Missed opportunity to remove a dead store #39873
Comments
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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) { if.then: if.end: declare void @alloc_sock(i32*) local_unnamed_addr #2 |
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Well, yes. I've illustrated the problem with an assembly excerpt, but this is indeed in IR. |
1 similar comment
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Well, yes. I've illustrated the problem with an assembly excerpt, but this is indeed in IR. |
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Changing component - not sure if anyone gets auto-cc'd on "scalar optimizations", but it's worth a try. :) |
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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. |
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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 |
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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 . |
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Added more people from CLs mentioned by Eli. |
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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. |
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MemorySSA backed DSE on trunk can remove the extra stores now (needs -mllvm -enable-dse-memoryssa): https://godbolt.org/z/x5YPsd |
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@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. |
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DSE + MemorySSA is the default now (51ff045). The store gets removed on current trunk without extra options: https://godbolt.org/z/avq4zz |
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."
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