[llvm-bugs] [Bug 52260] New: Dead Code Elimination Regression at -O3 (trunk vs 13.0.0)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52260

            Bug ID: 52260
           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 test.c       
void foo(void);

static int a;
static int *b = &a;
static int **c = &b;
static char e;

static unsigned d() {
    if (c)
      return a;
    else
      return 1;
}

int main() {
  int f = d();
  unsigned char g = f;
  e = g >> 4;
  if (e) {
    for (;;) {
      if (f)
        break;
      c = 0;
      foo();
    }
    for (;;)
      ;
  }
}


13.0.0 at -O3 can eliminate the call to foo but trunk cannot:


clang-13 -O3 -S -o /dev/stdout test.c
...
main:                                   # @main
        .cfi_startproc
# %bb.0:
        testb   $-16, a(%rip)
        je      .LBB0_2
        .p2align        4, 0x90
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
        jmp     .LBB0_1
.LBB0_2:
        xorl    %eax, %eax
        retq



clang-trunk -O3 -S -o /dev/stdout test.c
main:                                   # @main
        .cfi_startproc
# %bb.0:
        pushq   %rax
        .cfi_def_cfa_offset 16
        cmpq    $0, c(%rip)
        movl    $1, %eax
        je      .LBB0_2
# %bb.1:
        movl    a(%rip), %eax
.LBB0_2:
        cmpb    $16, %al
        jae     .LBB0_3
# %bb.6:
        xorl    %eax, %eax
        popq    %rcx
        .cfi_def_cfa_offset 8
        retq
.LBB0_3:
        .cfi_def_cfa_offset 16
        testl   %eax, %eax
        je      .LBB0_4
        .p2align        4, 0x90
.LBB0_5:                                # =>This Inner Loop Header: Depth=1
        jmp     .LBB0_5
        .p2align        4, 0x90
.LBB0_4:                                # =>This Inner Loop Header: Depth=1
        movq    $0, c(%rip)
        callq   foo
        jmp     .LBB0_4


clang-trunk -v
clang version 14.0.0 (https://github.com/llvm/llvm-project.git
eda2ebd7807376829eb880c39623f364b438971f)
Target: x86_64-unknown-linux-gnu
Thread model: posix

https://github.com/llvm/llvm-project/commit/f32c0fe8e50534f4210d878b9a1420793860c567
introduced this regression

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