cmd/compile/internal/ssa: optimise more branches with zero on riscv64

Optimise more branches with zero on riscv64. In particular, BLTU with
zero occurs with IsInBounds checks for index zero. This currently results
in two instructions and requires an additional register:

   li      t2, 0
   bltu    t2, t1, 0x174b4

This is equivalent to checking if the bounds is not equal to zero. With
this change:

   bnez    t1, 0x174c0

This removes more than 500 instructions from the Go binary on riscv64.

Change-Id: I6cd861d853e3ef270bd46dacecdfaa205b1c4644
Reviewed-on: https://go-review.googlesource.com/c/go/+/606715
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

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

@@ -558,14 +558,16 @@
 (BNEZ (SLTIU [x] y) yes no) => (BLTU y (MOVDconst [x]) yes no)
 
 // Convert branch with zero to more optimal branch zero.
-(BEQ (MOVDconst [0]) cond yes no) => (BEQZ cond yes no)
-(BEQ cond (MOVDconst [0]) yes no) => (BEQZ cond yes no)
-(BNE (MOVDconst [0]) cond yes no) => (BNEZ cond yes no)
-(BNE cond (MOVDconst [0]) yes no) => (BNEZ cond yes no)
-(BLT (MOVDconst [0]) cond yes no) => (BGTZ cond yes no)
-(BLT cond (MOVDconst [0]) yes no) => (BLTZ cond yes no)
-(BGE (MOVDconst [0]) cond yes no) => (BLEZ cond yes no)
-(BGE cond (MOVDconst [0]) yes no) => (BGEZ cond yes no)
+(BEQ  (MOVDconst [0]) cond yes no) => (BEQZ cond yes no)
+(BEQ  cond (MOVDconst [0]) yes no) => (BEQZ cond yes no)
+(BNE  (MOVDconst [0]) cond yes no) => (BNEZ cond yes no)
+(BNE  cond (MOVDconst [0]) yes no) => (BNEZ cond yes no)
+(BLT  (MOVDconst [0]) cond yes no) => (BGTZ cond yes no)
+(BLT  cond (MOVDconst [0]) yes no) => (BLTZ cond yes no)
+(BLTU (MOVDconst [0]) cond yes no) => (BNEZ cond yes no)
+(BGE  (MOVDconst [0]) cond yes no) => (BLEZ cond yes no)
+(BGE  cond (MOVDconst [0]) yes no) => (BGEZ cond yes no)
+(BGEU (MOVDconst [0]) cond yes no) => (BEQZ cond yes no)
 
 // Remove redundant NEG from SEQZ/SNEZ.
 (SEQZ (NEG x)) => (SEQZ x)

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

@@ -9403,6 +9403,18 @@ func rewriteBlockRISCV64(b *Block) bool {
 			b.resetWithControl(BlockRISCV64BGEZ, cond)
 			return true
 		}
+	case BlockRISCV64BGEU:
+		// match: (BGEU (MOVDconst [0]) cond yes no)
+		// result: (BEQZ cond yes no)
+		for b.Controls[0].Op == OpRISCV64MOVDconst {
+			v_0 := b.Controls[0]
+			if auxIntToInt64(v_0.AuxInt) != 0 {
+				break
+			}
+			cond := b.Controls[1]
+			b.resetWithControl(BlockRISCV64BEQZ, cond)
+			return true
+		}
 	case BlockRISCV64BLT:
 		// match: (BLT (MOVDconst [0]) cond yes no)
 		// result: (BGTZ cond yes no)
@@ -9426,6 +9438,18 @@ func rewriteBlockRISCV64(b *Block) bool {
 			b.resetWithControl(BlockRISCV64BLTZ, cond)
 			return true
 		}
+	case BlockRISCV64BLTU:
+		// match: (BLTU (MOVDconst [0]) cond yes no)
+		// result: (BNEZ cond yes no)
+		for b.Controls[0].Op == OpRISCV64MOVDconst {
+			v_0 := b.Controls[0]
+			if auxIntToInt64(v_0.AuxInt) != 0 {
+				break
+			}
+			cond := b.Controls[1]
+			b.resetWithControl(BlockRISCV64BNEZ, cond)
+			return true
+		}
 	case BlockRISCV64BNE:
 		// match: (BNE (MOVDconst [0]) cond yes no)
 		// result: (BNEZ cond yes no)

test/codegen/compare_and_branch.go

@@ -241,4 +241,14 @@ func ui64x0(x chan uint64) {
 	for <-x < 1 {
 		dummy()
 	}
+
+	// riscv64:"BNEZ"
+	for 0 < <-x {
+		dummy()
+	}
+
+	// riscv64:"BEQZ"
+	for 0 >= <-x {
+		dummy()
+	}
 }