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[libc] Use UMAXV.4S to reduce bcmp result. #99260

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Merged
merged 4 commits into from
Oct 13, 2025
Merged

[libc] Use UMAXV.4S to reduce bcmp result. #99260

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6f0bbb0
[𝘀𝗽𝗿] changes to main this commit is based on
DamonFool Jul 17, 2024
841c2e8
[𝘀𝗽𝗿] initial version
pcc Jul 17, 2024
776b912
[𝘀𝗽𝗿] changes introduced through rebase
pveras Oct 13, 2025
db12e3b
Rebase
pcc Oct 13, 2025
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18 changes: 6 additions & 12 deletions libc/src/string/memory_utils/op_aarch64.h
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Original file line number Diff line number Diff line change
Expand Up @@ -84,8 +84,7 @@ template <size_t Size> struct Bcmp {
uint8x16_t a = vld1q_u8(_p1);
uint8x16_t n = vld1q_u8(_p2);
uint8x16_t an = veorq_u8(a, n);
uint32x2_t an_reduced = vqmovn_u64(vreinterpretq_u64_u8(an));
return vmaxv_u32(an_reduced);
return vmaxvq_u32(vreinterpretq_u32_u8(an));
} else if constexpr (Size == 32) {
auto _p1 = as_u8(p1);
auto _p2 = as_u8(p2);
Expand All @@ -97,12 +96,9 @@ template <size_t Size> struct Bcmp {
uint8x16_t bo = veorq_u8(b, o);
// anbo = (a ^ n) | (b ^ o). At least one byte is nonzero if there is
// a difference between the two buffers. We reduce this value down to 4
// bytes in two steps. First, calculate the saturated move value when
// going from 2x64b to 2x32b. Second, compute the max of the 2x32b to get
// a single 32 bit nonzero value if a mismatch occurred.
// bytes using the UMAXV instruction to compute the max across the vector.
uint8x16_t anbo = vorrq_u8(an, bo);
uint32x2_t anbo_reduced = vqmovn_u64(vreinterpretq_u64_u8(anbo));
return vmaxv_u32(anbo_reduced);
return vmaxvq_u32(vreinterpretq_u32_u8(anbo));
} else if constexpr ((Size % BlockSize) == 0) {
for (size_t offset = 0; offset < Size; offset += BlockSize)
if (auto value = Bcmp<BlockSize>::block(p1 + offset, p2 + offset))
Expand All @@ -129,8 +125,7 @@ template <size_t Size> struct Bcmp {
uint8x16_t bo = veorq_u8(b, o);
// anbo = (a ^ n) | (b ^ o)
uint8x16_t anbo = vorrq_u8(an, bo);
uint32x2_t anbo_reduced = vqmovn_u64(vreinterpretq_u64_u8(anbo));
return vmaxv_u32(anbo_reduced);
return vmaxvq_u32(vreinterpretq_u32_u8(anbo));
} else if constexpr (Size == 32) {
auto _p1 = as_u8(p1);
auto _p2 = as_u8(p2);
Expand All @@ -150,9 +145,8 @@ template <size_t Size> struct Bcmp {
uint8x16_t cpdq = vorrq_u8(cp, dq);
// abnocpdq = ((a ^ n) | (b ^ o)) | ((c ^ p) | (d ^ q)). Reduce this to
// a nonzero 32 bit value if a mismatch occurred.
uint64x2_t abnocpdq = vreinterpretq_u64_u8(anbo | cpdq);
uint32x2_t abnocpdq_reduced = vqmovn_u64(abnocpdq);
return vmaxv_u32(abnocpdq_reduced);
uint8x16_t abnocpdq = anbo | cpdq;
return vmaxvq_u32(vreinterpretq_u32_u8(abnocpdq));
} else {
static_assert(cpp::always_false<decltype(Size)>, "SIZE not implemented");
}
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