Using the backtrace crate worsens codegen when building with opt-level='s'

[nmathewson]

My apologies if this is a known issue. I wanted to see whether the compiler would
optimize the following code in the way that I expected:

// lib.rs
pub struct Data { body: Box<[u8;509]> }

impl Data {
    pub fn some_bytes(&self) -> u16 {
	u16::from_be_bytes(
		self.body[3..5]
			.try_into()
			.expect("math fail")
	)
    }
}

It worked just fine, and compiled it down to a few simple instructions (according to cargo show-asm):

	mov rax, qword ptr [rdi]
	movzx eax, word ptr [rax + 3]
	rol ax, 8
	ret

But when I tried the same code in my own project, it generated a much worse implementation.

After some experimentation, I found that I could reproduce the issue by changing my test code as follows:

// lib.rs

use backtrace::Backtrace; // <--- ADDED THIS LINE

pub struct Data { body: Box<[u8;509]> }

impl Data {
    pub fn some_bytes(&self) -> u16 {
	u16::from_be_bytes(
		self.body[3..5]
			.try_into()
			.expect("math fail")
	)
    }
}

// Cargo.toml
[package]
name = "slice-playground"
version = "0.1.0"
edition = "2021"

[profile.release]
opt-level = 's'

[dependencies]
backtrace = "0.3.67"

With these changes, I got significantly worse code:

	.cfi_startproc
	push rax
	.cfi_def_cfa_offset 16
	mov rdi, qword ptr [rdi]
	add rdi, 3
	mov esi, 2
	call qword ptr [rip + core::array::<impl core::convert::TryFrom<&[T]> for [T; N]>::try_from@GOTPCREL]
	test al, 1
	jne .LBB1_1
	shr eax, 8
	rol ax, 8
	pop rcx
	.cfi_def_cfa_offset 8
	ret

.LBB1_1:
	.cfi_def_cfa_offset 16
	lea rdi, [rip + .L__unnamed_2]
	lea rcx, [rip + .L__unnamed_1]
	lea r8, [rip + .L__unnamed_3]
	mov rdx, rsp
	mov esi, 9
	call qword ptr [rip + core::result::unwrap_failed@GOTPCREL]

	ud2

Note that ALL of the changes are necessary to see the problem: I had to add the backtrace crate, I had to use it in the code, and I had to set opt-level = 's'.

I've reproduced this behavior with the following versions of Rust:

[2223]$ rustc +nightly --version --verbose 
rustc 1.71.0-nightly (c373194cb 2023-05-24)
binary: rustc
commit-hash: c373194cb6d882dc455a588bcc29c92a96b50252
commit-date: 2023-05-24
host: x86_64-unknown-linux-gnu
release: 1.71.0-nightly
LLVM version: 16.0.4
[2224]$ rustc --version --verbose
rustc 1.69.0 (84c898d65 2023-04-16)
binary: rustc
commit-hash: 84c898d65adf2f39a5a98507f1fe0ce10a2b8dbc
commit-date: 2023-04-16
host: x86_64-unknown-linux-gnu
release: 1.69.0
LLVM version: 15.0.7

Thanks for taking a look at this!

[nmathewson]

I've done some further investigation, trying to track this down. I'm not much practiced at compiler development, so this information might not actually be helpful. Here's what I found:

• While trying to minimize the issue further, I have found that in place of using backtrace, the bad code generation issue also occurs if I say use miniz_oxide.
• I can make the code generation issue go away again if instead I use a copy of miniz_oxide with the body of the function inflate::core::decompress commented out. (This function is very large.)
• When I use cargo show-asm to look at the MIR of compiling some_bytes in both cases, the MIR outputs are identical.
• When I use cargo show-asm to look at the "llvm-input" of some_bytes both cases, there is only one line difference:

-  %_9 = load ptr, ptr %self, align 8, !nonnull !2, !align !4, !noundef !2
+  %_9 = load ptr, ptr %self, align 8, !nonnull !2, !align !3, !noundef !2

I have no idea why the presence of miniz_oxide's decompress function would change the alignment on anything in the example some_bytes function, or why it would influence the behavior of LLVM passes on other code.

I hope this information is helpful somehow, or at least interesting.

[nikic]

Does -Z share-generics=off fix this?

[nmathewson]

Does -Z share-generics=off fix this?

It does!