From 9c7bf0807a56429a92c4518f90d3418609688cbb Mon Sep 17 00:00:00 2001 From: Brian Kessler Date: Wed, 9 Aug 2017 00:09:07 -0700 Subject: [PATCH] math/big: avoid unneeded sticky bit calculations As noted in the TODO comment, the sticky bit is only used when the rounding bit is zero or the rounding mode is ToNearestEven. This change makes that check explicit and will eliminate half the sticky bit calculations on average when rounding mode is not ToNearestEven. Change-Id: Ia4709f08f46e682bf97dabe5eb2a10e8e3d7af43 Reviewed-on: https://go-review.googlesource.com/54111 Reviewed-by: Robert Griesemer Run-TryBot: Robert Griesemer Run-TryBot: Ian Lance Taylor --- src/math/big/float.go | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/src/math/big/float.go b/src/math/big/float.go index 7e11f1aff5..afbed306f3 100644 --- a/src/math/big/float.go +++ b/src/math/big/float.go @@ -415,8 +415,9 @@ func (z *Float) round(sbit uint) { // bits > z.prec: mantissa too large => round r := uint(bits - z.prec - 1) // rounding bit position; r >= 0 rbit := z.mant.bit(r) & 1 // rounding bit; be safe and ensure it's a single bit - if sbit == 0 { - // TODO(gri) if rbit != 0 we don't need to compute sbit for some rounding modes (optimization) + // The sticky bit is only needed for rounding ToNearestEven + // or when the rounding bit is zero. Avoid computation otherwise. + if sbit == 0 && (rbit == 0 || z.mode == ToNearestEven) { sbit = z.mant.sticky(r) } sbit &= 1 // be safe and ensure it's a single bit