http://llvm.org/bugs/show_bug.cgi?id=19290
Bug ID: 19290
Summary: Missed optimization with "static const" local variable
initialized with function
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 12311
--> http://llvm.org/bugs/attachment.cgi?id=12311&action=edit
Complete bitcode output by clang for first program fragment
Take the following program fragment:
static int foo()
{
return 23;
}
int bar()
{
static const int FOO = foo();
return FOO;
}
When compiled with clang -r205174, -O3 and without thread-safe statics:
% ~/LLVM/build/Release+Asserts/bin/clang++ -S -emit-llvm -O3
-fno-threadsafe-statics clang.cpp
It is compiled to this (full output attached):
define i32 @_Z3barv() #0 {
entry:
%.b = load i1* @_ZGVZ3barvE3FOO, align 1
br i1 %.b, label %init.end, label %init.check
init.check: ; preds = %entry
store i32 23, i32* @_ZZ3barvE3FOO, align 4, !tbaa !1
%0 = tail call {}* @llvm.invariant.start(i64 4, i8* bitcast (i32*
@_ZZ3barvE3FOO to i8*))
store i1 true, i1* @_ZGVZ3barvE3FOO, align 1
br label %init.end
init.end: ; preds = %entry, %init.check
%1 = load i32* @_ZZ3barvE3FOO, align 4, !tbaa !1
ret i32 %1
}
As can be seen, despite the fact that "FOO" is initialized with a constant
value of 23, this initialization does not happen at compile-time or at least at
run-time during program startup. Instead there is a flag ("_ZGVZ3barvE3FOO")
that is checked each time <bar> is called to check if "FOO" has already been
initialized.
(With thread-safe statics there is additional code to make sure the flag is
checked and set in a thread-safe manner.)
When marking "foo" as "constexpr", the generated code is as expected:
define i32 @_Z3barv() #0 {
entry:
ret i32 23
}
However changing the function to "constexpr" is not always a possible solution,
e.g. when using external headers such as xmmintrin.h:
#include <xmmintrin.h>
float baz()
{
static const __m128 one = _mm_set_ss(1.0f);
return _mm_cvtss_f32(one);
}
Results in:
define float @_Z3bazv() #0 {
entry:
%.b = load i1* @_ZGVZ3bazvE3one, align 1
br i1 %.b, label %init.end, label %init.check
init.check: ; preds = %entry
store <4 x float> <float 1.000000e+00, float 0.000000e+00, float
0.000000e+00, float 0.000000e+00>, <4 x float>* @_ZZ3bazvE3one, align 16, !tbaa
!1
%0 = tail call {}* @llvm.invariant.start(i64 16, i8* bitcast (<4 x float>*
@_ZZ3bazvE3one to i8*))
store i1 true, i1* @_ZGVZ3bazvE3one, align 1
br label %init.end
init.end: ; preds = %entry, %init.check
%1 = load <4 x float>* @_ZZ3bazvE3one, align 16, !tbaa !1
%vecext.i = extractelement <4 x float> %1, i32 0
ret float %vecext.i
}
However, changing the code to the equivalent (essentially inlining _mm_set_ss):
float baz()
{
static const __m128 one = { 1.0f, 0.0f, 0.0f, 0.0f };
return _mm_cvtss_f32(one);
}
Results in the expected:
define float @_Z3bazv() #0 {
entry:
ret float 1.000000e+00
}
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