From 481ce36ec43dc699c6be18ec81dfce02ca24601c Mon Sep 17 00:00:00 2001
From: Matthew Dempsky <mdempsky@google.com>
Date: Thu, 18 Dec 2014 13:50:57 -0800
Subject: [PATCH] runtime: simplify cpuprof hash calculation
"x*41" computes the same value as "x*31 + x*7 + x*3" and (when
compiled by gc) requires just one multiply instruction instead of
three.
Alternatively, the expression could be written as "(x<<2+x)<<3 + x" to
use shifts instead of multiplies (which is how GCC optimizes "x*41").
But gc currently emits suboptimal instructions for this expression
anyway (e.g., separate SHL+ADD instructions rather than LEA on
386/amd64). Also, if such an optimization was worthwhile, it would
seem better to implement it as part of gc's strength reduction logic.
Change-Id: I7156b793229d723bbc9a52aa9ed6111291335277
Reviewed-on: https://go-review.googlesource.com/1830
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
---
src/runtime/cpuprof.go | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/runtime/cpuprof.go b/src/runtime/cpuprof.go
index d56678e21f..a9bb1add85 100644
--- a/src/runtime/cpuprof.go
+++ b/src/runtime/cpuprof.go
@@ -203,7 +203,7 @@ func (p *cpuProfile) add(pc []uintptr) {
h := uintptr(0)
for _, x := range pc {
h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1)))
- h += x*31 + x*7 + x*3
+ h += x * 41
}
p.count++
--
2.30.9