cmd/compile: fix CSE with commutative ops

CSE opportunities were being missed for commutative ops. We used to
order the args of commutative ops (by arg ID) once at the start of CSE.
But that may not be enough.

i1 = (Load ptr mem)
i2 = (Load ptr mem)
x1 = (Add i1 j)
x2 = (Add i2 j)

Equivalent commutative ops x1 and x2 may not get their args ordered in
the same way because because at the start of CSE, we don't know that
the i values will be CSEd. If x1 and x2 get opposite orders we won't
CSE them.

Instead, (re)order the args of commutative operations by their
equivalence class IDs each time we partition an equivalence class.

Change-Id: Ic609fa83b85299782a5e85bf93dc6023fccf4b0c
Reviewed-on: https://go-review.googlesource.com/33632
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>

1 files changed, 10 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/ssa/cse.go b/src/cmd/compile/internal/ssa/cse.go
index 4e07c89b88..39861b6e2a 100644
--- a/src/cmd/compile/internal/ssa/cse.go
+++ b/src/cmd/compile/internal/ssa/cse.go
@@ -39,10 +39,6 @@ func cse(f *Func) {
 			if v.Type.IsMemory() {
 				continue // memory values can never cse
 			}
-			if opcodeTable[v.Op].commutative && len(v.Args) == 2 && v.Args[1].ID < v.Args[0].ID {
-				// Order the arguments of binary commutative operations.
-				v.Args[0], v.Args[1] = v.Args[1], v.Args[0]
-			}
 			a = append(a, v)
 		}
 	}
@@ -92,6 +88,15 @@ func cse(f *Func) {
 		for i := 0; i < len(partition); i++ {
 			e := partition[i]
 
+			if opcodeTable[e[0].Op].commutative {
+				// Order the first two args before comparison.
+				for _, v := range e {
+					if valueEqClass[v.Args[0].ID] > valueEqClass[v.Args[1].ID] {
+						v.Args[0], v.Args[1] = v.Args[1], v.Args[0]
+					}
+				}
+			}
+
 			// Sort by eq class of arguments.
 			byArgClass.a = e
 			byArgClass.eqClass = valueEqClass
@@ -101,6 +106,7 @@ func cse(f *Func) {
 			splitPoints = append(splitPoints[:0], 0)
 			for j := 1; j < len(e); j++ {
 				v, w := e[j-1], e[j]
+				// Note: commutative args already correctly ordered by byArgClass.
 				eqArgs := true
 				for k, a := range v.Args {
 					b := w.Args[k]