MIRI says `reverse` is UB, so replace it with something LLVM can vectorize #90821

scottmcm · 2021-11-12T04:41:13Z

For small types with padding, the current implementation is UB because it does integer operations on uninit values.

error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/mod.rs:836:5
    |
836 | /     uint_impl! { u32, u32, i32, 32, 4294967295, 8, "0x10000b3", "0xb301", "0x12345678",
837 | |     "0x78563412", "0x1e6a2c48", "[0x78, 0x56, 0x34, 0x12]", "[0x12, 0x34, 0x56, 0x78]", "", "" }
    | |________________________________________________________________________________________________^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
            
    = note: inside `core::num::<impl u32>::rotate_left` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:211:13
    = note: inside `core::slice::<impl [Foo]>::reverse` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/mod.rs:701:58

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=340739f22ca5b457e1da6f361768edc6

But LLVM has gotten smarter since I wrote the previous implementation in 2017, so this PR removes all the manual magic and just writes it in such a way that LLVM will vectorize. This code is much simpler and has very little unsafe, and is actually faster to boot!

If you're curious to see the codegen: https://rust.godbolt.org/z/Pcn13Y9E3

Before:

running 7 tests
test slice::reverse_simd_f64x4                           ... bench:      17,940 ns/iter (+/- 481) = 58448 MB/s
test slice::reverse_u128                                 ... bench:      17,758 ns/iter (+/- 205) = 59048 MB/s
test slice::reverse_u16                                  ... bench:     158,234 ns/iter (+/- 6,876) = 6626 MB/s
test slice::reverse_u32                                  ... bench:      62,047 ns/iter (+/- 1,117) = 16899 MB/s
test slice::reverse_u64                                  ... bench:      31,582 ns/iter (+/- 552) = 33201 MB/s
test slice::reverse_u8                                   ... bench:      81,253 ns/iter (+/- 1,510) = 12905 MB/s
test slice::reverse_u8x3                                 ... bench:     270,615 ns/iter (+/- 11,463) = 3874 MB/s

After:

running 7 tests
test slice::reverse_simd_f64x4                           ... bench:      17,731 ns/iter (+/- 306) = 59137 MB/s
test slice::reverse_u128                                 ... bench:      17,919 ns/iter (+/- 239) = 58517 MB/s
test slice::reverse_u16                                  ... bench:      43,160 ns/iter (+/- 607) = 24295 MB/s
test slice::reverse_u32                                  ... bench:      21,065 ns/iter (+/- 371) = 49778 MB/s
test slice::reverse_u64                                  ... bench:      21,118 ns/iter (+/- 482) = 49653 MB/s
test slice::reverse_u8                                   ... bench:      76,878 ns/iter (+/- 1,688) = 13639 MB/s
test slice::reverse_u8x3                                 ... bench:     264,723 ns/iter (+/- 5,544) = 3961 MB/s

Those are the existing benches,

rust/library/alloc/benches/slice.rs

Lines 322 to 346 in 14a2fd6

    
           macro_rules! reverse { 
        
               ($name:ident, $ty:ty, $f:expr) => { 
        
                   #[bench] 
        
                   fn $name(b: &mut Bencher) { 
        
                       // odd length and offset by 1 to be as unaligned as possible 
        
                       let n = 0xFFFFF; 
        
                       let mut v: Vec<_> = (0..1 + (n / mem::size_of::<$ty>() as u64)).map($f).collect(); 
        
                       b.iter(|| black_box(&mut v[1..]).reverse()); 
        
                       b.bytes = n; 
        
                   } 
        
               }; 
        
           } 
        
           reverse!(reverse_u8, u8, |x| x as u8); 
        
           reverse!(reverse_u16, u16, |x| x as u16); 
        
           reverse!(reverse_u8x3, [u8; 3], |x| [x as u8, (x >> 8) as u8, (x >> 16) as u8]); 
        
           reverse!(reverse_u32, u32, |x| x as u32); 
        
           reverse!(reverse_u64, u64, |x| x as u64); 
        
           reverse!(reverse_u128, u128, |x| x as u128); 
        
           #[repr(simd)] 
        
           struct F64x4(f64, f64, f64, f64); 
        
           reverse!(reverse_simd_f64x4, F64x4, |x| { 
        
               let x = x as f64; 
        
               F64x4(x, x, x, x) 
        
           });

…LLVM can vectorize For small types with padding, the current implementation is UB because it does integer operations on uninit values. But LLVM has gotten smarter since I wrote the previous implementation in 2017, so remove all the manual magic and just write it in such a way that LLVM will vectorize. This code is much simpler (albeit nuanced) and has very little `unsafe`, and is actually faster to boot!

