Some getfield related fixes in inference

* Use the field index passed in in `lift_leaves`

    The caller has already done all the computation including bound checking.
    The `field` computed in this function is also affecting all the following iterations
    which is almost certainly wrong.

* Remove unnecessary type check on `field` in `lift_leaves` since it is always `Int`

* Move a branch disabling `return nothing` higher up

* Remove some duplicated calculation on field index in `getfield_elim_pass!`

* Fix `try_compute_fieldidx` to return `nothing` for non-`Int` `Integer` field index.

    This can cause `getfield_nothrow` to return incorrect result.
    It also gives the caller worse type info about the return value.

* Teach `getfield_nothrow` that `isbits` field cannot be undefined and getfield on such field cannot throw.

    This is already handled in `isdefined_tfunc`.

* Fix a few wrong use of `isbits` in dead branches

----

Ref JuliaLang#26948 (fa02d34)
Ref JuliaLang#27126 (9100329)

base/compiler/ssair/passes.jl

@@ -290,7 +290,7 @@ function lift_leaves(compact::IncrementalCompact, @nospecialize(stmt),
             else
                 def = compact[leaf]
             end
-            if is_tuple_call(compact, def) && isa(field, Int) && 1 <= field < length(def.args)
+            if is_tuple_call(compact, def) && 1 <= field < length(def.args)
                 lifted = def.args[1+field]
                 if is_old(compact, leaf) && isa(lifted, SSAValue)
                     lifted = OldSSAValue(lifted.id)
@@ -309,8 +309,6 @@ function lift_leaves(compact::IncrementalCompact, @nospecialize(stmt),
                 end
                 (isa(typ, DataType) && (!typ.abstract)) || return nothing
                 @assert !typ.mutable
-                field = try_compute_fieldidx_expr(typ, stmt)
-                field === nothing && return nothing
                 if length(def.args) < 1 + field
                     ftyp = fieldtype(typ, field)
                     if !isbitstype(ftyp)
@@ -325,7 +323,7 @@ function lift_leaves(compact::IncrementalCompact, @nospecialize(stmt),
                     compact[leaf] = nothing
                     for i = (length(def.args) + 1):(1+field)
                         ftyp = fieldtype(typ, i - 1)
-                        isbits(ftyp) || return nothing
+                        isbitstype(ftyp) || return nothing
                         push!(def.args, insert_node!(compact, leaf, result_t, Expr(:new, ftyp)))
                     end
                     compact[leaf] = def
@@ -344,22 +342,22 @@ function lift_leaves(compact::IncrementalCompact, @nospecialize(stmt),
             else
                 typ = compact_exprtype(compact, leaf)
                 if !isa(typ, Const)
+                    # Disabled since #27126
+                    return nothing
                     # If the leaf is an old ssa value, insert a getfield here
                     # We will revisit this getfield later when compaction gets
                     # to the appropriate point.
                     # N.B.: This can be a bit dangerous because it can lead to
                     # infinite loops if we accidentally insert a node just ahead
                     # of where we are
-                    if is_old(compact, leaf) && (isa(field, Int) || isa(field, Symbol))
+                    if is_old(compact, leaf)
                         (isa(typ, DataType) && (!typ.abstract)) || return nothing
                         @assert !typ.mutable
                         # If there's the potential for an undefref error on access, we cannot insert a getfield
-                        if field > typ.ninitialized && !isbits(fieldtype(typ, field))
-                            return nothing
+                        if field > typ.ninitialized && !isbitstype(fieldtype(typ, field))
                             lifted_leaves[leaf] = RefValue{Any}(insert_node!(compact, leaf, make_MaybeUndef(result_t), Expr(:call, :unchecked_getfield, SSAValue(leaf.id), field), true))
                             maybe_undef = true
                         else
-                            return nothing
                             lifted_leaves[leaf] = RefValue{Any}(insert_node!(compact, leaf, result_t, Expr(:call, getfield, SSAValue(leaf.id), field), true))
                         end
                         continue
@@ -672,7 +670,7 @@ function getfield_elim_pass!(ir::IRCode)
 
         isempty(leaves) && continue
 
-        field = try_compute_fieldidx_expr(struct_typ, stmt)
+        field = try_compute_fieldidx(struct_typ, field)
         field === nothing && continue
 
         r = lift_leaves(compact, stmt, result_t, field, leaves)
@@ -807,7 +805,7 @@ function getfield_elim_pass!(ir::IRCode)
                 for stmt in du.uses
                     ir[SSAValue(stmt)] = compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, stmt)
                 end
-                if !isbitstype(fieldtype(typ, fidx))
+                if !isbitstype(ftyp)
                     for (use, list) in preserve_uses
                         push!(list, compute_value_for_use(ir, domtree, allblocks, du, phinodes, fidx, use))
                     end

base/compiler/tfuncs.jl

@@ -695,7 +695,8 @@ function try_compute_fieldidx(typ::DataType, @nospecialize(field))
     if isa(field, Symbol)
         field = fieldindex(typ, field, false)
         field == 0 && return nothing
-    elseif isa(field, Integer)
+    elseif isa(field, Int)
+        # Numerical field name can only be of type `Int`
         max_fields = fieldcount_noerror(typ)
         max_fields === nothing && return nothing
         (1 <= field <= max_fields) || return nothing
@@ -742,7 +743,8 @@ function getfield_nothrow(@nospecialize(s00), @nospecialize(name), @nospecialize
         return false
     end
 
-    s = unwrap_unionall(widenconst(s00))
+    s0 = widenconst(s00)
+    s = unwrap_unionall(s0)
     if isa(s, Union)
         return getfield_nothrow(rewrap(s.a, s00), name, inbounds) &&
             getfield_nothrow(rewrap(s.b, s00), name, inbounds)
@@ -759,6 +761,8 @@ function getfield_nothrow(@nospecialize(s00), @nospecialize(name), @nospecialize
         field = try_compute_fieldidx(s, name.val)
         field === nothing && return false
         field <= s.ninitialized && return true
+        # `try_compute_fieldidx` already check for field index bound.
+        !isvatuple(s) && isbitstype(fieldtype(s0, field)) && return true
     end
 
     return false

test/compiler/irpasses.jl

@@ -315,6 +315,16 @@ let K = rand(2,2)
     @test test_29253(K) == 2
 end
 
+function no_op_refint(r)
+    r[]
+    return
+end
+let code = code_typed(no_op_refint,Tuple{Base.RefValue{Int}})[1].first.code
+    @test length(code) == 1
+    @test isa(code[1], Core.ReturnNode)
+    @test code[1].val === nothing
+end
+
 # check getfield elim handling of GlobalRef
 const _some_coeffs = (1,[2],3,4)
 splat_from_globalref(x) = (x, _some_coeffs...,)