Why does the Rust compiler not optimize code assuming that two mutable references cannot alias?

Question

As far as I know, reference/pointer aliasing can hinder the compiler's ability to generate optimized code, since they must ensure the generated binary behaves correctly in the case where the two references/pointers indeed alias. For instance, in the following C code,

void adds(int  *a, int *b) {
    *a += *b;
    *a += *b;
}

when compiled by clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final) with the -O3 flag, it emits

0000000000000000 <adds>:
   0:    8b 07                    mov    (%rdi),%eax
   2:    03 06                    add    (%rsi),%eax
   4:    89 07                    mov    %eax,(%rdi)  # The first time
   6:    03 06                    add    (%rsi),%eax
   8:    89 07                    mov    %eax,(%rdi)  # The second time
   a:    c3                       retq

Here the code stores back to (%rdi) twice in case int *a and int *b alias.

When we explicitly tell the compiler that these two pointers cannot alias with the restrict keyword:

void adds(int * restrict a, int * restrict b) {
    *a += *b;
    *a += *b;
}

Then Clang will emit a more optimized version of the binary code:

0000000000000000 <adds>:
   0:    8b 06                    mov    (%rsi),%eax
   2:    01 c0                    add    %eax,%eax
   4:    01 07                    add    %eax,(%rdi)
   6:    c3                       retq

Since Rust makes sure (except in unsafe code) that two mutable references cannot alias, I would think that the compiler should be able to emit the more optimized version of the code.

When I test with the code below and compile it with rustc 1.35.0 with -C opt-level=3 --emit obj,

#![crate_type = "staticlib"]
#[no_mangle]
fn adds(a: &mut i32, b: &mut i32) {
    *a += *b;
    *a += *b;
}

it generates:

0000000000000000 <adds>:
   0:    8b 07                    mov    (%rdi),%eax
   2:    03 06                    add    (%rsi),%eax
   4:    89 07                    mov    %eax,(%rdi)
   6:    03 06                    add    (%rsi),%eax
   8:    89 07                    mov    %eax,(%rdi)
   a:    c3                       retq

This does not take advantage of the guarantee that a and b cannot alias.

Is this because the current Rust compiler is still in development and has not yet incorporated alias analysis to do the optimization?

Is this because there is still a chance that a and b could alias, even in safe Rust?

Side remark: "Since Rust makes sure (except in unsafe code) that two mutable references cannot alias" -- it is worth mentioning that even in unsafe code, aliasing mutable references are not allowed and result in undefined behavior. You can have aliasing raw pointers, but unsafe code does not actually allow you to ignore Rust standard rules. It's just a common misconception and thus worth pointing out. — Lukas Kalbertodt
It took me a while to figure out what the example is getting at, because I'm not skilled at reading asm, so in case it helps anyone else: it boils down to whether the two += operations in the body of adds can be reinterpreted as *a = *a + *b + *b. If the pointers don't alias, they can, you can even see what amounts to b* + *b in the second asm listing: 2: 01 c0 add %eax,%eax. But if they do alias, they can't, because by the time you add *b for the second time, it will contain a different value than the first time around (the one you store on line 4: of the first asm listing). — dlukes

Shepmaster Shepmaster · Accepted Answer · 2019-07-29T18:13:37

Rust originally did enable LLVM's noalias attribute, but this caused miscompiled code. When all supported LLVM versions no longer miscompile the code, it will be re-enabled.

If you add -Zmutable-noalias=yes to the compiler options, you get the expected assembly:

adds:
        mov     eax, dword ptr [rsi]
        add     eax, eax
        add     dword ptr [rdi], eax
        ret

Simply put, Rust put the equivalent of C's restrict keyword everywhere, far more prevalent than any usual C program. This exercised corner cases of LLVM more than it was able to handle correctly. It turns out that C and C++ programmers simply don't use restrict as frequently as &mut is used in Rust.

This has happened multiple times.

Rust 1.0 through 1.7 — noalias enabled
Rust 1.8 through 1.27 — noalias disabled
Rust 1.28 through 1.29 — noalias enabled
Rust 1.30 through 1.54 — noalias disabled
Rust 1.54 through ??? — noalias conditionally enabled

Why does the Rust compiler not optimize code assuming that two mutable references cannot alias?

1 Answers

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