From: Martin Möhrmann Date: Wed, 7 Nov 2018 08:33:12 +0000 (+0100) Subject: runtime: replace division by span element size by multiply and shifts X-Git-Tag: go1.13beta1~1004 X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=6ca51f78978de7e6206d38b99eef8172fc8540d0;p=gostls13.git runtime: replace division by span element size by multiply and shifts Divisions are generally slow. The compiler can optimize a division to use a sequence of faster multiplies and shifts (magic constants) if the divisor is not know at compile time. The value of the span element size in mcentral.grow is not known at compile time but magic constants to compute n / span.elementsize are already stored in class_to_divmagic and mspan. They however need to be adjusted to work for (0 <= n <= span.npages * pagesize) instead of (0 <= n < span.npages * pagesize). Change-Id: Ieea59f1c94525a88d012f2557d43691967900deb Reviewed-on: https://go-review.googlesource.com/c/go/+/148057 Run-TryBot: Martin Möhrmann TryBot-Result: Gobot Gobot Reviewed-by: Austin Clements Reviewed-by: Keith Randall --- diff --git a/src/runtime/mcentral.go b/src/runtime/mcentral.go index a60eb9fd0c..cd5901054a 100644 --- a/src/runtime/mcentral.go +++ b/src/runtime/mcentral.go @@ -251,16 +251,16 @@ func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool { func (c *mcentral) grow() *mspan { npages := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) size := uintptr(class_to_size[c.spanclass.sizeclass()]) - n := (npages << _PageShift) / size s := mheap_.alloc(npages, c.spanclass, false, true) if s == nil { return nil } - p := s.base() - s.limit = p + size*n - + // Use division by multiplication and shifts to quickly compute: + // n := (npages << _PageShift) / size + n := (npages << _PageShift) >> s.divShift * uintptr(s.divMul) >> s.divShift2 + s.limit = s.base() + size*n heapBitsForAddr(s.base()).initSpan(s) return s } diff --git a/src/runtime/mksizeclasses.go b/src/runtime/mksizeclasses.go index b146dbcd6c..cacbb64207 100644 --- a/src/runtime/mksizeclasses.go +++ b/src/runtime/mksizeclasses.go @@ -171,7 +171,7 @@ func makeClasses() []class { // computeDivMagic computes some magic constants to implement // the division required to compute object number from span offset. // n / c.size is implemented as n >> c.shift * c.mul >> c.shift2 -// for all 0 <= n < c.npages * pageSize +// for all 0 <= n <= c.npages * pageSize func computeDivMagic(c *class) { // divisor d := c.size @@ -180,7 +180,7 @@ func computeDivMagic(c *class) { } // maximum input value for which the formula needs to work. - max := c.npages*pageSize - 1 + max := c.npages * pageSize if powerOfTwo(d) { // If the size is a power of two, heapBitsForObject can divide even faster by masking. diff --git a/src/runtime/sizeclasses.go b/src/runtime/sizeclasses.go index 9e17b001d3..9c1b44fe0b 100644 --- a/src/runtime/sizeclasses.go +++ b/src/runtime/sizeclasses.go @@ -90,6 +90,6 @@ type divMagic struct { baseMask uint16 } -var class_to_divmagic = [_NumSizeClasses]divMagic{{0, 0, 0, 0}, {3, 0, 1, 