[llvm-bugs] [Bug 52525] New: Dead Code Elimination Regression at -O3 (trunk vs 13.0.0)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52525

            Bug ID: 52525
           Summary: Dead Code Elimination Regression at -O3 (trunk vs
                    13.0.0)
           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 char a(char l) { return l; }
static char c(char l) { return l == 0; }

static int b, d;
static int *g;

int main() {
  if (g) {
    unsigned char f = b;
    char n = f >> 2;
    if (c(n)) {
      d = a(b) || d;
    } else if (!b)
      foo();
  }
  g = &b;
}


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
        .cfi_startproc
# %bb.0:
        cmpq    $0, g(%rip)
        je      .LBB0_3
# %bb.1:
        movl    b(%rip), %eax
        testb   $-4, %al
        je      .LBB0_2
.LBB0_3:
        movq    $b, g(%rip)
        xorl    %eax, %eax
        retq
.LBB0_2:
        testb   %al, %al
        setne   %al
        cmpl    $0, d(%rip)
        setne   %cl
        orb     %al, %cl
        movzbl  %cl, %eax
        movl    %eax, d(%rip)
        movq    $b, g(%rip)
        xorl    %eax, %eax
        retq
.Lfunc_end0:


clang-trunk -O3 -S -o /dev/stdout case.c
main:                                   # @main
        .cfi_startproc
# %bb.0:
        pushq   %rax
        .cfi_def_cfa_offset 16
        cmpq    $0, g(%rip)
        je      .LBB0_5
# %bb.1:
        movl    b(%rip), %eax
        cmpb    $3, %al
        jbe     .LBB0_2
# %bb.3:
        testl   %eax, %eax
        je      .LBB0_4
.LBB0_5:
        movq    $b, g(%rip)
        xorl    %eax, %eax
        popq    %rcx
        .cfi_def_cfa_offset 8
        retq
.LBB0_2:
        .cfi_def_cfa_offset 16
        testb   %al, %al
        setne   %al
        cmpl    $0, d(%rip)
        setne   %cl
        orb     %al, %cl
        movzbl  %cl, %eax
        movl    %eax, d(%rip)
        movq    $b, g(%rip)
        xorl    %eax, %eax
        popq    %rcx
        .cfi_def_cfa_offset 8
        retq
.LBB0_4:
        .cfi_def_cfa_offset 16
        callq   foo
        movq    $b, g(%rip)
        xorl    %eax, %eax
        popq    %rcx
        .cfi_def_cfa_offset 8
        retq
.Lfunc_end0:

clang-trunk -v
clang version 14.0.0 (https://github.com/llvm/llvm-project.git
1ee32055ea1dd4db70d1939cbd4f5105c2dce160)
Target: x86_64-unknown-linux-gnu
Thread model: posix


Started with
https://github.com/llvm/llvm-project/commit/f32c0fe8e50534f4210d878b9a1420793860c567

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