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refactor: Align RISC-V implementation with AArch64 branchless mask extraction #232

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Merged
danilak-G merged 1 commit into from
May 9, 2026
Merged

refactor: Align RISC-V implementation with AArch64 branchless mask extraction #232

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8 changes: 7 additions & 1 deletion snappy.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1375,7 +1375,13 @@ inline uint32_t ExtractOffset(uint32_t val, size_t tag_type) {
reinterpret_cast<const char*>(&kExtractMasksCombined) + 2 * tag_type,
sizeof(result));
return val & result;
#elif defined(__aarch64__)
// For AArch64 and RISC-V, use a bit-twiddling trick to extract the mask from a
// single combined constant instead of a lookup table. The constant packs multiple
// 16-bit masks based on tag_type (see implementation below). The code calculates
// the shift amount from tag_type, right-shifts the constant to move the desired
// mask to the LSB position, then extracts it with & 0xFFFF. This branchless
// approach is often more performant on modern CPUs.
#elif defined(__aarch64__) || (defined(__riscv) && (__riscv_xlen == 64))
constexpr uint64_t kExtractMasksCombined = 0x0000FFFF00FF0000ull;
return val & static_cast<uint32_t>(
(kExtractMasksCombined >> (tag_type * 16)) & 0xFFFF);
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