cmd/compile: generalize strength reduction of mulq

* This is an improved version of an earlier patch.
* Verified with gcc up to 100.
* Limited to two instructions based on costs from
https://gmplib.org/~tege/x86-timing.pdf

Change-Id: Ib7c37de6fd8e0ba554459b15c7409508cbcf6728
Reviewed-on: https://go-review.googlesource.com/21103
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 45 insertions, 0 deletions

diff --git a/test/strength.go b/test/strength.go
new file mode 100644
index 0000000000..94d589c240
--- /dev/null
+++ b/test/strength.go
@@ -0,0 +1,45 @@
+// runoutput
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Generate test of strength reduction for multiplications
+// with contstants. Especially useful for amd64/386.
+
+package main
+
+import "fmt"
+
+func testMul(fact, bits int) string {
+	n := fmt.Sprintf("testMul_%d_%d", fact, bits)
+	fmt.Printf("func %s(s int%d) {\n", n, bits)
+
+	want := 0
+	for i := 0; i < 200; i++ {
+		fmt.Printf(`	if want, got := int%d(%d), s*%d; want != got {
+		failed = true
+		fmt.Printf("got %d * %%d == %%d, wanted %d\n",  s, got)
+	}
+`, bits, want, i, i, want)
+		want += fact
+	}
+
+	fmt.Printf("}\n")
+	return fmt.Sprintf("%s(%d)", n, fact)
+}
+
+func main() {
+	fmt.Printf("package main\n")
+	fmt.Printf("import \"fmt\"\n")
+	fmt.Printf("var failed = false\n")
+
+	f1 := testMul(17, 32)
+	f2 := testMul(131, 64)
+
+	fmt.Printf("func main() {\n")
+	fmt.Println(f1)
+	fmt.Println(f2)
+	fmt.Printf("if failed {\n	panic(\"multiplication failed\")\n}\n")
+	fmt.Printf("}\n")
+}