cmd/compile: combine doubling with shift on riscv64

Change-Id: I4bee2770fedf97e35b5a5b9187a8ba3c41f9ec2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/702697
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Keith Randall <[email protected]>

Author: mengzhuo

src/cmd/compile/internal/ssa/_gen/RISCV64.rules

@@ -782,6 +782,10 @@
 (SRLI [x] (MOVDconst [y])) => (MOVDconst [int64(uint64(y) >> uint32(x))])
 (SRAI [x] (MOVDconst [y])) => (MOVDconst [int64(y) >> uint32(x)])
 
+// Combine doubling via addition with shift.
+(SLLI <t> [c] (ADD x x)) && c < t.Size() * 8 - 1 => (SLLI <t> [c+1] x)
+(SLLI <t> [c] (ADD x x)) && c >= t.Size() * 8 - 1 => (MOVDconst [0])
+
 // SLTI/SLTIU with constants.
 (SLTI  [x] (MOVDconst [y])) => (MOVDconst [b2i(int64(y) < int64(x))])
 (SLTIU [x] (MOVDconst [y])) => (MOVDconst [b2i(uint64(y) < uint64(x))])

src/cmd/compile/internal/ssa/rewriteRISCV64.go

@@ -122,27 +122,41 @@ func rshConst64x32(v int64) int64 {
 func lshConst32x1Add(x int32) int32 {
 	// amd64:"SHLL\t[$]2"
 	// loong64:"SLL\t[$]2"
+	// riscv64:"SLLI\t[$]2"
 	return (x + x) << 1
 }
 
 func lshConst64x1Add(x int64) int64 {
 	// amd64:"SHLQ\t[$]2"
 	// loong64:"SLLV\t[$]2"
+	// riscv64:"SLLI\t[$]2"
 	return (x + x) << 1
 }
 
 func lshConst32x2Add(x int32) int32 {
 	// amd64:"SHLL\t[$]3"
 	// loong64:"SLL\t[$]3"
+	// riscv64:"SLLI\t[$]3"
 	return (x + x) << 2
 }
 
 func lshConst64x2Add(x int64) int64 {
 	// amd64:"SHLQ\t[$]3"
 	// loong64:"SLLV\t[$]3"
+	// riscv64:"SLLI\t[$]3"
 	return (x + x) << 2
 }
 
+func lshConst32x31Add(x int32) int32 {
+	// riscv64:-"SLLI","MOV\t[$]0"
+	return (x + x) << 31
+}
+
+func lshConst64x63Add(x int64) int64 {
+	// riscv64:-"SLLI","MOV\t[$]0"
+	return (x + x) << 63
+}
+
 // ------------------ //
 //   masked shifts    //
 // ------------------ //