Dead Code Elimination Regression

[thetheodor]

Bugzilla Link	51090
Version	trunk
OS	Linux
CC	@fhahn,@ttheodor

Extended Description

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 1a02aae 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

[xgupta]

This issue seems to be fixed on trunk - https://godbolt.org/z/4PMjGTcqo.