From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Tue, 21 Jan 2020 03:42:11 +0000 (-0800)
Subject: cmd/compile: document non-commutative rule detection
X-Git-Tag: go1.15beta1~1171
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=2dfdd85c4f3a5ca1b127644e8c7c6b2ba5a89bca;p=gostls13.git

cmd/compile: document non-commutative rule detection

This documentation was lost in CL 213703.
This change restores it.

Change-Id: I544f15771d8a7390893efbda93478b46095ccf3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/215541
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
---

diff --git a/src/cmd/compile/internal/ssa/gen/rulegen.go b/src/cmd/compile/internal/ssa/gen/rulegen.go
index 7c6e1b29dd..47cf5f15da 100644
--- a/src/cmd/compile/internal/ssa/gen/rulegen.go
+++ b/src/cmd/compile/internal/ssa/gen/rulegen.go
@@ -966,9 +966,16 @@ func genMatch0(rr *RuleRewrite, arch arch, match, v string, cnt map[string]int)
 	commutative := op.commutative
 	if commutative {
 		if args[0] == args[1] {
+			// When we have (Add x x), for any x,
+			// even if there are other uses of x besides these two,
+			// and even if x is not a variable,
+			// we can skip the commutative match.
 			commutative = false
 		}
 		if cnt[args[0]] == 1 && cnt[args[1]] == 1 {
+			// When we have (Add x y) with no other uses
+			// of x and y in the matching rule and condition,
+			// then we can skip the commutative match (Add y x).
 			commutative = false
 		}
 	}