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decompression: optimize ExtractOffset for Arm 

Inspired by kExtractMasksCombined, this patch uses shift
to replace table lookup. On Arm the codegen is 2 shift ops
(lsl+lsr). Comparing to previous ldr which requires 4 cycles
latency, the lsl+lsr only need 2 cycles.
Slight (~0.3%) uplift observed on N1, and ~3% on A72.

Signed-off-by: Jun He <jun.he@arm.com>
Change-Id: I5b53632d22d9e5cf1a49d0c5cdd16265a15de23b
This commit is contained in:
Jun He 2021-08-06 14:46:53 +08:00
parent f2db8f77ce
commit f52721b2b4
1 changed files with 3 additions and 0 deletions
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@ -1081,6 +1081,9 @@ 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__)
constexpr uint64_t kExtractMasksCombined = 0x0000FFFF00FF0000ull;
return val & (uint32_t)((kExtractMasksCombined >> (tag_type * 16)) & 0xFFFF);
#else
static constexpr uint32_t kExtractMasks[4] = {0, 0xFF, 0xFFFF, 0};
return val & kExtractMasks[tag_type];