rust-highfive · 2021-11-12T04:41:16Z

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

Mark-Simulacrum · 2021-11-14T15:27:38Z

@bors r+ rollup=never

bors · 2021-11-14T15:27:39Z

📌 Commit 71f5cfb has been approved by Mark-Simulacrum

bors · 2021-11-14T18:21:22Z

⌛ Testing commit 71f5cfb with merge 834126ba1b45171372be457c925e7209624feeee...

bors · 2021-11-14T18:28:01Z

💔 Test failed - checks-actions

scottmcm · 2021-11-14T20:53:40Z

I'm quite confident in the unsafe in here, so this is scary

rustc exited with signal: 11 (core dumped)
error: could not compile `bitflags`

I'll just close my eyes and
@bors retry

Mark-Simulacrum · 2021-11-14T21:16:33Z

This is #90812, almost certainly.

bors · 2021-11-14T21:50:21Z

⌛ Testing commit 71f5cfb with merge 9e811bc4c0e96a0a804668a9f743d510ef7f2b2a...

bors · 2021-11-14T21:52:07Z

💔 Test failed - checks-actions

Mark-Simulacrum · 2021-11-14T21:56:46Z

@bors retry missing log

bors · 2021-11-14T21:58:21Z

⌛ Testing commit 71f5cfb with merge ea32ff4f0641626a78ae29916df2610c1b6a2c0e...

bors · 2021-11-14T22:15:01Z

💔 Test failed - checks-actions

bors · 2021-11-15T10:04:33Z

⌛ Testing commit f541dd1 with merge 8a3d50db3dad17308528f63668a9dbe628bdaa06...

