @test BLAS.get_num_threads(:osxblas) === 2
+        BLAS.set_num_threads(1, _blas=:osxblas)
+        @test BLAS.get_num_threads(_blas=:osxblas) === 1
+        BLAS.set_num_threads(2, _blas=:osxblas)
+        @test BLAS.get_num_threads(_blas=:osxblas) === 2
     end
 end
 

From 9eabcb6b559bbd86f4c43418a7950789729a4536 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 07:50:06 +0200
Subject: [PATCH 21/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index ca96069628898..09b1c4b0a0d59 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -127,7 +127,9 @@ The exact heuristic is an implementation detail.
 
 On exotic variants of `BLAS` this function can fail.
 """
-function set_num_threads(n::Integer; _blas=guess_vendor())::Nothing
+set_num_threads(n)::Nothing = _set_num_threads(n)
+
+function _set_num_threads(n::Integer; _blas = guess_vendor())
     if _blas === :openblas || _blas == :openblas64
         return ccall((@blasfunc(openblas_set_num_threads), libblas), Cvoid, (Cint,), n)
     elseif _blas === :mkl

From 920b90bb8f4f0806cefd3b4ed15e61f150b1fccd Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 07:50:17 +0200
Subject: [PATCH 22/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 09b1c4b0a0d59..18bf16e1a5a03 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -147,7 +147,7 @@ end
 
 _tryparse_env_int(key) = tryparse(Int, get(ENV, key, ""))
 
-function set_num_threads(::Nothing; _blas=guess_vendor())
+function _set_num_threads(::Nothing; _blas = guess_vendor())
     n = something(
         _tryparse_env_int("OPENBLAS_NUM_THREADS"),
         _tryparse_env_int("OMP_NUM_THREADS"),

From b011dc6bfd7a6cea5479de4d87b95fd8e60c6899 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 07:50:33 +0200
Subject: [PATCH 23/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 18bf16e1a5a03..95cc55c41d5d7 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -163,7 +163,9 @@ Get the number of threads the BLAS library is using.
 
 On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
 """
-function get_num_threads(;_blas=guess_vendor())::Union{Int, Nothing}
+get_num_threads(;_blas=guess_vendor())::Union{Int, Nothing} = _get_num_threads()
+
+function _get_num_threads(; _blas = guess_vendor())::Union{Int, Nothing}
     if _blas === :openblas || _blas === :openblas64
         return Int(ccall((@blasfunc(openblas_get_num_threads), libblas), Cint, ()))
     elseif _blas === :mkl

From 95ccdf90f8fb004384a2d7f16e327c99851427f2 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 07:50:45 +0200
Subject: [PATCH 24/30] Update stdlib/LinearAlgebra/test/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/test/blas.jl | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index 26867e0928315..f93dad8f34a6c 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -564,10 +564,22 @@ end
 
     if BLAS.guess_vendor() !== :osxblas
         # test osxblas which is not covered by CI
-        BLAS.set_num_threads(1, _blas=:osxblas)
-        @test BLAS.get_num_threads(_blas=:osxblas) === 1
-        BLAS.set_num_threads(2, _blas=:osxblas)
-        @test BLAS.get_num_threads(_blas=:osxblas) === 2
+        withenv() do
+            @test_logs (:warn,) match_mode=:any BLAS._set_num_threads(1, _blas=:osxblas)
+            @test @test_logs(
+                (:warn,),
+                (:warn,),
+                match_mode=:any,
+                BLAS._get_num_threads(_blas=:osxblas),
+            ) === 1
+            @test_logs (:warn,) match_mode=:any BLAS._set_num_threads(2, _blas=:osxblas)
+            @test @test_logs(
+                (:warn,),
+                (:warn,),
+                match_mode=:any,
+                BLAS._get_num_threads(_blas=:osxblas),
+            ) === 2
+        end
     end
 end
 

From 72ef30ef7da12ef249e2345ae34a0ef6a5b43c94 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 07:50:57 +0200
Subject: [PATCH 25/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index 95cc55c41d5d7..b338a023d7ba2 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -110,7 +110,7 @@ function guess_vendor()
     # like determine_vendor, but guesses blas in some cases
     # where determine_vendor returns :unknown
     ret = vendor()
-    if Sys.isapple() && (ret == :unknown )
+    if Sys.isapple() && (ret == :unknown)
         ret = :osxblas
     end
     ret

From 4cf2628104e1e605da894de9e342a9227cf35fc4 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 11:59:30 +0200
Subject: [PATCH 26/30] Update stdlib/LinearAlgebra/test/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/test/blas.jl | 1 +
 1 file changed, 1 insertion(+)

diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index f93dad8f34a6c..c8cce0079bb34 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -562,6 +562,7 @@ end
     BLAS.set_num_threads(default)
     @test BLAS.get_num_threads() === default
 
+    @test_logs (:warn,) match_mode=:any BLAS._set_num_threads(1, _blas=:unknown)
     if BLAS.guess_vendor() !== :osxblas
         # test osxblas which is not covered by CI
         withenv() do

From bb699315d0460fa53d6b1426c4a1cd5163c709c2 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 12:00:09 +0200
Subject: [PATCH 27/30] Update stdlib/LinearAlgebra/test/blas.jl

