[LLVMbugs] [Bug 20354] New: Miscompile of fabs due to vecotrization

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20354

            Bug ID: 20354
           Summary: Miscompile of fabs due to vecotrization
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: nicolasweber at gmx.de
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Nicos-MacBook-Pro:clang thakis$ cat test.cc
#include <math.h>
#include <stdio.h>

int main() {
  float v[2] = {-1, -1};
  printf("%p\n", v); // (needed to trigger the bug, else not needed)
  float f = fabs(v[0]);
  printf("%f\n", f < fabs(v[1]) ? f : 0);
  return 0;
}
Nicos-MacBook-Pro:clang thakis$ ~/src/llvm-build/bin/clang -o foo -O2 test.cc
-isysroot $(xcrun --show-sdk-path) 
Nicos-MacBook-Pro:clang thakis$ ./foo
0x7fff53f3aaf8
-1.000000


What! Works fine with minor changes to the code.


Generated llvm looks fine:
Nicos-MacBook-Pro:clang thakis$ ~/src/llvm-build/bin/clang -o foo -O2 test.cc
-isysroot $(xcrun --show-sdk-path) -S -o - -emit-llvm
; ModuleID = 'test.cc'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.9.0"

@.str = private unnamed_addr constant [4 x i8] c"%p\0A\00", align 1
@.str1 = private unnamed_addr constant [4 x i8] c"%f\0A\00", align 1

; Function Attrs: nounwind ssp uwtable
define i32 @main() #0 {
entry:
  %v = alloca i64, align 8
  store i64 -4647714812233515008, i64* %v, align 8
  %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([4 x i8]*
@.str, i64 0, i64 0), i64* %v)
  %0 = load i64* %v, align 8
  %1 = bitcast i64 %0 to <2 x float>
  %2 = call <2 x float> @llvm.fabs.v2f32(<2 x float> %1)
  %3 = extractelement <2 x float> %2, i32 0
  %4 = extractelement <2 x float> %2, i32 1
  %cmp = fcmp olt float %3, %4
  %5 = fpext float %3 to double
  %conv7 = select i1 %cmp, double %5, double 0.000000e+00
  %call8 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([4 x i8]*
@.str1, i64 0, i64 0), double %conv7)
  ret i32 0
}

; Function Attrs: nounwind
declare i32 @printf(i8* nocapture readonly, ...) #1

; Function Attrs: nounwind readnone
declare <2 x float> @llvm.fabs.v2f32(<2 x float>) #2

attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #1 = { nounwind "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
"use-soft-float"="false" }
attributes #2 = { nounwind readnone }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"clang version 3.5.0 (213220)"}


Generated asm:
Nicos-MacBook-Pro:clang thakis$ ~/src/llvm-build/bin/clang -o foo -O2 test.cc
-isysroot $(xcrun --show-sdk-path) -S -o -
    .section    __TEXT,__text,regular,pure_instructions
    .macosx_version_min 10, 9
    .globl    _main
    .align    4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:                                ## %entry
    pushq    %rbp
Ltmp0:
    .cfi_def_cfa_offset 16
Ltmp1:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp2:
    .cfi_def_cfa_register %rbp
    subq    $16, %rsp
    movabsq    $-4647714812233515008, %rax ## imm = 0xBF800000BF800000
    movq    %rax, -8(%rbp)
    leaq    L_.str(%rip), %rdi
    leaq    -8(%rbp), %rsi
    xorl    %eax, %eax
    callq    _printf
    movabsq    $9223372036854775807, %rax ## imm = 0x7FFFFFFFFFFFFFFF
    andq    -8(%rbp), %rax
    movd    %rax, %xmm0
    pshufd    $1, %xmm0, %xmm1        ## xmm1 = xmm0[1,0,0,0]
    ucomiss    %xmm0, %xmm1
    ja    LBB0_1
## BB#2:                                ## %entry
    pxor    %xmm0, %xmm0
    jmp    LBB0_3
LBB0_1:
    cvtss2sd    %xmm0, %xmm0
LBB0_3:                                 ## %entry
    leaq    L_.str1(%rip), %rdi
    movb    $1, %al
    callq    _printf
    xorl    %eax, %eax
    addq    $16, %rsp
    popq    %rbp
    retq
    .cfi_endproc

    .section    __TEXT,__cstring,cstring_literals
L_.str:                                 ## @.str
    .asciz    "%p\n"

L_.str1:                                ## @.str1
    .asciz    "%f\n"


.subsections_via_symbols

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