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Dead Code Elimination Regression at -O3 (trunk vs 13.0.0) #51885

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llvmbot opened this issue Nov 18, 2021 · 4 comments
Closed

Dead Code Elimination Regression at -O3 (trunk vs 13.0.0) #51885

llvmbot opened this issue Nov 18, 2021 · 4 comments
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@llvmbot
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llvmbot commented Nov 18, 2021

Bugzilla Link 52543
Resolution FIXED
Resolved on Nov 20, 2021 12:39
Version trunk
OS Linux
Reporter LLVM Bugzilla Contributor
CC @rotateright
Fixed by commit(s) 337948a

Extended Description

cat case.c
void foo(void);

static unsigned char f(int *x) { return *x; }

static int b = -1;

int main() {
unsigned char c = f(&b);
unsigned char r = c >= 2 ? 1 : 1 << c;
if (!r)
foo();
b = 0;
}

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:

movb	$1, b(%rip)
xorl	%eax, %eax
retq

.Lfunc_end0:

clang-trunk -O3 -S -o /dev/stdout case.c
main: # @​main
.cfi_startproc

%bb.0:

pushq	%rax
.cfi_def_cfa_offset 16
movb	b(%rip), %al
xorb	$1, %al
movzbl	%al, %ecx
movl	%ecx, %edx
andl	$1, %edx
negl	%edx
andb	$1, %cl
negb	%cl
movl	$1, %eax
                                    # kill: def $cl killed $cl killed $ecx
shll	%cl, %eax
testb	$-2, %dl
jne	.LBB0_3

%bb.1:

testb	%al, %al
jne	.LBB0_3

%bb.2:

callq	foo

.LBB0_3:
movb $1, b(%rip)
xorl %eax, %eax
popq %rcx
.cfi_def_cfa_offset 8
retq
.Lfunc_end0:

clang-trunk -v
clang version 14.0.0 (https://github.com/llvm/llvm-project 693b020)
Target: x86_64-unknown-linux-gnu
Thread model: posix

Started with acabad9
@llvmbot
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llvmbot commented Nov 18, 2021

assigned to @rotateright

@rotateright
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Started with
https://github.com/llvm/llvm-project/commit/
acabad9

The irony is that patch solved a previous bug similar to this one - bug 52260. :)

We're missing some kind of canonicalization/analysis in instcombine/value tracking.

This case reduces to:
https://alive2.llvm.org/ce/z/qMCISF

define i1 @​src(i1 %b) {
%not..b = xor i1 %b, true
%t0 = sext i1 %not..b to i32
%conv = and i32 %t0, 255
%t1 = and i32 %t0, 254
%cmp = icmp eq i32 %t1, 0
%shl = shl nuw i32 1, %conv
%t2 = trunc i32 %shl to i8
%tobool.not5 = icmp eq i8 %t2, 0
%r = select i1 %cmp, i1 %tobool.not5, i1 false
ret i1 %r
}

define i1 @​tgt(i1 %b) {
ret i1 0
}

@rotateright
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The sequence in comment 1 actually shows multiple failures. Fixing any of them may solve the larger example, so I forked some over to new bug reports for independent tracking.

Extra use prevents converting and-of-sext to select:
https://alive2.llvm.org/ce/z/ZpyTyi

declare void @​use(i32)

define i32 @​src(i1 %b) {
%s = sext i1 %b to i32
call void @​use(i32 %s) ; extra use is preventing an existing fold
%a = and i32 %s, 254
ret i32 %a
}

define i32 @​tgt(i1 %b) {
%s = sext i1 %b to i32
call void @​use(i32 %s)
%r = select i1 %b, i32 254, i32 0
ret i32 %r
}

I'll keep this one here for now. I'm not sure yet if converting to select if the mask is not a constant is worthwhile - that could potentially lead to other problems because that's not reversible:
https://alive2.llvm.org/ce/z/KLS2GM

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Should be fixed with:
https://reviews.llvm.org/rG337948ac6e22

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 11, 2021
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