cyb70289 commented on a change in pull request #9635:
URL: https://github.com/apache/arrow/pull/9635#discussion_r592108324
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File path: cpp/src/arrow/util/bit_run_reader.h
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@@ -424,48 +452,15 @@ inline uint64_t
BaseSetBitRunReader<true>::ConsumeBits(uint64_t word, int32_t nu
return word << num_bits;
}
-template <>
-inline BaseSetBitRunReader<false>::BaseSetBitRunReader(const uint8_t* bitmap,
- int64_t start_offset,
- int64_t length)
- : bitmap_(bitmap + start_offset / 8), length_(length), remaining_(length_)
{
- const int8_t bit_offset = static_cast<int8_t>(start_offset % 8);
- if (length > 0 && bit_offset) {
- // Get MSBs from first byte
- current_num_bits_ =
- std::min(static_cast<int32_t>(length), static_cast<int32_t>(8 -
bit_offset));
- current_word_ = LoadPartialWord(bit_offset, current_num_bits_);
- } else {
- current_num_bits_ = 0;
- current_word_ = 0;
- }
-}
-
-template <>
-inline BaseSetBitRunReader<true>::BaseSetBitRunReader(const uint8_t* bitmap,
- int64_t start_offset,
- int64_t length)
- : bitmap_(bitmap + (start_offset + length) / 8),
- length_(length),
- remaining_(length_) {
- const int8_t end_bit_offset = static_cast<int8_t>((start_offset + length) %
8);
- if (length > 0 && end_bit_offset) {
- // Get LSBs from last byte
- ++bitmap_;
- current_num_bits_ =
- std::min(static_cast<int32_t>(length),
static_cast<int32_t>(end_bit_offset));
- current_word_ = LoadPartialWord(8 - end_bit_offset, current_num_bits_);
- } else {
- current_num_bits_ = 0;
- current_word_ = 0;
- }
-}
-
using SetBitRunReader = BaseSetBitRunReader</*Reverse=*/false>;
using ReverseSetBitRunReader = BaseSetBitRunReader</*Reverse=*/true>;
// Functional-style bit run visitors.
+// XXX: Try to make this function small so the compiler can inline and optimize
+// the `visit` function, which is normally a hot loop with vectorizable code.
+// - don't inline SetBitRunReader constructor, it doesn't hurt performance
+// - un-inline NextRun hurts 'many null' cases a bit, but improves normal cases
Review comment:
Compiler failed to inline VisitSetBitRuns function as it contains too
complex codes from BaseSetBitRunReader. It turns `visit` into a function call,
and compiler cannot optimize it (unroll loops, vectorize, etc.).
Un-inline non performance critical code (BaseSetBitRunReader constructor and
NextRun()) makes this function small. It does helps the compiler to inline
`visit` and generates optimized code.
It works cleanly for clang.
Gcc doesn't respect this change, it still cannot inline `visit`. I tried
some tricks to make gcc happy, but the changes are fragile and not explainable,
so I will ignore them.
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