mapleFU commented on code in PR #41690:
URL: https://github.com/apache/arrow/pull/41690#discussion_r1605673886
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cpp/src/arrow/util/bit_util.h:
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@@ -282,15 +282,19 @@ static inline int Log2(uint64_t x) {
// Bitmask selecting the k-th bit in a byte
// static constexpr uint8_t kBitmask[] = {1, 2, 4, 8, 16, 32, 64, 128};
-static constexpr uint8_t GetBitMask(uint8_t index) {
+template <typename T>
+static constexpr uint8_t GetBitMask(T index) {
// DCHECK(index >= 0 && index <= 7);
+ ARROW_COMPILER_ASSUME(index >= 0 && index <= 7);
return static_cast<uint8_t>(1) << index;
}
// the bitwise complement version of kBitmask
// static constexpr uint8_t kFlippedBitmask[] = {254, 253, 251, 247, 239, 223,
191, 127};
-static constexpr uint8_t GetFlippedBitMask(uint8_t index) {
+template <typename T>
+static constexpr uint8_t GetFlippedBitMask(T index) {
// DCHECK(index >= 0 && index <= 7);
+ ARROW_COMPILER_ASSUME(index >= 0 && index <= 7);
Review Comment:
I'm not fully understand this, `signed_index % 8` happens before the call to
GetBitmask doesn't means the code of `GetBitMask` cannot benifit from this? Or
you means the system might analyze `% 8` and do the assume itself?
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