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Auto merge of #94412 - scottmcm:cfg-out-miri-from-swap, r=oli-obk

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For MIRI, cfg out the swap vectorization logic from 94212

Because of #69488 the swap logic from #94212 doesn't currently work in MIRI.

Copying in smaller pieces is probably much worse for its performance anyway, so it'd probably rather just use the simple path regardless.

Part of #94371, though another PR will be needed for the CTFE aspect.

r? `@oli-obk`
cc `@RalfJung`
This commit is contained in:
bors 2022-02-27 17:42:48 +00:00
commit 6a70556616
3 changed files with 64 additions and 13 deletions
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27
src/test/codegen/swap-large-types.rs
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@ -39,6 +39,9 @@ pub fn swap_std(x: &mut KeccakBuffer, y: &mut KeccakBuffer) {
swap(x, y)
}
// Verify that types with usize alignment are swapped via vectored usizes,
// not falling back to byte-level code.
// CHECK-LABEL: @swap_slice
#[no_mangle]
pub fn swap_slice(x: &mut [KeccakBuffer], y: &mut [KeccakBuffer]) {
@ -50,6 +53,8 @@ pub fn swap_slice(x: &mut [KeccakBuffer], y: &mut [KeccakBuffer]) {
}
}
// But for a large align-1 type, vectorized byte copying is what we want.
type OneKilobyteBuffer = [u8; 1024];
// CHECK-LABEL: @swap_1kb_slices
@ -62,3 +67,25 @@ pub fn swap_1kb_slices(x: &mut [OneKilobyteBuffer], y: &mut [OneKilobyteBuffer])
x.swap_with_slice(y);
}
}
// This verifies that the 2×read + 2×write optimizes to just 3 memcpys
// for an unusual type like this. It's not clear whether we should do anything
// smarter in Rust for these, so for now it's fine to leave these up to the backend.
// That's not as bad as it might seem, as for example, LLVM will lower the
// memcpys below to VMOVAPS on YMMs if one enables the AVX target feature.
// Eventually we'll be able to pass `align_of::<T>` to a const generic and
// thus pick a smarter chunk size ourselves without huge code duplication.
#[repr(align(64))]
pub struct BigButHighlyAligned([u8; 64 * 3]);
// CHECK-LABEL: @swap_big_aligned
#[no_mangle]
pub fn swap_big_aligned(x: &mut BigButHighlyAligned, y: &mut BigButHighlyAligned) {
// CHECK-NOT: call void @llvm.memcpy
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* noundef nonnull align 64 dereferenceable(192)
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* noundef nonnull align 64 dereferenceable(192)
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* noundef nonnull align 64 dereferenceable(192)
// CHECK-NOT: call void @llvm.memcpy
swap(x, y)
}