From aa6c3d0bde78bdbed6f6ff2ee18cff2942c83d07 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Mon, 24 Jul 2017 22:39:29 -0400
Subject: [PATCH] treat String as immutable in `===`. part of #22193

---
 src/builtins.c        | 13 +++++++++++++
 src/codegen.cpp       |  3 ++-
 test/strings/basic.jl | 10 ++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/src/builtins.c b/src/builtins.c
index 3a0014144a107..b4b6cf4fbd43c 100644
--- a/src/builtins.c
+++ b/src/builtins.c
@@ -121,6 +121,12 @@ JL_DLLEXPORT int jl_egal(jl_value_t *a, jl_value_t *b)
         jl_datatype_t *dtb = (jl_datatype_t*)b;
         return dta->name == dtb->name && compare_svec(dta->parameters, dtb->parameters);
     }
+    if (dt == jl_string_type) {
+        size_t l = jl_string_len(a);
+        if (jl_string_len(b) != l)
+            return 0;
+        return !memcmp(jl_string_data(a), jl_string_data(b), l);
+    }
     if (dt->mutabl) return 0;
     size_t sz = jl_datatype_size(dt);
     if (sz == 0) return 1;
@@ -186,6 +192,13 @@ static uintptr_t jl_object_id_(jl_value_t *tv, jl_value_t *v)
 #else
     if (v == jl_ANY_flag) return 0x8ee30bdd;
 #endif
+    if (dt == jl_string_type) {
+#ifdef _P64
+        return memhash_seed(jl_string_data(v), jl_string_len(v), 0xedc3b677);
+#else
+        return memhash32_seed(jl_string_data(v), jl_string_len(v), 0xedc3b677);
+#endif
+    }
     if (dt->mutabl) return inthash((uintptr_t)v);
     size_t sz = jl_datatype_size(tv);
     uintptr_t h = jl_object_id(tv);
diff --git a/src/codegen.cpp b/src/codegen.cpp
index e36e6b2c0fdd0..ac0c1e664c5f4 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -2349,7 +2349,8 @@ static Value *emit_f_is(jl_codectx_t &ctx, const jl_cgval_t &arg1, const jl_cgva
     if ((jl_is_type_type(rt1) && jl_is_leaf_type(jl_tparam0(rt1))) ||
         (jl_is_type_type(rt2) && jl_is_leaf_type(jl_tparam0(rt2)))) // can compare leaf types by pointer
         ptr_comparable = 1;
-    if (rt1 == (jl_value_t*)jl_simplevector_type && rt2 == (jl_value_t*)jl_simplevector_type)
+    if ((rt1 == (jl_value_t*)jl_string_type && rt2 == (jl_value_t*)jl_string_type) ||
+        (rt1 == (jl_value_t*)jl_simplevector_type && rt2 == (jl_value_t*)jl_simplevector_type))
         ptr_comparable = 0; // technically mutable, but compared by contents
     if (ptr_comparable) {
         Value *varg1 = arg1.constant ? literal_pointer_val(ctx, arg1.constant) : arg1.V;
diff --git a/test/strings/basic.jl b/test/strings/basic.jl
index 2f2dff88546a1..a65a57af8fe14 100644
--- a/test/strings/basic.jl
+++ b/test/strings/basic.jl
@@ -8,6 +8,16 @@
 @test eltype(GenericString) == Char
 @test start("abc") == 1
 @test cmp("ab","abc") == -1
+@test "abc" === "abc"
+@test "ab"  !== "abc"
+@test string("ab", 'c') === "abc"
+codegen_egal_of_strings(x, y) = (x===y, x!==y)
+@test codegen_egal_of_strings(string("ab", 'c'), "abc") === (true, false)
+let strs = ["", "a", "a b c", "до свидания"]
+    for x in strs, y in strs
+        @test (x === y) == (object_id(x) == object_id(y))
+    end
+end
 
 # {starts,ends}with
 @test startswith("abcd", 'a')