65528}, {4, 0, 1, 65520}, {5, 0, 1, 65504}, {4, 9, 171, 0}, {6, 0, 1, 65472}, {4, 10, 205, 0}, {5, 9, 171, 0}, {4, 11, 293, 0}, {7, 0, 1, 65408}, {4, 9, 57, 0}, {5, 10, 205, 0}, {4, 12, 373, 0}, {6, 7, 43, 0}, {4, 13, 631, 0}, {5, 11, 293, 0}, {4, 13, 547, 0}, {8, 0, 1, 65280}, {5, 9, 57, 0}, {6, 9, 103, 0}, {5, 12, 373, 0}, {7, 7, 43, 0}, {5, 10, 79, 0}, {6, 10, 147, 0}, {5, 11, 137, 0}, {9, 0, 1, 65024}, {6, 9, 57, 0}, {7, 6, 13, 0}, {6, 11, 187, 0}, {8, 5, 11, 0}, {7, 8, 37, 0}, {10, 0, 1, 64512}, {7, 9, 57, 0}, {8, 6, 13, 0}, {7, 11, 187, 0}, {9, 5, 11, 0}, {8, 8, 37, 0}, {11, 0, 1, 63488}, {8, 9, 57, 0}, {7, 10, 49, 0}, {10, 5, 11, 0}, {7, 10, 41, 0}, {7, 9, 19, 0}, {12, 0, 1, 61440}, {8, 9, 27, 0}, {8, 10, 49, 0}, {11, 5, 11, 0}, {7, 13, 161, 0}, {7, 13, 155, 0}, {8, 9, 19, 0}, {13, 0, 1, 57344}, {8, 12, 111, 0}, {9, 9, 27, 0}, {11, 6, 13, 0}, {7, 14, 193, 0}, {12, 3, 3, 0}, {8, 13, 155, 0}, {11, 8, 37, 0}, {14, 0, 1, 49152}, {11, 8, 29, 0}, {7, 13, 55, 0}, {12, 5, 7, 0}, {8, 14, 193, 0}, {13, 3, 3, 0}, {7, 14, 77, 0}, {12, 7, 19, 0}, {15, 0, 1, 32768}} +var class_to_divmagic = [_NumSizeClasses]divMagic{{0, 0, 0, 0}, {3, 0, 1, 65528}, {4, 0, 1, 65520}, {5, 0, 1, 65504}, {4, 11, 683, 0}, {6, 0, 1, 65472}, {4, 10, 205, 0}, {5, 9, 171, 0}, {4, 11, 293, 0}, {7, 0, 1, 65408}, {4, 13, 911, 0}, {5, 10, 205, 0}, {4, 12, 373, 0}, {6, 9, 171, 0}, {4, 13, 631, 0}, {5, 11, 293, 0}, {4, 13, 547, 0}, {8, 0, 1, 65280}, {5, 9, 57, 0}, {6, 9, 103, 0}, {5, 12, 373, 0}, {7, 7, 43, 0}, {5, 10, 79, 0}, {6, 10, 147, 0}, {5, 11, 137, 0}, {9, 0, 1, 65024}, {6, 9, 57, 0}, {7, 9, 103, 0}, {6, 11, 187, 0}, {8, 7, 43, 0}, {7, 8, 37, 0}, {10, 0, 1, 64512}, {7, 9, 57, 0}, {8, 6, 13, 0}, {7, 11, 187, 0}, {9, 5, 11, 0}, {8, 8, 37, 0}, {11, 0, 1, 63488}, {8, 9, 57, 0}, {7, 10, 49, 0}, {10, 5, 11, 0}, {7, 10, 41, 0}, {7, 9, 19, 0}, {12, 0, 1, 61440}, {8, 9, 27, 0}, {8, 10, 49, 0}, {11, 5, 11, 0}, {7, 13, 161, 0}, {7, 13, 155, 0}, {8, 9, 19, 0}, {13, 0, 1, 57344}, {8, 12, 111, 0}, {9, 9, 27, 0}, {11, 6, 13, 0}, {7, 14, 193, 0}, {12, 3, 3, 0}, {8, 13, 155, 0}, {11, 8, 37, 0}, {14, 0, 1, 49152}, {11, 8, 29, 0}, {7, 13, 55, 0}, {12, 5, 7, 0}, {8, 14, 193, 0}, {13, 3, 3, 0}, {7, 14, 77, 0}, {12, 7, 19, 0}, {15, 0, 1, 32768}} var size_to_class8 = [smallSizeMax/smallSizeDiv + 1]uint8{0, 1, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 29, 29, 29, 29, 29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31} var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv + 1]uint8{31, 32, 33, 34, 35, 36, 36, 37, 37, 38, 38, 39, 39, 39, 40, 40, 40, 41, 42, 42, 43, 43, 43, 43, 43, 44, 44, 44, 44, 44, 44, 45, 45, 45, 45, 46, 46, 46, 46, 46, 46, 47, 47, 47, 48, 48, 49, 50, 50, 50, 50, 50, 50, 50, 50, 50, 50, 51, 51, 51, 51, 51, 51, 51, 51, 51, 51, 52, 52, 53, 53, 53, 53, 54, 54, 54, 54, 54, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 57, 57, 57, 57, 57, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 60, 60, 60, 60, 60, 61, 61, 61, 61, 61, 61, 61, 61, 61, 61, 61, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66}