Missed opportunity to remove a dead store

[ramosian-glider]

Bugzilla Link	40527
Resolution	FIXED
Resolved on	Oct 22, 2020 06:00
Version	trunk
OS	Linux
CC	@topperc,@davidbolvansky,@KernelAddress,@efriedma-quic,@fhahn,@kcc,@bryant,@jfbastien,@RKSimon,@rjmccall,@rotateright,@vitalybuka
Fixed by commit(s)	51ff045

Extended Description

When compiling the following code:

============================
void alloc_sock(int *err);
int try_send();

int send(int len) {
int err;
if (len) {
err = -1;
alloc_sock(&err);
err = try_send();
}
return -1;
}

with
$ clang -O2 -g -ftrivial-auto-var-init=pattern -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang -c t.c

Clang produces an extra store of $0xaaaaaaaa to |err|, which is introduced by the -ftrivial-auto-var-init and not removed by other optimizations in the pipeline:

$ objdump -d t.o

0000000000000000 :
0: 50 push %rax
1: c7 44 24 04 aa aa aa movl $0xaaaaaaaa,0x4(%rsp)
8: aa
9: 85 ff test %edi,%edi
b: 74 1d je 2a <send+0x2a>
d: c7 44 24 04 ff ff ff movl $0xffffffff,0x4(%rsp)
14: ff
15: 48 8d 7c 24 04 lea 0x4(%rsp),%rdi
1a: e8 00 00 00 00 callq 1f <send+0x1f>
1f: 31 c0 xor %eax,%eax
21: e8 00 00 00 00 callq 26 <send+0x26>
26: 89 44 24 04 mov %eax,0x4(%rsp)
2a: b8 ff ff ff ff mov $0xffffffff,%eax
2f: 59 pop %rcx
30: c3 retq

As noted by JF Bastien here: https://reviews.llvm.org/D54604, "Seems like the optimizer should sink the store enough that it appears only once on each path."

[rotateright]

Isn't this a job for -dse or some other pass in the IR optimizer rather than a backend problem?

define i32 @​send(i32 %len) {
entry:
%err = alloca i32, align 4
store i32 -1431655766, i32* %err, align 4 <--- dead store
%tobool = icmp eq i32 %len, 0
br i1 %tobool, label %if.end, label %if.then

if.then:
store i32 -1, i32* %err, align 4
call void @​alloc_sock(i32* nonnull %err) #​3
%call = call i32 (...) @​try_send() #​3
store i32 %call, i32* %err, align 4
br label %if.end

if.end:
ret i32 -1
}

declare void @​alloc_sock(i32*) local_unnamed_addr #​2
declare i32 @​try_send(...) local_unnamed_addr #​2

[ramosian-glider]

Well, yes. I've illustrated the problem with an assembly excerpt, but this is indeed in IR.

[ramosian-glider]

Well, yes. I've illustrated the problem with an assembly excerpt, but this is indeed in IR.

[rotateright]

Changing component - not sure if anyone gets auto-cc'd on "scalar optimizations", but it's worth a try. :)

[efriedma-quic]

There's been work in the past to do cross-block DSE, but it's currently not in trunk. See https://reviews.llvm.org/D13363 https://reviews.llvm.org/D22568 etc.

[jfbastien]

Apologies, I thought the code had a diamond and we needed to DSE on one side, and not on the other (some min-cut). This missed optimization is even worst than what I'd understood.

For Apple folks, I mirrored this bug in rdar://problem/47671216

[efriedma-quic]

There's also a patch for partial DSE, which never got merged: https://reviews.llvm.org/D29866 / https://reviews.llvm.org/D29865 . But the store in the given testcase is fully redundant, yes.

There's also slightly more recent WIP patch for cross-block DSE: https://reviews.llvm.org/D29624 .

[ramosian-glider]

Added more people from CLs mentioned by Eli.
Folks, do you think reviving these CLs may help here?

[fhahn]

FYI I tried to revive MemorySSA backed DSE and broke down the patches in relatively small chunks: https://reviews.llvm.org/D72146 (and linked)

The patch series I posted still does not quite cover this case, but I know what's missing (exploring multiple memorydefs) and that should also be addressed at some point soon.

[fhahn]

MemorySSA backed DSE on trunk can remove the extra stores now (needs -mllvm -enable-dse-memoryssa): https://godbolt.org/z/x5YPsd

[davidbolvansky]

@​fhahn, close and commit testcase?

[fhahn]

@​fhahn, close and commit testcase?

I think there are already plenty of tests that cover the scenario in tree. The unnecessary store is now removed without extra options, but I was planning on waiting with closing the issues a bit to wait until the new default sticks.

[fhahn]

DSE + MemorySSA is the default now (51ff045).

The store gets removed on current trunk without extra options: https://godbolt.org/z/avq4zz