bors · 2021-11-15T10:08:42Z

💔 Test failed - checks-actions

rust-log-analyzer · 2021-11-15T10:11:18Z

The job dist-mips64el-linux failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling cmake v0.1.44
error: could not compile `cc`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustc --crate-name cc --edition=2018 /cargo/registry/src/github.com-1ecc6299db9ec823/cc-1.0.69/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=0 -C metadata=9022b5d7a536baba -C extra-filename=-9022b5d7a536baba --out-dir /checkout/obj/build/bootstrap/debug/deps -L dependency=/checkout/obj/build/bootstrap/debug/deps --cap-lints allow -Wrust_2018_idioms -Wunused_lifetimes -Wsemicolon_in_expressions_from_macros -Dwarnings` (signal: 11, SIGSEGV: invalid memory reference)
error: build failed
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml --locked
Build completed unsuccessfully in 0:00:55
make: *** [prepare] Error 1
---
   Compiling globset v0.4.5
error: could not compile `regex`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustc --crate-name regex --edition=2018 /cargo/registry/src/github.com-1ecc6299db9ec823/regex-1.5.4/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=0 --cfg 'feature="aho-corasick"' --cfg 'feature="default"' --cfg 'feature="memchr"' --cfg 'feature="perf"' --cfg 'feature="perf-cache"' --cfg 'feature="perf-dfa"' --cfg 'feature="perf-inline"' --cfg 'feature="perf-literal"' --cfg 'feature="std"' --cfg 'feature="unicode"' --cfg 'feature="unicode-age"' --cfg 'feature="unicode-bool"' --cfg 'feature="unicode-case"' --cfg 'feature="unicode-gencat"' --cfg 'feature="unicode-perl"' --cfg 'feature="unicode-script"' --cfg 'feature="unicode-segment"' -C metadata=b0c184cb911fb168 -C extra-filename=-b0c184cb911fb168 --out-dir /checkout/obj/build/bootstrap/debug/deps -L dependency=/checkout/obj/build/bootstrap/debug/deps --extern aho_corasick=/checkout/obj/build/bootstrap/debug/deps/libaho_corasick-6acdc16a2f6d6338.rmeta --extern memchr=/checkout/obj/build/bootstrap/debug/deps/libmemchr-9d65e5e3abdde0ba.rmeta --extern regex_syntax=/checkout/obj/build/bootstrap/debug/deps/libregex_syntax-74207c8016af6cf7.rmeta --cap-lints allow -Wrust_2018_idioms -Wunused_lifetimes -Wsemicolon_in_expressions_from_macros -Dwarnings` (signal: 11, SIGSEGV: invalid memory reference)
error: build failed
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml --locked
Build completed unsuccessfully in 0:00:03
make: *** [prepare] Error 1
---
[RUSTC-TIMING] adler test:false 0.155
[RUSTC-TIMING] unwind test:false 0.075
[RUSTC-TIMING] libc test:false 0.721
[RUSTC-TIMING] compiler_builtins test:false 0.897
rustc exited with signal: 11 (core dumped)
error: could not compile `compiler_builtins`
Caused by:
Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --crate-name compiler_builtins /cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.49/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=10000 -C debuginfo=1 --cfg 'feature="c"' --cfg 'feature="cc"' --cfg 'feature="compiler-builtins"' --cfg 'feature="core"' --cfg 'feature="default"' --cfg 'feature="rustc-dep-of-std"' -C metadata=6bdc4f8396c65e15 -C extra-filename=-6bdc4f8396c65e15 --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --extern core=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/librustc_std_workspace_core-fd44d8b17c2359b1.rmeta --cap-lints allow --cfg=bootstrap -Zsymbol-mangling-version=legacy -Zmacro-backtrace '-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Cprefer-dynamic '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' -Z binary-dep-depinfo -L native=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/build/compiler_builtins-399cb2af88d0d65a/out --cfg 'feature="unstable"' --cfg '__absvdi2="optimized-c"' --cfg '__absvsi2="optimized-c"' --cfg '__absvti2="optimized-c"' --cfg '__addvdi3="optimized-c"' --cfg '__addvsi3="optimized-c"' --cfg '__addvti3="optimized-c"' --cfg '__clzdi2="optimized-c"' --cfg '__clzsi2="optimized-c"' --cfg '__clzti2="optimized-c"' --cfg '__cmpdi2="optimized-c"' --cfg '__cmpti2="optimized-c"' --cfg '__ctzdi2="optimized-c"' --cfg '__ctzsi2="optimized-c"' --cfg '__ctzti2="optimized-c"' --cfg '__divdc3="optimized-c"' --cfg '__divsc3="optimized-c"' --cfg '__divxc3="optimized-c"' --cfg '__extendhfsf2="optimized-c"' --cfg '__ffsti2="optimized-c"' --cfg '__floatdisf="optimized-c"' --cfg '__floatdixf="optimized-c"' --cfg '__floatundidf="optimized-c"' --cfg '__floatundisf="optimized-c"' --cfg '__floatundixf="optimized-c"' --cfg '__int_util="optimized-c"' --cfg '__muldc3="optimized-c"' --cfg '__mulsc3="optimized-c"' --cfg '__mulvdi3="optimized-c"' --cfg '__mulvsi3="optimized-c"' --cfg '__mulvti3="optimized-c"' --cfg '__mulxc3="optimized-c"' --cfg '__negdf2="optimized-c"' --cfg '__negdi2="optimized-c"' --cfg '__negsf2="optimized-c"' --cfg '__negti2="optimized-c"' --cfg '__negvdi2="optimized-c"' --cfg '__negvsi2="optimized-c"' --cfg '__negvti2="optimized-c"' --cfg '__paritydi2="optimized-c"' --cfg '__paritysi2="optimized-c"' --cfg '__parityti2="optimized-c"' --cfg '__popcountdi2="optimized-c"' --cfg '__popcountsi2="optimized-c"' --cfg '__popcountti2="optimized-c"' --cfg '__powixf2="optimized-c"' --cfg '__subvdi3="optimized-c"' --cfg '__subvsi3="optimized-c"' --cfg '__subvti3="optimized-c"' --cfg '__truncdfhf2="optimized-c"' --cfg '__truncdfsf2="optimized-c"' --cfg '__truncsfhf2="optimized-c"' --cfg '__ucmpdi2="optimized-c"' --cfg '__ucmpti2="optimized-c"' --cfg 'apple_versioning="optimized-c"' -l static=compiler-rt` (exit status: 254)
[RUSTC-TIMING] memchr test:false 1.375
[RUSTC-TIMING] rustc_demangle test:false 1.515
[RUSTC-TIMING] alloc test:false 2.293
[RUSTC-TIMING] core test:false 16.420

Mark-Simulacrum · 2021-11-15T16:02:25Z

@bors retry #90812

bors · 2021-11-15T20:19:27Z

⌛ Testing commit f541dd1 with merge 891ca5f...

bors · 2021-11-15T23:23:33Z

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 891ca5f to master...

rust-timer · 2021-11-16T00:54:24Z

Finished benchmarking commit (891ca5f): comparison url.

Summary: This change led to moderate relevant regressions 😿 in compiler performance.

