[llvm-bugs] [Bug 52543] New: Dead Code Elimination Regression at -O3 (trunk vs 13.0.0)

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Nov 18 02:58:33 PST 2021


            Bug ID: 52543
           Summary: Dead Code Elimination Regression at -O3 (trunk vs
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: theodort at inf.ethz.ch
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

cat case.c
void foo(void);

static unsigned char f(int *x) { return *x; }

static int b = -1;

int main() {
  unsigned char c = f(&b);
  unsigned char r = c >= 2 ? 1 : 1 << c;
  if (!r)
  b = 0;

trunk cannot eliminate the call to foo but 13.0.0 can:

clang-13.0.0 -O3 -S -o /dev/stdout case.c
main:                                   # @main
# %bb.0:
        movb    $1, b(%rip)
        xorl    %eax, %eax

clang-trunk -O3 -S -o /dev/stdout case.c
main:                                   # @main
# %bb.0:
        pushq   %rax
        .cfi_def_cfa_offset 16
        movb    b(%rip), %al
        xorb    $1, %al
        movzbl  %al, %ecx
        movl    %ecx, %edx
        andl    $1, %edx
        negl    %edx
        andb    $1, %cl
        negb    %cl
        movl    $1, %eax
                                        # kill: def $cl killed $cl killed $ecx
        shll    %cl, %eax
        testb   $-2, %dl
        jne     .LBB0_3
# %bb.1:
        testb   %al, %al
        jne     .LBB0_3
# %bb.2:
        callq   foo
        movb    $1, b(%rip)
        xorl    %eax, %eax
        popq    %rcx
        .cfi_def_cfa_offset 8

clang-trunk -v
clang version 14.0.0 (https://github.com/llvm/llvm-project
Target: x86_64-unknown-linux-gnu
Thread model: posix

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