[llvm-bugs] [Bug 52535] New: Dead Code Elimination Regression at -O3 (trunk vs 13.0.0)
via llvm-bugs
llvm-bugs at lists.llvm.org
Wed Nov 17 09:51:24 PST 2021
https://bugs.llvm.org/show_bug.cgi?id=52535
Bug ID: 52535
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
static int a = 1, b;
void foo(void);
int main() {
for (unsigned d = 0; d < 19; ++d) {
if ((char)a <= d)
for (; b < 1; b++)
foo();
else
break;
}
a = 128;
}
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:
xorl %eax, %eax
retq
.Lfunc_end0:
clang-trunk -O3 -S -o /dev/stdout case.c
main: # @main
.cfi_startproc
# %bb.0:
pushq %rbp
.cfi_def_cfa_offset 16
pushq %rbx
.cfi_def_cfa_offset 24
pushq %rax
.cfi_def_cfa_offset 32
.cfi_offset %rbx, -24
.cfi_offset %rbp, -16
xorl %ebx, %ebx
movl $-128, %ebp
jmp .LBB0_1
.p2align 4, 0x90
.LBB0_4: # in Loop: Header=BB0_1 Depth=1
addl $1, %ebx
cmpl $19, %ebx
je .LBB0_5
.LBB0_1: # =>This Loop Header: Depth=1
# Child Loop BB0_3 Depth 2
cmpb $0, a(%rip)
movl $1, %eax
cmovnel %ebp, %eax
cmpl %ebx, %eax
ja .LBB0_5
# %bb.2: # in Loop: Header=BB0_1 Depth=1
cmpl $0, b(%rip)
jg .LBB0_4
.p2align 4, 0x90
.LBB0_3: # Parent Loop BB0_1 Depth=1
# => This Inner Loop Header: Depth=2
callq foo
movl b(%rip), %eax
leal 1(%rax), %ecx
movl %ecx, b(%rip)
testl %eax, %eax
js .LBB0_3
jmp .LBB0_4
.LBB0_5:
movb $1, a(%rip)
xorl %eax, %eax
addq $8, %rsp
.cfi_def_cfa_offset 24
popq %rbx
.cfi_def_cfa_offset 16
popq %rbp
.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/3af8a11bc66b93dc3f63ce7192647a27189aeefc
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