[LLVMbugs] [Bug 19290] New: Missed optimization with "static const" local variable initialized with function

[bugzilla-daemon at llvm.org]

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: unassignedclangbugs at nondot.org
          Reporter: jonathan.sauer at gmx.de
                CC: llvmbugs at cs.uiuc.edu
    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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