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decompress: add hint to remove extra AND 

Clang doesn't realize the load with free zero-extension,
and emits another extra 'and xn, xm, 0xff' to calc offset.
With this change ,this extra op is removed, and consistent
1.7% performance uplift is observed.

Signed-off-by: Jun He <jun.he@arm.com>
Change-Id: Ica4617852c4b93eadc6c5c551dc3961ffbadb8f0
This commit is contained in:
Jun He 2021-08-03 15:36:20 +08:00
parent f2db8f77ce
commit d643b9a988
1 changed files with 9 additions and 0 deletions
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snappy.cc
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@ -1108,6 +1108,15 @@ std::pair<const uint8_t*, ptrdiff_t> DecompressBranchless(
// ip points just past the tag and we are touching at maximum kSlopBytes
// in an iteration.
size_t tag = ip[-1];
#if defined(__clang__) && defined(__aarch64__)
// Workaround for https://bugs.llvm.org/show_bug.cgi?id=51317
// when loading 1 byte, clang for aarch64 doesn't realize that it(ldrb)
// comes with free zero-extension, so clang generates another
// 'and xn, xm, 0xff' before it use that as the offset. This 'and' is
// redundant and can be removed by adding this dummy asm, which gives
// clang a hint that we're doing the zero-extension at the load.
asm("" ::"r"(tag));
#endif
do {
// The throughput is limited by instructions, unrolling the inner loop
// twice reduces the amount of instructions checking limits and also