cmd/compile: move constant divide strength reduction to SSA rules

Currently the conversion from constant divides to multiplies is mostly
done during the walk pass.  This is suboptimal because SSA can
determine that the value being divided by is constant more often
(e.g. after inlining).

Change-Id: If1a9b993edd71be37396b9167f77da271966f85f
Reviewed-on: https://go-review.googlesource.com/37015
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/magic_test.go

@@ -0,0 +1,205 @@
+// Copyright 2017 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.
+
+package ssa
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestMagicExhaustive8(t *testing.T) {
+	testMagicExhaustive(t, 8)
+}
+func TestMagicExhaustive8U(t *testing.T) {
+	testMagicExhaustiveU(t, 8)
+}
+func TestMagicExhaustive16(t *testing.T) {
+	if testing.Short() {
+		t.Skip("slow test; skipping")
+	}
+	testMagicExhaustive(t, 16)
+}
+func TestMagicExhaustive16U(t *testing.T) {
+	if testing.Short() {
+		t.Skip("slow test; skipping")
+	}
+	testMagicExhaustiveU(t, 16)
+}
+
+// exhaustive test of magic for n bits
+func testMagicExhaustive(t *testing.T, n uint) {
+	min := -int64(1) << (n - 1)
+	max := int64(1) << (n - 1)
+	for c := int64(1); c < max; c++ {
+		if !smagicOK(n, int64(c)) {
+			continue
+		}
+		m := int64(smagic(n, c).m)
+		s := smagic(n, c).s
+		for i := min; i < max; i++ {
+			want := i / c
+			got := (i * m) >> (n + uint(s))
+			if i < 0 {
+				got++
+			}
+			if want != got {
+				t.Errorf("signed magic wrong for %d / %d: got %d, want %d (m=%d,s=%d)\n", i, c, got, want, m, s)
+			}
+		}
+	}
+}
+func testMagicExhaustiveU(t *testing.T, n uint) {
+	max := uint64(1) << n
+	for c := uint64(1); c < max; c++ {
+		if !umagicOK(n, int64(c)) {
+			continue
+		}
+		m := umagic(n, int64(c)).m
+		s := umagic(n, int64(c)).s
+		for i := uint64(0); i < max; i++ {
+			want := i / c
+			got := (i * (max + m)) >> (n + uint(s))
+			if want != got {
+				t.Errorf("unsigned magic wrong for %d / %d: got %d, want %d (m=%d,s=%d)\n", i, c, got, want, m, s)
+			}
+		}
+	}
+}
+
+func TestMagicUnsigned(t *testing.T) {
+	One := new(big.Int).SetUint64(1)
+	for _, n := range [...]uint{8, 16, 32, 64} {
+		TwoN := new(big.Int).Lsh(One, n)
+		Max := new(big.Int).Sub(TwoN, One)
+		for _, c := range [...]uint64{
+			3,
+			5,
+			6,
+			7,
+			9,
+			10,
+			11,
+			12,
+			13,
+			14,
+			15,
+			17,
+			1<<8 - 1,
+			1<<8 + 1,
+			1<<16 - 1,
+			1<<16 + 1,
+			1<<32 - 1,
+			1<<32 + 1,
+			1<<64 - 1,
+		} {
+			if c>>n != 0 {
+				continue // not appropriate for the given n.
+			}
+			if !umagicOK(n, int64(c)) {
+				t.Errorf("expected n=%d c=%d to pass\n", n, c)
+			}
+			m := umagic(n, int64(c)).m
+			s := umagic(n, int64(c)).s
+
+			C := new(big.Int).SetUint64(c)
+			M := new(big.Int).SetUint64(m)
+			M.Add(M, TwoN)
+
+			// Find largest multiple of c.
+			Mul := new(big.Int).Div(Max, C)
+			Mul.Mul(Mul, C)
+			mul := Mul.Uint64()
+
+			// Try some input values, mostly around multiples of c.
+			for _, x := range [...]uint64{0, 1,
+				c - 1, c, c + 1,
+				2*c - 1, 2 * c, 2*c + 1,
+				mul - 1, mul, mul + 1,
+				uint64(1)<<n - 1,
+			} {
+				X := new(big.Int).SetUint64(x)
+				if X.Cmp(Max) > 0 {
+					continue
+				}
+				Want := new(big.Int).Quo(X, C)
+				Got := new(big.Int).Mul(X, M)
+				Got.Rsh(Got, n+uint(s))
+				if Want.Cmp(Got) != 0 {
+					t.Errorf("umagic for %d/%d n=%d doesn't work, got=%s, want %s\n", x, c, n, Got, Want)
+				}
+			}
+		}
+	}
+}
+
+func TestMagicSigned(t *testing.T) {
+	One := new(big.Int).SetInt64(1)
+	for _, n := range [...]uint{8, 16, 32, 64} {
+		TwoNMinusOne := new(big.Int).Lsh(One, n-1)
+		Max := new(big.Int).Sub(TwoNMinusOne, One)
+		Min := new(big.Int).Neg(TwoNMinusOne)
+		for _, c := range [...]int64{
+			3,
+			5,
+			6,
+			7,
+			9,
+			10,
+			11,
+			12,
+			13,
+			14,
+			15,
+			17,
+			1<<7 - 1,
+			1<<7 + 1,
+			1<<15 - 1,
+			1<<15 + 1,
+			1<<31 - 1,
+			1<<31 + 1,
+			1<<63 - 1,
+		} {
+			if c>>(n-1) != 0 {
+				continue // not appropriate for the given n.
+			}
+			if !smagicOK(n, int64(c)) {
+				t.Errorf("expected n=%d c=%d to pass\n", n, c)
+			}
+			m := smagic(n, int64(c)).m
+			s := smagic(n, int64(c)).s
+
+			C := new(big.Int).SetInt64(c)
+			M := new(big.Int).SetUint64(m)
+
+			// Find largest multiple of c.
+			Mul := new(big.Int).Div(Max, C)
+			Mul.Mul(Mul, C)
+			mul := Mul.Int64()
+
+			// Try some input values, mostly around multiples of c.
+			for _, x := range [...]int64{
+				-1, 1,
+				-c - 1, -c, -c + 1, c - 1, c, c + 1,
+				-2*c - 1, -2 * c, -2*c + 1, 2*c - 1, 2 * c, 2*c + 1,
+				-mul - 1, -mul, -mul + 1, mul - 1, mul, mul + 1,
+				int64(1)<<n - 1, -int64(1)<<n + 1,
+			} {
+				X := new(big.Int).SetInt64(x)
+				if X.Cmp(Min) < 0 || X.Cmp(Max) > 0 {
+					continue
+				}
+				Want := new(big.Int).Quo(X, C)
+				Got := new(big.Int).Mul(X, M)
+				Got.Rsh(Got, n+uint(s))
+				if x < 0 {
+					Got.Add(Got, One)
+				}
+				if Want.Cmp(Got) != 0 {
+					t.Errorf("smagic for %d/%d n=%d doesn't work, got=%s, want %s\n", x, c, n, Got, Want)
+				}
+			}
+		}
+	}
+}