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Optimize tag extraction for ARM with conditional increment instruction generation (csinc). For codegen see https://gcc.godbolt.org/z/a8z9j95Pv 

PiperOrigin-RevId: 382688740
This commit is contained in:
Snappy Team 2021-07-02 07:52:56 +00:00 committed by Victor Costan
parent b3fb0b5b4b
commit b4888f7616
1 changed files with 25 additions and 2 deletions
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snappy.cc
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@ -1004,7 +1004,26 @@ void MemMove(ptrdiff_t dst, const void* src, size_t size) {
}
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
size_t AdvanceToNextTag(const uint8_t** ip_p, size_t* tag) {
size_t AdvanceToNextTagARMOptimized(const uint8_t** ip_p, size_t* tag) {
const uint8_t*& ip = *ip_p;
// This section is crucial for the throughput of the decompression loop.
// The latency of an iteration is fundamentally constrained by the
// following data chain on ip.
// ip -> c = Load(ip) -> delta1 = (c & 3) -> ip += delta1 or delta2
// delta2 = ((c >> 2) + 1) ip++
// This is different from X86 optimizations because ARM has conditional add
// instruction (csinc) and it removes several register moves.
const size_t literal_tag_offset = (*tag >> 2) + 1;
const size_t tag_type = *tag & 3;
const bool is_literal = (tag_type == 0);
*tag = is_literal ? ip[literal_tag_offset] : ip[tag_type];
ip += is_literal ? literal_tag_offset : tag_type;
ip++;
return tag_type;
}
SNAPPY_ATTRIBUTE_ALWAYS_INLINE
size_t AdvanceToNextTagX86Optimized(const uint8_t** ip_p, size_t* tag) {
const uint8_t*& ip = *ip_p;
// This section is crucial for the throughput of the decompression loop.
// The latency of an iteration is fundamentally constrained by the
@ -1084,7 +1103,11 @@ std::pair<const uint8_t*, ptrdiff_t> DecompressBranchless(
// For literals tag_type = 0, hence we will always obtain 0 from
// ExtractLowBytes. For literals offset will thus be kLiteralOffset.
ptrdiff_t len_min_offset = table.length_minus_offset[tag];
size_t tag_type = AdvanceToNextTag(&ip, &tag);
#if defined(__aarch64__)
size_t tag_type = AdvanceToNextTagARMOptimized(&ip, &tag);
#else
size_t tag_type = AdvanceToNextTagX86Optimized(&ip, &tag);
#endif
uint32_t next = LittleEndian::Load32(old_ip);
size_t len = len_min_offset & 0xFF;
len_min_offset -= ExtractOffset(next, tag_type);