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Partially-undef constants cause a large size regression in src/test/run-make/wasm-panic-small #84565

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erikdesjardins opened this issue Apr 25, 2021 · 1 comment · Fixed by #94130
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Partially-undef constants cause a large size regression in src/test/run-make/wasm-panic-small #84565

erikdesjardins opened this issue Apr 25, 2021 · 1 comment · Fixed by #94130
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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. I-heavy Issue: Problems and improvements with respect to binary size of generated code.

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@erikdesjardins
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erikdesjardins commented Apr 25, 2021 •
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Opening this to track a limitation of #83698, which does not generate partially-undef constants (controlled by -Zpartially-uninit-const-threshold=1234) due to this issue.

The root cause is that LLVM is unable to const prop (?) a load from a partially-undef constant, which prevents all the panic formatting code from being removed in the src/test/run-make/wasm-panic-small test, which causes the test to fail.

Minimal repro (from #83698 (comment)): https://godbolt.org/z/3vYqM5865

per nikic:

https://godbolt.org/z/1Mff6zrvd shows one of the ways this could be fixed on the LLVM side. We should be dropping trailing zeros from bitcasted GEPs.
@rustbot rustbot added I-heavy Issue: Problems and improvements with respect to binary size of generated code. O-wasm Target: WASM (WebAssembly), http://webassembly.org/ labels Apr 25, 2021
@erikdesjardins erikdesjardins changed the title Undef in constants causes a large size regression in src/test/run-make/wasm-panic-small Partially-undef constants cause a large size regression in src/test/run-make/wasm-panic-small Apr 28, 2021
@rustbot rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. and removed O-wasm Target: WASM (WebAssembly), http://webassembly.org/ labels Apr 28, 2021
@erikdesjardins erikdesjardins mentioned this issue Aug 7, 2021
Merged
bors added a commit to rust-lang-ci/rust that referenced this issue Aug 26, 2021
@bors
Auto merge of rust-lang#83698 - erikdesjardins:undefconst, r=RalfJung…
20997f6 
…,oli-obk

Use undef for uninitialized bytes in constants

Fixes rust-lang#83657

This generates good code when the const is fully uninit, e.g.

```rust
#[no_mangle]
pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
    const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit();
    M
}
```
generates
```asm
fully_uninit:
	ret
```

as you would expect.

There is no improvement, however, when it's partially uninit, e.g.

```rust
pub struct PartiallyUninit {
    x: u64,
    y: MaybeUninit<[u8; 10]>
}

#[no_mangle]
pub const fn partially_uninit() -> PartiallyUninit {
    const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeefcafe, y: MaybeUninit::uninit() };
    X
}
```
generates
```asm
partially_uninit:
	mov	rax, rdi
	mov	rcx, qword ptr [rip + .L__unnamed_1+16]
	mov	qword ptr [rdi + 16], rcx
	movups	xmm0, xmmword ptr [rip + .L__unnamed_1]
	movups	xmmword ptr [rdi], xmm0
	ret

.L__unnamed_1:
	.asciz	"\376\312\357\276\255\336\000"
	.zero	16
	.size	.L__unnamed_1, 24
```
which copies a bunch of zeros in place of the undef bytes, the same as before this change.

Edit: generating partially-undef constants isn't viable at the moment anyways due to rust-lang#84565, so it's disabled
@erikdesjardins
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Underlying issue should be fixed in the next LLVM release by https://reviews.llvm.org/D111023. Thanks @nikic!

When LLVM 14 is out, I will revisit enabling this.

@erikdesjardins erikdesjardins mentioned this issue Feb 18, 2022
Merged
@bors bors closed this as completed in ece55d4 Feb 25, 2022
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Use undef for (some) partially-uninit constants erikdesjardins/rust
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