cmd/compile: add more generic rewrite rules to reassociate (op (op y C) x|C)

With this patch, opt pass can expose more obvious constant-folding
opportunites.

Example:
func test(i int) int {return (i+8)-(i+4)}

The previous version:
  MOVD	"".i(FP), R0
  ADD	$8, R0, R1
  ADD	$4, R0, R0
  SUB	R0, R1, R0
  MOVD	R0, "".~r1+8(FP)
  RET	(R30)

The optimized version:
  MOVD	$4, R0
  MOVD	R0, "".~r1+8(FP)
  RET	(R30)

This patch removes some existing reassociation rules, such as "x+(z-C)",
because the current generic rewrite rules will canonicalize "x-const"
to "x+(-const)", making "x+(z-C)" equal to "x+(z+(-C))".

This patch also adds test cases.

Change-Id: I857108ba0b5fcc18a879eeab38e2551bc4277797
Reviewed-on: https://go-review.googlesource.com/c/go/+/237137
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 30 insertions, 1 deletions

diff --git a/test/codegen/arithmetic.go b/test/codegen/arithmetic.go
index 45fdb68903..afd4d66bd9 100644
--- a/test/codegen/arithmetic.go
+++ b/test/codegen/arithmetic.go
@@ -462,7 +462,6 @@ func addSpecial(a, b, c uint32) (uint32, uint32, uint32) {
 	return a, b, c
 }
 
-
 // Divide -> shift rules usually require fixup for negative inputs.
 // If the input is non-negative, make sure the fixup is eliminated.
 func divInt(v int64) int64 {
@@ -472,3 +471,33 @@ func divInt(v int64) int64 {
 	// amd64:-`.*SARQ.*63,`, -".*SHRQ", ".*SARQ.*[$]9,"
 	return v / 512
 }
+
+// The reassociate rules "x - (z + C) -> (x - z) - C" and
+// "(z + C) -x -> C + (z - x)" can optimize the following cases.
+func constantFold1(i0, j0, i1, j1, i2, j2, i3, j3 int) (int, int, int, int) {
+	// arm64:"SUB","ADD\t[$]2"
+	r0 := (i0 + 3) - (j0 + 1)
+	// arm64:"SUB","SUB\t[$]4"
+	r1 := (i1 - 3) - (j1 + 1)
+	// arm64:"SUB","ADD\t[$]4"
+	r2 := (i2 + 3) - (j2 - 1)
+	// arm64:"SUB","SUB\t[$]2"
+	r3 := (i3 - 3) - (j3 - 1)
+	return r0, r1, r2, r3
+}
+
+// The reassociate rules "x - (z + C) -> (x - z) - C" and
+// "(C - z) - x -> C - (z + x)" can optimize the following cases.
+func constantFold2(i0, j0, i1, j1 int) (int, int) {
+	// arm64:"ADD","MOVD\t[$]2","SUB"
+	r0 := (3 - i0) - (j0 + 1)
+	// arm64:"ADD","MOVD\t[$]4","SUB"
+	r1 := (3 - i1) - (j1 - 1)
+	return r0, r1
+}
+
+func constantFold3(i, j int) int {
+	// arm64: "MOVD\t[$]30","MUL",-"ADD",-"LSL"
+	r := (5 * i) * (6 * j)
+	return r
+}