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Missed code elimination with std::mem::replace/std::mem::swap #44701
Description
Edit: As pointed out by @oyvindln, the problem appears to be a regression introduced in 1.20.
Demonstration: https://godbolt.org/g/5uuzVL
Version: rustc 1.22.0-nightly (277476c 2017-09-16) , -C opt-level=3 -C target-cpu=native
Code:
use std::rc::Rc;
use std::cell::RefCell;
pub struct Buffer {
buf: Vec<u8>,
pool: Rc<RefCell<Vec<Vec<u8>>>>,
}
impl Drop for Buffer {
fn drop(&mut self) {
self.pool.borrow_mut()
.push(std::mem::replace(&mut self.buf, vec![]));
}
}Expected result: An optimal code would move self.buf directly into self.pool and then reset self.buf in-place. An acceptable code would moveself.buf into a temporary on the stack, move the temporary into self.pool and reset self.buf in-place.
Observed result:
- Space for two
std::Vec<u8>(each 24 bytes) is allocated on the stack,-48(%rbp)(A) and-96(%rbp)(B). self.bufis copied to A.self.bufis reset in-place.- A is copied to B.
- B is copied to A.
- A is inserted into
self.pool.
Steps 4 and 5 is a completely unnecessary copying of 48 bytes and could be safely removed. Replacing std::mem::replace with an equivalent std::mem::swap call produces a slightly different code with the same basic problem.
Compiler output:
<example::Buffer as core::ops::drop::Drop>::drop:
pushq %rbp
movq %rsp, %rbp
pushq %rbx
subq $88, %rsp
movq %rdi, %rax
movq 24(%rax), %rbx
cmpq $0, 16(%rbx)
jne .LBB6_6
leaq 16(%rbx), %rcx
movq $-1, 16(%rbx)
leaq 24(%rbx), %rdi
movq %rdi, -64(%rbp)
movq %rcx, -56(%rbp)
;2
movq 16(%rax), %rcx
movq %rcx, -32(%rbp)
vmovups (%rax), %xmm0
vmovaps %xmm0, -48(%rbp)
;3
movq $1, (%rax)
vxorps %xmm0, %xmm0, %xmm0
vmovups %xmm0, 8(%rax)
;4
movq -32(%rbp), %rax
movq %rax, -80(%rbp)
vmovaps -48(%rbp), %xmm0
vmovaps %xmm0, -96(%rbp)
;5
movq -80(%rbp), %rax
movq %rax, -32(%rbp)
vmovaps -96(%rbp), %xmm0
vmovaps %xmm0, -48(%rbp)
;6
movq 40(%rbx), %rax
cmpq 32(%rbx), %rax
jne .LBB6_4
callq <alloc::raw_vec::RawVec<T, A>>::double
movq 40(%rbx), %rax
.LBB6_4:
movq 24(%rbx), %rcx
leaq (%rax,%rax,2), %rax
movq -32(%rbp), %rdx
movq %rdx, 16(%rcx,%rax,8)
vmovaps -48(%rbp), %xmm0
vmovups %xmm0, (%rcx,%rax,8)
incq 40(%rbx)
movq $0, 16(%rbx)
addq $88, %rsp
popq %rbx
popq %rbp
retq
.LBB6_6:
callq core::result::unwrap_failed
movq %rax, %rbx
leaq -48(%rbp), %rdi
callq core::ptr::drop_in_place
leaq -64(%rbp), %rdi
callq core::ptr::drop_in_place
movq %rbx, %rdi
callq _Unwind_Resume@PLT
Activity
oyvindln commentedon Sep 20, 2017
Haven't studied the ASM in detail, but with 1.19 and earlier the drop function contains a lot less assembly output. Maybe this is related to #40454 ?
batonius commentedon Sep 20, 2017
Indeed, 1.19 doesn't use a temporary, and #40454 looks suspicious, so I've investigated a little: https://godbolt.org/g/Fo2bZ1 .
BufferOlduses the old version ofstd::swap, andBufferNew- the relevant part of the new version.With 1.20, beta and nightly,
BufferNew::dropcompiles as described above, butBufferOld::dropuses only a single on-stack temporary and doesn't waste time copying data around, so at first it looks like the problem is the newstd::swap. Yet with 1.19, bothBufferNew::dropandBufferOld::dropcompile to the same, more efficient code that doesn't use temporaries at all.So it looks like there has been a regression in the optimizer since 1.20, and the new
std::swap, while not the problem itself, suffers from it unproportionally.batonius commentedon Sep 21, 2017
@aidanhs The label should probably be changed since the issue appears to be a recent regression.
saethlin commentedon Jul 16, 2023
@scottmcm is this related to #112733?
scottmcm commentedon Jul 20, 2023
Yes, this looks almost identical to the
replace_stringcodegen test in that PR: https://github.com/rust-lang/rust/pull/112733/files#diff-aeacafb3161bf010bd64adaedc5709ad328f4e1f222cbbc3b3def7c6ac4e4385R49scottmcm commentedon Jul 20, 2023
Actually, if this is about
allocas duringmem::replace, then I think it's actually already fixed with https://github.com/rust-lang/rust/pull/111010/files#diff-24ed0a5d700ac2b1c9e1ef53e400e7f92ec49f8165fc5bf2647d1efb69e21c80R16