cmd/compile: combine x*n - y*n into (x-y)*n

Do the similar thing to CL 55143 to reduce IMUL.

Change-Id: I1bd38f618058e3cd74fac181f003610ea13f2294
Reviewed-on: https://go-review.googlesource.com/56252
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 42 insertions, 6 deletions

diff --git a/test/mergemul.go b/test/mergemul.go
index 86fbe676cb..a23115b612 100644
--- a/test/mergemul.go
+++ b/test/mergemul.go
@@ -24,7 +24,7 @@ import "fmt"
 //   if a8 != b8 {
 // 	   // print error msg and panic
 //   }
-func makeMergeTest(m1, m2, k int, size string) string {
+func makeMergeAddTest(m1, m2, k int, size string) string {
 
 	model := "    a" + size + ", b" + size
 	model += fmt.Sprintf(" = %%d*n%s + %%d*(n%s+%%d), (%%d+%%d)*n%s + (%%d*%%d)", size, size, size)
@@ -32,7 +32,39 @@ func makeMergeTest(m1, m2, k int, size string) string {
 	test := fmt.Sprintf(model, m1, m2, k, m1, m2, m2, k)
 	test += fmt.Sprintf(`
     if a%s != b%s {
-        fmt.Printf("MergeTest(%d, %d, %d, %s) failed\n")
+        fmt.Printf("MergeAddTest(%d, %d, %d, %s) failed\n")
+        fmt.Printf("%%d != %%d\n", a%s, b%s)
+        panic("FAIL")
+    }
+`, size, size, m1, m2, k, size, size, size)
+	return test + "\n"
+}
+
+// Check that expressions like (c*n - d*(n+k)) get correctly merged by
+// the compiler into (c-d)*n - d*k (with c-d and d*k computed at
+// compile time).
+//
+// The merging is performed by a combination of the multiplication
+// merge rules
+//  (c*n - d*n) -> (c-d)*n
+// and the distributive multiplication rules
+//  c * (d-x)  ->  c*d - c*x
+
+// Generate a MergeTest that looks like this:
+//
+//   a8, b8 = m1*n8 - m2*(n8+k), (m1-m2)*n8 - m2*k
+//   if a8 != b8 {
+// 	   // print error msg and panic
+//   }
+func makeMergeSubTest(m1, m2, k int, size string) string {
+
+	model := "    a" + size + ", b" + size
+	model += fmt.Sprintf(" = %%d*n%s - %%d*(n%s+%%d), (%%d-%%d)*n%s - (%%d*%%d)", size, size, size)
+
+	test := fmt.Sprintf(model, m1, m2, k, m1, m2, m2, k)
+	test += fmt.Sprintf(`
+    if a%s != b%s {
+        fmt.Printf("MergeSubTest(%d, %d, %d, %s) failed\n")
         fmt.Printf("%%d != %%d\n", a%s, b%s)
         panic("FAIL")
     }
@@ -42,10 +74,14 @@ func makeMergeTest(m1, m2, k int, size string) string {
 
 func makeAllSizes(m1, m2, k int) string {
 	var tests string
-	tests += makeMergeTest(m1, m2, k, "8")
-	tests += makeMergeTest(m1, m2, k, "16")
-	tests += makeMergeTest(m1, m2, k, "32")
-	tests += makeMergeTest(m1, m2, k, "64")
+	tests += makeMergeAddTest(m1, m2, k, "8")
+	tests += makeMergeAddTest(m1, m2, k, "16")
+	tests += makeMergeAddTest(m1, m2, k, "32")
+	tests += makeMergeAddTest(m1, m2, k, "64")
+	tests += makeMergeSubTest(m1, m2, k, "8")
+	tests += makeMergeSubTest(m1, m2, k, "16")
+	tests += makeMergeSubTest(m1, m2, k, "32")
+	tests += makeMergeSubTest(m1, m2, k, "64")
 	tests += "\n"
 	return tests
 }