Dead Code Elimination Regression at -O3 (trunk vs. 14.0.4)

[thetheodor]

static short a;
static int b;

void foo(void);
void bar9_(void);
int main() {
    char c;
    b = a = 23;
    for (; b <= 1;)
        a = 1;
    bar9_();
    c = a >= 18 || 2 > 2 >> a ? 1 : 1 << a;
    if (c == 0)
        foo();
}

llvm-4c2bccfda3892ae13e97b6bfdbc99ec8cf5d095d (trunk) -O3 can not eliminate foo but llvm-llvmorg-14.0.4 -O3 can.

Target: x86_64-unknown-linux-gnu

---

llvm-4c2bccfda3892ae13e97b6bfdbc99ec8cf5d095d (trunk) -O3 [-emit-llvm] -S -o /dev/stdout case.c

Reduced assembly

main:                                   # @main
	.cfi_startproc
# %bb.0:
	pushq	%rax
	.cfi_def_cfa_offset 16
	movb	$1, a(%rip)
	callq	bar9_@PLT
	xorl	%eax, %eax
	cmpb	$0, a(%rip)
	movl	$23, %ecx
	cmovel	%eax, %ecx
	movl	$2, %eax
	shrl	%cl, %eax
	addl	$-8, %ecx
	cmpw	$9, %cx
	ja	.LBB0_3
# %bb.1:
	cmpl	$2, %eax
	jb	.LBB0_3
# %bb.2:
	callq	foo@PLT
.LBB0_3:
	xorl	%eax, %eax
	popq	%rcx
	.cfi_def_cfa_offset 8
	retq
.Lfunc_end0:
	.size	main, .Lfunc_end0-main

---

llvm-llvmorg-14.0.4 -O3 [-emit-llvm] -S -o /dev/stdout case.c

Reduced assembly

main:                                   # @main
	.cfi_startproc
# %bb.0:
	pushq	%rax
	.cfi_def_cfa_offset 16
	movb	$1, a(%rip)
	callq	bar9_
	xorl	%eax, %eax
	popq	%rcx
	.cfi_def_cfa_offset 8
	retq
.Lfunc_end0:
	.size	main, .Lfunc_end0-main

---

Bisection

Bisected to: c0e06c7
Committed by: @nikic

[rotateright]

I'm guessing this will be solved if we add an icmp fold that is similar to some recent patches for shift+mask:
https://alive2.llvm.org/ce/z/fEDQXU

[rotateright]

On 2nd thought, we can generalize further than the above. As long as we're shifting a power-of-2 constant, the compare constant can be almost anything - it's just a check of leading zeros:
https://alive2.llvm.org/ce/z/TyGiT0
...and there should be a sibling fold for 'ult'.

[rotateright]

Should be fixed with:
0399473

[fhahn]

Thanks for the fix!