Co-authored-by: Takafumi Arakaki <aka.tkf@gmail.com>
---
 stdlib/LinearAlgebra/test/blas.jl | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/stdlib/LinearAlgebra/test/blas.jl b/stdlib/LinearAlgebra/test/blas.jl
index c8cce0079bb34..cefb0625d00d5 100644
--- a/stdlib/LinearAlgebra/test/blas.jl
+++ b/stdlib/LinearAlgebra/test/blas.jl
@@ -565,21 +565,17 @@ end
     @test_logs (:warn,) match_mode=:any BLAS._set_num_threads(1, _blas=:unknown)
     if BLAS.guess_vendor() !== :osxblas
         # test osxblas which is not covered by CI
-        withenv() do
-            @test_logs (:warn,) match_mode=:any BLAS._set_num_threads(1, _blas=:osxblas)
+        withenv("VECLIB_MAXIMUM_THREADS" => nothing) do
             @test @test_logs(
                 (:warn,),
                 (:warn,),
                 match_mode=:any,
                 BLAS._get_num_threads(_blas=:osxblas),
-            ) === 1
-            @test_logs (:warn,) match_mode=:any BLAS._set_num_threads(2, _blas=:osxblas)
-            @test @test_logs(
-                (:warn,),
-                (:warn,),
-                match_mode=:any,
-                BLAS._get_num_threads(_blas=:osxblas),
-            ) === 2
+            ) === nothing
+            @test_logs BLAS._set_num_threads(1, _blas=:osxblas)
+            @test @test_logs(BLAS._get_num_threads(_blas=:osxblas)) === 1
+            @test_logs BLAS._set_num_threads(2, _blas=:osxblas)
+            @test @test_logs(BLAS._get_num_threads(_blas=:osxblas)) === 2
         end
     end
 end

From 06550d6ca62fd65bb394012710f2d00b5b7ff826 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 12:12:23 +0200
Subject: [PATCH 28/30] Update stdlib/LinearAlgebra/src/blas.jl

Co-authored-by: Okon Samuel <39421418+OkonSamuel@users.noreply.github.com>
---
 stdlib/LinearAlgebra/src/blas.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index b338a023d7ba2..e0758a26998ea 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -153,7 +153,7 @@ function _set_num_threads(::Nothing; _blas = guess_vendor())
         _tryparse_env_int("OMP_NUM_THREADS"),
         max(1, Sys.CPU_THREADS ÷ 2),
     )
-    set_num_threads(n; _blas)
+    _set_num_threads(n; _blas)
 end
 
 """

From b6aa07629440b59363f750cccc303d51fa179b9b Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 13:41:36 +0200
Subject: [PATCH 29/30] improve docstrings

---
 stdlib/LinearAlgebra/src/blas.jl | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/stdlib/LinearAlgebra/src/blas.jl b/stdlib/LinearAlgebra/src/blas.jl
index e0758a26998ea..fca99ceee3f78 100644
--- a/stdlib/LinearAlgebra/src/blas.jl
+++ b/stdlib/LinearAlgebra/src/blas.jl
@@ -122,10 +122,26 @@ end
     set_num_threads(::Nothing)
 
 Set the number of threads the BLAS library should use equal to `n::Integer`.
-If `nothing` is passed to this function, julia tries to figure out the optimial number of threads.
-The exact heuristic is an implementation detail.
 
-On exotic variants of `BLAS` this function can fail.
+Also accepts `nothing`, in which case julia tries to guess the default number of threads.
+Passing `nothing` is discouraged and mainly exists for the following reason:
+
+On exotic variants of BLAS, `nothing` may be returned by `get_num_threads()`.
+Thus on exotic variants of BLAS, the following pattern may fail to set the number of threads:
+
+```julia
+old = get_num_threads()
+set_num_threads(1)
+@threads for i in 1:10
+    # single-threaded BLAS calls
+end
+set_num_threads(old)
+```
+Because `set_num_threads` accepts `nothing`, this code can still run
+on exotic variants of BLAS without error. Warnings will be raised instead.
+
+!!! compat "Julia 1.6"
+    `set_num_threads(::Nothing)` requires at least Julia 1.6.
 """
 set_num_threads(n)::Nothing = _set_num_threads(n)
 
@@ -162,6 +178,9 @@ end
 Get the number of threads the BLAS library is using.
 
 On exotic variants of `BLAS` this function can fail, which is indicated by returning `nothing`.
+
+!!! compat "Julia 1.6"
+    `get_num_threads` requires at least Julia 1.6.
 """
 get_num_threads(;_blas=guess_vendor())::Union{Int, Nothing} = _get_num_threads()
 

From c524c5351def7268c11fa21beb63e3b36192ad81 Mon Sep 17 00:00:00 2001
From: Jan Weidner <jw3126@gmail.com>
Date: Tue, 23 Jun 2020 23:14:31 +0200
Subject: [PATCH 30/30] add to NEWS.md

---
 NEWS.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/NEWS.md b/NEWS.md
index c9d73854e0f9d..748acc4f87863 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -57,6 +57,7 @@ Standard library changes
 #### LinearAlgebra
 * New method `LinearAlgebra.issuccess(::CholeskyPivoted)` for checking whether pivoted Cholesky factorization was successful ([#36002]).
 * `UniformScaling` can now be indexed into using ranges to return dense matrices and vectors ([#24359]).
+* New function `LinearAlgebra.BLAS.get_num_threads()` for getting the number of BLAS threads. ([#36360])
 
 #### Markdown