From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Fri, 24 Apr 2020 16:54:13 +0000 (-0700)
Subject: cmd/compile: improve generated eq algs for structs containing strings
X-Git-Tag: go1.15beta1~360
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=5029c3671dec0e086fdefbd5a5bb43afa9711905;p=gostls13.git

cmd/compile: improve generated eq algs for structs containing strings

type T struct {
    s string
    i int
}

Prior to this change, we generated this equality algorithm for T:

func eqT(p, q *T) bool {
    return len(p.s) == len(q.s) &&
      runtime.memequal(p.s.ptr, q.s.ptr, len(p.s)) &&
      p.i == q.i
}

This change splits the two halves of the string equality,
so that we can do the cheap (length) half early and the expensive
(contents) half late. We now generate:

func eqT(p, q *T) bool {
    return len(p.s) == len(q.s) &&
      p.i == q.i &&
      runtime.memequal(p.s.ptr, q.s.ptr, len(p.s))
}

The generated code for these functions tends to be a bit shorter. Examples:

runtime
.eq."".Frame 274 -> 272  (-0.73%)
.eq."".funcinl 249 -> 247  (-0.80%)
.eq."".modulehash 207 -> 205  (-0.97%)

Change-Id: I4efac9f7d410f0a11a94dcee2bf9c0b49b60e301
Reviewed-on: https://go-review.googlesource.com/c/go/+/230205
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
---

diff --git a/src/cmd/compile/internal/gc/alg.go b/src/cmd/compile/internal/gc/alg.go
index 1130a4c17b..33e8eca54e 100644
--- a/src/cmd/compile/internal/gc/alg.go
+++ b/src/cmd/compile/internal/gc/alg.go
@@ -578,7 +578,16 @@ func geneq(t *types.Type) *obj.LSym {
 
 			// Compare non-memory fields with field equality.
 			if !IsRegularMemory(f.Type) {
-				and(eqfield(np, nq, f.Sym))
+				p := nodSym(OXDOT, np, f.Sym)
+				q := nodSym(OXDOT, nq, f.Sym)
+				switch {
+				case f.Type.IsString():
+					eqlen, eqmem := eqstring(p, q)
+					and(eqlen)
+					and(eqmem)
+				default:
+					and(nod(OEQ, p, q))
+				}
 				i++
 				continue
 			}