Moderate regression in instruction counts (up to 0.8% on full builds of deeply-nested)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

riking · 2021-11-26T20:53:05Z

Instruction count increase seems expected for a vectorization.

Stop manually SIMDing in `swap_nonoverlapping` Like I previously did for `reverse` (rust-lang#90821), this leaves it to LLVM to pick how to vectorize it, since it can know better the chunk size to use, compared to the "32 bytes always" approach we currently have. A variety of codegen tests are included to confirm that the various cases are still being vectorized. It does still need logic to type-erase in some cases, though, as while LLVM is now smart enough to vectorize over slices of things like `[u8; 4]`, it fails to do so over slices of `[u8; 3]`. As a bonus, this change also means one no longer gets the spurious `memcpy`(s?) at the end up swapping a slice of `__m256`s: <https://rust.godbolt.org/z/joofr4v8Y> <details> <summary>ASM for this example</summary> ## Before (from godbolt) note the `push`/`pop`s and `memcpy` ```x86 swap_m256_slice: push r15 push r14 push r13 push r12 push rbx sub rsp, 32 cmp rsi, rcx jne .LBB0_6 mov r14, rsi shl r14, 5 je .LBB0_6 mov r15, rdx mov rbx, rdi xor eax, eax .LBB0_3: mov rcx, rax vmovaps ymm0, ymmword ptr [rbx + rax] vmovaps ymm1, ymmword ptr [r15 + rax] vmovaps ymmword ptr [rbx + rax], ymm1 vmovaps ymmword ptr [r15 + rax], ymm0 add rax, 32 add rcx, 64 cmp rcx, r14 jbe .LBB0_3 sub r14, rax jbe .LBB0_6 add rbx, rax add r15, rax mov r12, rsp mov r13, qword ptr [rip + memcpy@GOTPCREL] mov rdi, r12 mov rsi, rbx mov rdx, r14 vzeroupper call r13 mov rdi, rbx mov rsi, r15 mov rdx, r14 call r13 mov rdi, r15 mov rsi, r12 mov rdx, r14 call r13 .LBB0_6: add rsp, 32 pop rbx pop r12 pop r13 pop r14 pop r15 vzeroupper ret ``` ## After (from my machine) Note no `rsp` manipulation, sorry for different ASM syntax ```x86 swap_m256_slice: cmpq %r9, %rdx jne .LBB1_6 testq %rdx, %rdx je .LBB1_6 cmpq $1, %rdx jne .LBB1_7 xorl %r10d, %r10d jmp .LBB1_4 .LBB1_7: movq %rdx, %r9 andq $-2, %r9 movl $32, %eax xorl %r10d, %r10d .p2align 4, 0x90 .LBB1_8: vmovaps -32(%rcx,%rax), %ymm0 vmovaps -32(%r8,%rax), %ymm1 vmovaps %ymm1, -32(%rcx,%rax) vmovaps %ymm0, -32(%r8,%rax) vmovaps (%rcx,%rax), %ymm0 vmovaps (%r8,%rax), %ymm1 vmovaps %ymm1, (%rcx,%rax) vmovaps %ymm0, (%r8,%rax) addq $2, %r10 addq $64, %rax cmpq %r10, %r9 jne .LBB1_8 .LBB1_4: testb $1, %dl je .LBB1_6 shlq $5, %r10 vmovaps (%rcx,%r10), %ymm0 vmovaps (%r8,%r10), %ymm1 vmovaps %ymm1, (%rcx,%r10) vmovaps %ymm0, (%r8,%r10) .LBB1_6: vzeroupper retq ``` </details> This does all its copying operations as either the original type or as `MaybeUninit`s, so as far as I know there should be no potential abstract machine issues with reading padding bytes as integers. <details> <summary>Perf is essentially unchanged</summary> Though perhaps with more target features this would help more, if it could pick bigger chunks ## Before ``` running 10 tests test slice::swap_with_slice_4x_usize_30 ... bench: 894 ns/iter (+/- 11) test slice::swap_with_slice_4x_usize_3000 ... bench: 99,476 ns/iter (+/- 2,784) test slice::swap_with_slice_5x_usize_30 ... bench: 1,257 ns/iter (+/- 7) test slice::swap_with_slice_5x_usize_3000 ... bench: 139,922 ns/iter (+/- 959) test slice::swap_with_slice_rgb_30 ... bench: 328 ns/iter (+/- 27) test slice::swap_with_slice_rgb_3000 ... bench: 16,215 ns/iter (+/- 176) test slice::swap_with_slice_u8_30 ... bench: 312 ns/iter (+/- 9) test slice::swap_with_slice_u8_3000 ... bench: 5,401 ns/iter (+/- 123) test slice::swap_with_slice_usize_30 ... bench: 368 ns/iter (+/- 3) test slice::swap_with_slice_usize_3000 ... bench: 28,472 ns/iter (+/- 3,913) ``` ## After ``` running 10 tests test slice::swap_with_slice_4x_usize_30 ... bench: 868 ns/iter (+/- 36) test slice::swap_with_slice_4x_usize_3000 ... bench: 99,642 ns/iter (+/- 1,507) test slice::swap_with_slice_5x_usize_30 ... bench: 1,194 ns/iter (+/- 11) test slice::swap_with_slice_5x_usize_3000 ... bench: 139,761 ns/iter (+/- 5,018) test slice::swap_with_slice_rgb_30 ... bench: 324 ns/iter (+/- 6) test slice::swap_with_slice_rgb_3000 ... bench: 15,962 ns/iter (+/- 287) test slice::swap_with_slice_u8_30 ... bench: 281 ns/iter (+/- 5) test slice::swap_with_slice_u8_3000 ... bench: 5,324 ns/iter (+/- 40) test slice::swap_with_slice_usize_30 ... bench: 275 ns/iter (+/- 5) test slice::swap_with_slice_usize_3000 ... bench: 28,277 ns/iter (+/- 277) ``` </detail>

