[LLVMbugs] [Bug 23583] New: instcombine miscompile after r237539

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23583

            Bug ID: 23583
           Summary: instcombine miscompile after r237539
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: hans at chromium.org
                CC: james.molloy at arm.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Consider this reduced test case from a 64-bit Chromium build on Linux:

int f() __attribute__((noinline));
int f() { volatile int x = -1; return x; }

void ParseUniformName(unsigned long *outSubscript) {
    int index = f();
    if (index >= 0) {
        *outSubscript = index;
    } else {
        *outSubscript = 0xffffffffu;
    }
}

#include <stdio.h>
int main() {
  unsigned long index = 0xdeadbeef;
  ParseUniformName(&index);
  printf("index = 0x%zx\n", index);

  return 0;
}

The expected output is 0xffffffff, but compiling with ToT Clang at -O3 yields
0xffffffffffffffff instead.

This seems to have regressed at r237539.


Asm before:

_Z16ParseUniformNamePm:                 # @_Z16ParseUniformNamePm
  .cfi_startproc
# BB#0:                                 # %entry
  pushq %rbx
.Ltmp0:
  .cfi_def_cfa_offset 16
.Ltmp1:
  .cfi_offset %rbx, -16
  movq  %rdi, %rbx
  callq _Z1fv
  testl %eax, %eax
  cltq
  movl  $4294967295, %ecx       # imm = 0xFFFFFFFF
  cmovnsq %rax, %rcx
  movq  %rcx, (%rbx)
  popq  %rbx
  retq

Asm after:

_Z16ParseUniformNamePm:                 # @_Z16ParseUniformNamePm
  .cfi_startproc
# BB#0:                                 # %entry
  pushq %rbx
.Ltmp0:
  .cfi_def_cfa_offset 16
.Ltmp1:
  .cfi_offset %rbx, -16
  movq  %rdi, %rbx
  callq _Z1fv
  cmpl  $-2, %eax
  movl  $-1, %ecx
  cmovgl  %eax, %ecx
  movslq  %ecx, %rax
  movq  %rax, (%rbx)
  popq  %rbx
  retq

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