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CRC32C dispatch: prefer scalar Zbc over Zvbc (fix PR3332 small-buffer regression) #3601

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Merged
chenBright merged 1 commit into from
Oct 18, 2026
+25 −3
Merged

CRC32C dispatch: prefer scalar Zbc over Zvbc (fix PR3332 small-buffer regression) #3601

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28 changes: 25 additions & 3 deletions src/butil/crc32c.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -814,19 +814,41 @@ static bool isSSE42() {

typedef uint32_t (*Function)(uint32_t, const char*, size_t);

#if defined(__riscv) && (__riscv_xlen == 64) && defined(__riscv_zbc) && defined(__riscv_zvbc)
// Both Zbc and Zvbc: scalar is cheaper for small inputs, the vector fold
// wins from ~256 B on.
static constexpr size_t kRiscvVclmulThresholdBytes = 256;
static uint32_t RiscvExtend(uint32_t crc, const char* buf, size_t size) {
return (size >= kRiscvVclmulThresholdBytes ? rv_crc32c_vclmul : rv_crc32c_clmul)(crc, buf, size);
}
#endif

static inline Function Choose_Extend() {
#ifdef __SSE4_2__
if (isSSE42()) {
return (Function)ExtendImpl<FastCRC32Functor>;
}
#endif
#if defined(__riscv) && (__riscv_xlen == 64) && (defined(__riscv_zbc) || defined(__riscv_zvbc))
#if defined(__riscv_zvbc)
#if defined(__riscv_zbc) && defined(__riscv_zvbc)
{
static const bool has_zbc = is_zbc();
static const bool has_zvbc = is_zvbc();
if (has_zbc && has_zvbc) {
return (Function)RiscvExtend;
}
if (has_zvbc) {
return (Function)rv_crc32c_vclmul;
}
if (has_zbc) {
return (Function)rv_crc32c_clmul;
}
}
#elif defined(__riscv_zvbc)

@Felix-Gong Felix-Gong Oct 14, 2026 •
edited
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Changed to #elif defined(__riscv_zbc). The outer condition already requires zbc or zvbc, so behavior is unchanged, but writing it out makes the intent clear if that condition is ever edited.

if (is_zvbc()) {
return (Function)rv_crc32c_vclmul;
}
#endif
#if defined(__riscv_zbc)
#elif defined(__riscv_zbc)
if (is_zbc()) {
return (Function)rv_crc32c_clmul;
}
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