From c011270fa56303c0f16a8cfa9b37f37f470a45bb Mon Sep 17 00:00:00 2001
From: Joel Sing <joel@sing.id.au>
Date: Mon, 29 Aug 2022 20:36:51 +1000
Subject: [PATCH] cmd/compile: improve Slicemask on riscv64

Implement Slicemask the same way every other architecture does - negate
then arithmetic right shift. This sets or clears the sign bit, before
extending it to the entire register.

Removes around 2,500 instructions from the Go binary on linux/riscv64.

Change-Id: I4d675b826e7eb23fe2b1e6e46b95dcd49ab49733
Reviewed-on: https://go-review.googlesource.com/c/go/+/426354
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
---
 src/cmd/compile/internal/ssa/gen/RISCV64.rules |  9 +--------
 src/cmd/compile/internal/ssa/rewriteRISCV64.go | 13 +++++--------
 2 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/src/cmd/compile/internal/ssa/gen/RISCV64.rules b/src/cmd/compile/internal/ssa/gen/RISCV64.rules
index 9d3cb06697..0207fb45d6 100644
--- a/src/cmd/compile/internal/ssa/gen/RISCV64.rules
+++ b/src/cmd/compile/internal/ssa/gen/RISCV64.rules
@@ -142,14 +142,7 @@
 (Round32F ...) => (Copy ...)
 (Round64F ...) => (Copy ...)
 
-// From genericOps.go:
-// "0 if arg0 == 0, -1 if arg0 > 0, undef if arg0<0"
-//
-// Like other arches, we compute ~((x-1) >> 63), with arithmetic right shift.
-// For positive x, bit 63 of x-1 is always 0, so the result is -1.
-// For zero x, bit 63 of x-1 is 1, so the result is 0.
-//
-(Slicemask <t> x) => (NOT (SRAI <t> [63] (ADDI <t> [-1] x)))
+(Slicemask <t> x) => (SRAI [63] (NEG <t> x))
 
 // Truncations
 // We ignore the unused high parts of registers, so truncates are just copies.
diff --git a/src/cmd/compile/internal/ssa/rewriteRISCV64.go b/src/cmd/compile/internal/ssa/rewriteRISCV64.go
index 6244488992..908456b0aa 100644
--- a/src/cmd/compile/internal/ssa/rewriteRISCV64.go
+++ b/src/cmd/compile/internal/ssa/rewriteRISCV64.go
@@ -7329,17 +7329,14 @@ func rewriteValueRISCV64_OpSlicemask(v *Value) bool {
 	v_0 := v.Args[0]
 	b := v.Block
 	// match: (Slicemask <t> x)
-	// result: (NOT (SRAI <t> [63] (ADDI <t> [-1] x)))
+	// result: (SRAI [63] (NEG <t> x))
 	for {
 		t := v.Type
 		x := v_0
-		v.reset(OpRISCV64NOT)
-		v0 := b.NewValue0(v.Pos, OpRISCV64SRAI, t)
-		v0.AuxInt = int64ToAuxInt(63)
-		v1 := b.NewValue0(v.Pos, OpRISCV64ADDI, t)
-		v1.AuxInt = int64ToAuxInt(-1)
-		v1.AddArg(x)
-		v0.AddArg(v1)
+		v.reset(OpRISCV64SRAI)
+		v.AuxInt = int64ToAuxInt(63)
+		v0 := b.NewValue0(v.Pos, OpRISCV64NEG, t)
+		v0.AddArg(x)
 		v.AddArg(v0)
 		return true
 	}