b954d4d1b5
mem::swap the obvious way for types smaller than the SIMD optimization's block size
LLVM isn't able to remove the alloca for the unaligned block in the post-SIMD tail in some cases, so doing this helps SRoA work in cases where it currently doesn't. Found in the `replace_with` RFC discussion.
Examples of the improvements:
<details>
<summary>swapping `[u16; 3]` takes 1/3 fewer instructions and no stackalloc</summary>
```rust
type Demo = [u16; 3];
pub fn swap_demo(x: &mut Demo, y: &mut Demo) {
std::mem::swap(x, y);
}
```
nightly:
```asm
_ZN4blah9swap_demo17ha1732a9b71393a7eE:
.seh_proc _ZN4blah9swap_demo17ha1732a9b71393a7eE
sub rsp, 32
.seh_stackalloc 32
.seh_endprologue
movzx eax, word ptr [rcx + 4]
mov word ptr [rsp + 4], ax
mov eax, dword ptr [rcx]
mov dword ptr [rsp], eax
movzx eax, word ptr [rdx + 4]
mov word ptr [rcx + 4], ax
mov eax, dword ptr [rdx]
mov dword ptr [rcx], eax
movzx eax, word ptr [rsp + 4]
mov word ptr [rdx + 4], ax
mov eax, dword ptr [rsp]
mov dword ptr [rdx], eax
add rsp, 32
ret
.seh_handlerdata
.section .text,"xr",one_only,_ZN4blah9swap_demo17ha1732a9b71393a7eE
.seh_endproc
```
this PR:
```asm
_ZN4blah9swap_demo17ha1732a9b71393a7eE:
mov r8d, dword ptr [rcx]
movzx r9d, word ptr [rcx + 4]
movzx eax, word ptr [rdx + 4]
mov word ptr [rcx + 4], ax
mov eax, dword ptr [rdx]
mov dword ptr [rcx], eax
mov word ptr [rdx + 4], r9w
mov dword ptr [rdx], r8d
ret
```
</details>
<details>
<summary>`replace_with` optimizes down much better</summary>
Inspired by https://github.com/rust-lang/rfcs/pull/2490,
```rust
fn replace_with<T, F>(x: &mut Option<T>, f: F)
where F: FnOnce(Option<T>) -> Option<T>
{
*x = f(x.take());
}
pub fn inc_opt(mut x: &mut Option<i32>) {
replace_with(&mut x, |i| i.map(|j| j + 1));
}
```
Rust 1.26.0:
```asm
_ZN4blah7inc_opt17heb0acb64c51777cfE:
mov rax, qword ptr [rcx]
movabs r8, 4294967296
add r8, rax
shl rax, 32
movabs rdx, -4294967296
and rdx, r8
xor r8d, r8d
test rax, rax
cmove rdx, rax
setne r8b
or rdx, r8
mov qword ptr [rcx], rdx
ret
```
Nightly (better thanks to ScalarPair, maybe?):
```asm
_ZN4blah7inc_opt17h66df690be0b5899dE:
mov r8, qword ptr [rcx]
mov rdx, r8
shr rdx, 32
xor eax, eax
test r8d, r8d
setne al
add edx, 1
mov dword ptr [rcx], eax
mov dword ptr [rcx + 4], edx
ret
```
This PR:
```asm
_ZN4blah7inc_opt17h1426dc215ecbdb19E:
xor eax, eax
cmp dword ptr [rcx], 0
setne al
mov dword ptr [rcx], eax
add dword ptr [rcx + 4], 1
ret
```
Where that add is beautiful -- using an addressing mode to not even need to explicitly go through a register -- and the remaining imperfection is well-known (https://github.com/rust-lang/rust/pull/49420#issuecomment-376805721).
</details>
This directory contains the source code of the rust project, including:
rustcand its testslibstd- Various submodules for tools, like rustdoc, rls, etc.
For more information on how various parts of the compiler work, see the rustc guide.
Their is also useful content in the following READMEs, which are gradually being moved over to the guide:
- https://github.com/rust-lang/rust/tree/master/src/librustc/ty/query
- https://github.com/rust-lang/rust/tree/master/src/librustc/dep_graph
- https://github.com/rust-lang/rust/blob/master/src/librustc/infer/region_constraints
- https://github.com/rust-lang/rust/tree/master/src/librustc/infer/higher_ranked
- https://github.com/rust-lang/rust/tree/master/src/librustc/infer/lexical_region_resolve