[llvm-bugs] [Bug 51090] New: Dead Code Elimination Regression

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51090

            Bug ID: 51090
           Summary: Dead Code Elimination Regression
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: theodoros.theodoridis at inf.ethz.ch
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

extern void foo(void);
int a;
short b(unsigned short c, int d) {
 return d < 0 || c > (600 >> d) ? c : c << d;
}
int main() {
 char f;
 int g = 300;
 while (a) {
  f = g;
  if (!b(f, 2)) {
   foo();
   g = 0;
  }
 }
 return 0;
}

commit 1a02aaeaa4f8675490da38ee8cb0d4a6d39815dd introduced a regression
(for Target: x86_64-unknown-linux-gnu).

The generated asm (clang -S -c -O3) for main before that commit was:
main:                                   # @main
.Lmain$local:
        .cfi_startproc
# %bb.0:
        cmpl    $0, .La$local(%rip)
        je      .LBB1_2
        .p2align        4, 0x90
.LBB1_1:                                # =>This Inner Loop Header: Depth=1
        jmp     .LBB1_1
.LBB1_2:
        xorl    %eax, %eax
        retq
.Lfunc_end1:
        .size   main, .Lfunc_end1-main
        .cfi_endproc


and after:

main:                                   # @main
.Lmain$local:
        .cfi_startproc
# %bb.0:
        pushq   %rax
        .cfi_def_cfa_offset 16
        movl    .La$local(%rip), %eax
        testl   %eax, %eax
        je      .LBB1_5
# %bb.1:
        movl    $300, %edx              # imm = 0x12C
        jmp     .LBB1_2
        .p2align        4, 0x90
.LBB1_4:                                #   in Loop: Header=BB1_2 Depth=1
        testl   %eax, %eax
        je      .LBB1_5
.LBB1_2:                                # =>This Inner Loop Header: Depth=1
        movsbl  %dl, %ecx
        movzwl  %cx, %esi
        cmpl    $151, %esi
        setb    %cl
        addb    %cl, %cl
        shll    %cl, %esi
        testw   %si, %si
        jne     .LBB1_4
# %bb.3:                                #   in Loop: Header=BB1_2 Depth=1
        callq   foo
        xorl    %edx, %edx
        movl    .La$local(%rip), %eax
        jmp     .LBB1_4
.LBB1_5:
        xorl    %eax, %eax
        popq    %rcx
        .cfi_def_cfa_offset 8
        retq
.Lfunc_end1:
        .size   main, .Lfunc_end1-main
        .cfi_endproc

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20210714/c2b567c9/attachment.html>