Worse codegen with `mem::take(vec)` than on stable

[clubby789]

With this code

pub fn foo(t: &mut Vec<usize>) {
    let mut taken = std::mem::take(t);
    taken.pop();
    *t = taken;
}

Stable produces

playground::foo:
	sub	rsp, 24
	movups	xmm0, xmmword ptr [rdi]
	movaps	xmmword ptr [rsp], xmm0
	mov	rax, qword ptr [rdi + 16]
	xor	ecx, ecx
	sub	rax, 1
	cmovae	rcx, rax
	mov	qword ptr [rdi + 16], rcx
	add	rsp, 24
	ret

Whereas beta/nightly produces

playground::foo:
	push	r15
	push	r14
	push	rbx
	mov	rbx, rdi
	mov	r14, qword ptr [rdi + 8]
	mov	r15, qword ptr [rdi + 16]
	xorps	xmm0, xmm0
	movups	xmmword ptr [rdi + 8], xmm0
	mov	rsi, qword ptr [rdi + 8]
	test	rsi, rsi
	je	.LBB0_2
	shl	rsi, 3
	mov	edi, 8
	mov	edx, 8
	call	qword ptr [rip + __rust_dealloc@GOTPCREL]

.LBB0_2:
	xor	eax, eax
	sub	r15, 1
	cmovae	rax, r15
	mov	qword ptr [rbx + 8], r14
	mov	qword ptr [rbx + 16], rax
	pop	rbx
	pop	r14
	pop	r15
	ret

searched nightlies: from nightly-2022-07-02 to nightly-2022-07-03
regressed nightly: nightly-2022-07-03
searched commit range: 46b8c23...f2d9393
regressed commit: 0075bb4

bisected with cargo-bisect-rustc v0.6.4

Host triple: x86_64-unknown-linux-gnu

@rustbot label +regression-from-stable-to-nightly +A-mir-opt-inlining

[nikic]

Godbolt: https://rust.godbolt.org/z/4GTrh1EGx

Result IR can be further optimized by GVN, so this might be addressable on the LLVM side.

[nikic]

Looks like this got a bit worse on LLVM main because an additional assume is being preserved: https://llvm.godbolt.org/z/95eMe6j7q

Anyway, there is a phase ordering problem here. MemCpyOpt runs after GVN, and only at that point do we convert the memcpy into a memset, which makes the following load from it easy to fold.

An easy fix would probably be to support memset in InstCombine load store forwarding. But this is no longer going to fix this issue due to the aforementioned assume issue. Ugh.

[nikic]

Upstream patch for InstCombine: https://reviews.llvm.org/D137323

An alternative solution would be to move MemCpyOpt prior to GVN, but I'm not sure whether that would cause other issues.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[nikic]

Upstream patch for SimplifyCFG: https://reviews.llvm.org/D137339

Together these produce the following final IR:

define void @_ZN7example3foo17h9f11ae7042742a8dE(ptr noalias nocapture noundef align 8 dereferenceable(24) %t) unnamed_addr #0 personality ptr @rust_eh_personality {
start:
  %taken.sroa.6.0.t.sroa_idx = getelementptr inbounds i8, ptr %t, i64 8
  %taken.sroa.6.0.copyload5 = load i64, ptr %taken.sroa.6.0.t.sroa_idx, align 8, !alias.scope !2, !noalias !6
  %taken.sroa.7.0.t.sroa_idx = getelementptr inbounds i8, ptr %t, i64 16
  %taken.sroa.7.0.copyload6 = load i64, ptr %taken.sroa.7.0.t.sroa_idx, align 8, !alias.scope !2, !noalias !6
  tail call void @llvm.memset.p0.i64(ptr noundef nonnull align 8 dereferenceable(16) %taken.sroa.6.0.t.sroa_idx, i8 0, i64 16, i1 false)
  %0 = icmp eq i64 %taken.sroa.7.0.copyload6, 0
  %1 = add i64 %taken.sroa.7.0.copyload6, -1
  %spec.select = select i1 %0, i64 0, i64 %1
  store i64 %taken.sroa.6.0.copyload5, ptr %taken.sroa.6.0.t.sroa_idx, align 8
  store i64 %spec.select, ptr %taken.sroa.7.0.t.sroa_idx, align 8
  ret void
}

Ignoring the opportunity to form a usub.sat, this is optimal.

[clubby789]

Nightly now compiles to

example::foo:
        mov     rax, qword ptr [rdi + 16]
        xor     ecx, ecx
        sub     rax, 1
        cmovae  rcx, rax
        mov     qword ptr [rdi + 16], rcx
        ret