codegen: Unnecessary `memcpy` when returning a mutated `mut self`

[Filiprogrammer]

When implementing an operator by delegating to its Assign version, two formulations that should be equivalent produce different assembly. Using mut self generates a redundant memcpy compared to using let mut ret = self.

Produces suboptimal assembly (Godbolt):

use std::ops::{BitAnd, BitAndAssign};

struct Foo([u128; 1]);

impl BitAndAssign<&Self> for Foo {
    #[unsafe(no_mangle)]
    fn bitand_assign(&mut self, other: &Self) {
        self.0[0] &= other.0[0];
    }
}

impl BitAnd<&Self> for Foo {
    type Output = Foo;
    #[unsafe(no_mangle)]
    fn bitand(mut self, other: &Self) -> Self::Output {
        self &= other;
        self
    }
}

Generated LLVM IR

define void @bitand(ptr dead_on_unwind noalias nofree noundef writable writeonly sret([16 x i8]) align 16 captures(none) dereferenceable(16) initializes((0, 16)) %_0, ptr dead_on_return noalias nofree noundef align 16 captures(none) dereferenceable(16) %self, ptr noalias nofree noundef readonly align 16 captures(none) dereferenceable(16) %other) unnamed_addr {
start:
  tail call void @llvm.experimental.noalias.scope.decl(metadata !29)
  tail call void @llvm.experimental.noalias.scope.decl(metadata !33)
  %_3.i = load i128, ptr %other, align 16
  %0 = load i128, ptr %self, align 16
  %1 = and i128 %0, %_3.i
  store i128 %1, ptr %self, align 16
  tail call void @llvm.memcpy.p0.p0.i64(ptr noundef nonnull align 16 dereferenceable(16) %_0, ptr noundef nonnull align 16 dereferenceable(16) %self, i64 16, i1 false)
  ret void
}

define void @bitand_assign(ptr noalias nofree noundef align 16 captures(none) dereferenceable(16) %self, ptr noalias nofree noundef readonly align 16 captures(none) dereferenceable(16) %other) unnamed_addr {
start:
  %_3 = load i128, ptr %other, align 16
  %0 = load i128, ptr %self, align 16
  %1 = and i128 %0, %_3
  store i128 %1, ptr %self, align 16
  ret void
}

declare void @llvm.memcpy.p0.p0.i64(ptr noalias writeonly captures(none), ptr noalias readonly captures(none), i64, i1 immarg) #2

declare void @llvm.experimental.noalias.scope.decl(metadata) #3

Generated amd64 assembly:

bitand:
        mov     rax, rdi
        movaps  xmm0, xmmword ptr [rsi]
        andps   xmm0, xmmword ptr [rdx]
        movaps  xmmword ptr [rsi], xmm0   ; store to self
        movaps  xmm0, xmmword ptr [rsi]   ; load from self
        movaps  xmmword ptr [rdi], xmm0   ; store to output (should have been direct)
        ret

bitand_assign:
        movaps  xmm0, xmmword ptr [rdi]
        andps   xmm0, xmmword ptr [rsi]
        movaps  xmmword ptr [rdi], xmm0
        ret

Produces optimal assembly (Godbolt):

use std::ops::{BitAnd, BitAndAssign};

struct Foo([u128; 1]);

impl BitAndAssign<&Self> for Foo {
    #[unsafe(no_mangle)]
    fn bitand_assign(&mut self, other: &Self) {
        self.0[0] &= other.0[0];
    }
}

impl BitAnd<&Self> for Foo {
    type Output = Foo;
    #[unsafe(no_mangle)]
    fn bitand(self, other: &Self) -> Self::Output {
        let mut ret = self;
        ret &= other;
        ret
    }
}

Generated LLVM IR

define void @bitand(ptr dead_on_unwind noalias nofree noundef writable writeonly sret([16 x i8]) align 16 captures(none) dereferenceable(16) initializes((0, 16)) %_0, ptr dead_on_return noalias nofree noundef readonly align 16 captures(none) dereferenceable(16) %self, ptr noalias nofree noundef readonly align 16 captures(none) dereferenceable(16) %other) unnamed_addr {
start:
  %ret.sroa.0.0.copyload = load i128, ptr %self, align 16
  %_3.i = load i128, ptr %other, align 16
  %0 = and i128 %_3.i, %ret.sroa.0.0.copyload
  store i128 %0, ptr %_0, align 16
  ret void
}

define void @bitand_assign(ptr noalias nofree noundef align 16 captures(none) dereferenceable(16) %self, ptr noalias nofree noundef readonly align 16 captures(none) dereferenceable(16) %other) unnamed_addr {
start:
  %_3 = load i128, ptr %other, align 16
  %0 = load i128, ptr %self, align 16
  %1 = and i128 %0, %_3
  store i128 %1, ptr %self, align 16
  ret void
}

Generated amd64 assembly:

bitand:
        mov     rax, rdi
        movaps  xmm0, xmmword ptr [rdx]
        andps   xmm0, xmmword ptr [rsi]
        movaps  xmmword ptr [rdi], xmm0
        ret

bitand_assign:
        movaps  xmm0, xmmword ptr [rdi]
        andps   xmm0, xmmword ptr [rsi]
        movaps  xmmword ptr [rdi], xmm0
        ret

---

Observed on rustc 1.98.0-nightly (cb46fbb 2026-06-08)

[i-like-ICEs]

I bisected this, and it shows that it's in 2022?

searched toolchains nightly-2021-06-01 through nightly-2026-06-10

---

Regression in nightly-2022-03-05

---

looking for regression commit between 2022-03-04 and 2022-03-05
fetching (via remote github) commits from max(10913c0, 2022-03-02) to 9fcbc32
ending github query because we found starting sha: 10913c0
get_commits_between returning commits, len: 10
commit[0] 2022-03-03: Auto merge of #87835 - xFrednet:rfc-2383-expect-attribute-with-ids, r=wesleywiser
commit[1] 2022-03-03: Auto merge of #94571 - matthiaskrgr:rollup-4ul5ydb, r=matthiaskrgr
commit[2] 2022-03-04: Auto merge of #94009 - compiler-errors:gat-rustdoc, r=GuillaumeGomez
commit[3] 2022-03-04: Auto merge of #94588 - Dylan-DPC:rollup-7pxd0i3, r=Dylan-DPC
commit[4] 2022-03-04: Auto merge of #94096 - cjgillot:ensure-stability, r=lcnr
commit[5] 2022-03-04: Auto merge of #94159 - erikdesjardins:align-load, r=nikic
commit[6] 2022-03-04: Auto merge of #94539 - tmiasko:string-attributes, r=nikic
commit[7] 2022-03-04: Auto merge of #94570 - shampoofactory:reopen-91719, r=workingjubilee
commit[8] 2022-03-04: Auto merge of #94612 - matthiaskrgr:rollup-2jm5wkr, r=matthiaskrgr
commit[9] 2022-03-04: Auto merge of #94494 - jonhoo:bump-autocfg, r=Mark-Simulacrum

I'm not sure what could have caused it.

[Filiprogrammer]

Thank you for narrowing this down. I bisected further and identified b4bf56c as the first commit that generates the unnecessary memcpy.