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[dev.garbage] runtime: logic that uses count trailing zero (ctz)
Most (all?) processors that Go supports supply a hardware instruction that takes a byte and returns the number of zeros trailing the first 1 encountered, or 8 if no ones are found. This is the index within the byte of the first 1 encountered. CTZ should improve the performance of the nextFreeIndex function. Since nextFreeIndex wants the next unmarked (0) bit a bit-wise complement is needed before calling ctz. Furthermore unmarked bits associated with previously allocated objects need to be ignored. Instead of writing a 1 as we allocate the code masks all bits less than the freeindex after loading the byte. While this CL does not actual execute a CTZ instruction it supplies a ctz function with the appropiate signature along with the logic to execute it. Change-Id: I5c55ce0ed48ca22c21c4dd9f969b0819b4eadaa7 Reviewed-on: https://go-review.googlesource.com/20169 Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Austin Clements <austin@google.com>
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@ -186,12 +186,22 @@ func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
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return markBits{&s.allocBits[whichByte], uint8(1 << whichBit), allocBitIndex}
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}
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// A temporary stand in for the count trailing zero ctz instruction.
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func ctz(markBits byte) uint8 {
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tz := uint8(0) // trailing zero count.
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if markBits == 0 {
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return 8 // 8
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}
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for mask := byte(1); mask&markBits == 0; mask, tz = mask<<1, tz+1 {
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}
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return tz
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}
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// nextFreeIndex returns the index of the next free object in s at or
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// after the index'th object.
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// There are hardware instructions that can be used to make this
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// faster if profiling warrants it.
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func (s *mspan) nextFreeIndex(index uintptr) uintptr {
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var mask uint8
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if index == s.nelems {
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return index
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}
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@ -200,47 +210,34 @@ func (s *mspan) nextFreeIndex(index uintptr) uintptr {
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}
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whichByte := index / 8
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theByte := s.allocBits[whichByte]
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// Optimize for the first byte holding a free object.
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if theByte != 0xff {
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mask = 1 << (index % 8)
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for index < s.nelems {
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if mask&theByte == 0 {
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return index
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}
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if mask == 1<<7 {
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break
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}
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mask = mask << 1
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index++
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}
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}
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maxByteIndex := (s.nelems - 1) / 8
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theByte = 0xff // Free bit not found in this byte above so set to 0xff.
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// If there was a 0 bit before incoming index then the byte would not be 0xff.
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for theByte == 0xff {
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whichByte++
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if whichByte > maxByteIndex {
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theBitMask := uint8(1<<(index%8) - 1)
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// theBitMask holds a 1 for every bit < index which have already been allocated.
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// Flip the masked marked bits so 1 means a free bit.
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theByte = ^(theByte | theBitMask)
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tz := ctz(theByte)
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if tz != 8 {
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result := uintptr(tz) + whichByte*8
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if result >= s.nelems {
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return s.nelems
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}
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if uintptr(len(s.allocBits)) <= whichByte {
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throw("whichByte > len(s.allocBits")
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}
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theByte = s.allocBits[whichByte]
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return result
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}
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index = whichByte * 8
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mask = uint8(1)
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for index < s.nelems {
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if mask&theByte == 0 {
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return index
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whichByte++
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index = (whichByte) * 8
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for ; index < s.nelems; index += 8 {
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theByte = ^s.allocBits[whichByte]
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tz = ctz(theByte)
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if tz != 8 {
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result := uintptr(tz) + whichByte*8
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if result >= s.nelems {
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return s.nelems
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}
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return result
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}
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if mask == 1<<7 {
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break
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}
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mask = mask << 1
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index++
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whichByte++
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}
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return index
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return s.nelems
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}
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func (s *mspan) isFree(index uintptr) bool {
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