regression test for reverse() unsoundness Cc rust-lang/rust#90821

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hkratz mentioned this pull request Nov 15, 2021

Nondeterministic(?) SIGSEGV&co. in CI #90812

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bors added the merged-by-bors This PR was explicitly merged by bors. label Nov 15, 2021

bors merged commit 891ca5f into rust-lang:master Nov 15, 2021

rustbot added this to the 1.58.0 milestone Nov 15, 2021

rustbot added the perf-regression Performance regression. label Nov 16, 2021

scottmcm deleted the new-slice-reverse branch November 17, 2021 07:13

scottmcm mentioned this pull request Nov 27, 2021

Suboptimal code generation for omittable bound checks #91109

Open

scottmcm mentioned this pull request Jan 17, 2022

slice::rotate_left/right is not simplified for compile-time-constant small rotates #89714

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scottmcm mentioned this pull request Feb 21, 2022

Stop manually SIMDing in swap_nonoverlapping #94212

Merged

dtolnay added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Feb 21, 2022

RalfJung mentioned this pull request Mar 22, 2022

regression test for reverse() unsoundness rust-lang/miri#2036

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bors added a commit to rust-lang/miri that referenced this pull request Mar 24, 2022

Auto merge of #2036 - RalfJung:vec, r=RalfJung

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regression test for reverse() unsoundness Cc rust-lang/rust#90821

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MIRI says `reverse` is UB, so replace it with something LLVM can vectorize #90821

MIRI says `reverse` is UB, so replace it with something LLVM can vectorize #90821

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riking commented Nov 26, 2021

	macro_rules! reverse {
	($name:ident, $ty:ty, $f:expr) => {
	#[bench]
	fn $name(b: &mut Bencher) {
	// odd length and offset by 1 to be as unaligned as possible
	let n = 0xFFFFF;
	let mut v: Vec<_> = (0..1 + (n / mem::size_of::<$ty>() as u64)).map($f).collect();
	b.iter(\|\| black_box(&mut v[1..]).reverse());
	b.bytes = n;
	}
	};
	}

	reverse!(reverse_u8, u8, \|x\| x as u8);
	reverse!(reverse_u16, u16, \|x\| x as u16);
	reverse!(reverse_u8x3, [u8; 3], \|x\| [x as u8, (x >> 8) as u8, (x >> 16) as u8]);
	reverse!(reverse_u32, u32, \|x\| x as u32);
	reverse!(reverse_u64, u64, \|x\| x as u64);
	reverse!(reverse_u128, u128, \|x\| x as u128);
	#[repr(simd)]
	struct F64x4(f64, f64, f64, f64);
	reverse!(reverse_simd_f64x4, F64x4, \|x\| {
	let x = x as f64;
	F64x4(x, x, x, x)
	});

MIRI says reverse is UB, so replace it with something LLVM can vectorize #90821

MIRI says reverse is UB, so replace it with something LLVM can vectorize #90821

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riking commented Nov 26, 2021

MIRI says `reverse` is UB, so replace it with something LLVM can vectorize #90821

MIRI says `reverse` is UB, so replace it with something LLVM can vectorize #90821