cmd/compile: constant fold 128-bit multiplies

The full 64x64->128 multiply comes up when using bits.Mul64.
The 64x64->64+overflow multiply comes up in unsafe.Slice when using
a constant length.

Change-Id: I298515162ca07d804b2d699d03bc957ca30a4ebc
Reviewed-on: https://go-review.googlesource.com/c/go/+/667175
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: randall77

src/cmd/compile/internal/ssa/_gen/generic.rules

@@ -139,6 +139,10 @@
 (Mul64  (Const64 [c])  (Const64 [d]))  => (Const64 [c*d])
 (Mul32F (Const32F [c]) (Const32F [d])) && c*d == c*d => (Const32F [c*d])
 (Mul64F (Const64F [c]) (Const64F [d])) && c*d == c*d => (Const64F [c*d])
+(Mul32uhilo (Const32 [c]) (Const32 [d])) => (MakeTuple (Const32 <typ.UInt32> [bitsMulU32(c, d).hi]) (Const32 <typ.UInt32> [bitsMulU32(c,d).lo]))
+(Mul64uhilo (Const64 [c]) (Const64 [d])) => (MakeTuple (Const64 <typ.UInt64> [bitsMulU64(c, d).hi]) (Const64 <typ.UInt64> [bitsMulU64(c,d).lo]))
+(Mul32uover (Const32 [c]) (Const32 [d])) => (MakeTuple (Const32 <typ.UInt32> [bitsMulU32(c, d).lo]) (ConstBool <typ.Bool> [bitsMulU32(c,d).hi != 0]))
+(Mul64uover (Const64 [c]) (Const64 [d])) => (MakeTuple (Const64 <typ.UInt64> [bitsMulU64(c, d).lo]) (ConstBool <typ.Bool> [bitsMulU64(c,d).hi != 0]))
 
 (And8   (Const8 [c])   (Const8 [d]))   => (Const8  [c&d])
 (And16  (Const16 [c])  (Const16 [d]))  => (Const16 [c&d])

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

@@ -2584,3 +2584,14 @@ func bitsAdd64(x, y, carry int64) (r struct{ sum, carry int64 }) {
 	r.sum, r.carry = int64(s), int64(c)
 	return
 }
+
+func bitsMulU64(x, y int64) (r struct{ hi, lo int64 }) {
+	hi, lo := bits.Mul64(uint64(x), uint64(y))
+	r.hi, r.lo = int64(hi), int64(lo)
+	return
+}
+func bitsMulU32(x, y int32) (r struct{ hi, lo int32 }) {
+	hi, lo := bits.Mul32(uint32(x), uint32(y))
+	r.hi, r.lo = int32(hi), int32(lo)
+	return
+}

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

@@ -6,7 +6,10 @@
 
 package codegen
 
-import "math/bits"
+import (
+	"math/bits"
+	"unsafe"
+)
 
 // ----------------------- //
 //    bits.LeadingZeros    //
@@ -957,6 +960,17 @@ func Mul64LoOnly(x, y uint64) uint64 {
 	return lo
 }
 
+func Mul64Const() (uint64, uint64) {
+	// 7133701809754865664 == 99<<56
+	// arm64:"MOVD\t[$]7133701809754865664, R1", "MOVD\t[$]88, R0"
+	return bits.Mul64(99+88<<8, 1<<56)
+}
+
+func MulUintOverflow(p *uint64) []uint64 {
+	// arm64:"CMP\t[$]72"
+	return unsafe.Slice(p, 9)
+}
+
 // --------------- //
 //    bits.Div*    //
 // --------------- //