cmd/compile: fix misscompilation of Rsh with 32bits rhs on arm64

A copy paste mistake means we used a 64bits compare to check the
32bits shiftend.

Fixes #79886

Change-Id: Idcd2213e87ac2bece22fa817880af26354beba59
Reviewed-on: https://go-review.googlesource.com/c/go/+/787920
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/ssa/_gen/ARM64.rules

diff --git a/src/cmd/compile/internal/ssa/_gen/ARM64.rules b/src/cmd/compile/internal/ssa/_gen/ARM64.rules
index 3c773e1..4c6c437 100644
--- a/src/cmd/compile/internal/ssa/_gen/ARM64.rules
+++ b/src/cmd/compile/internal/ssa/_gen/ARM64.rules
@@ -174,8 +174,8 @@
 (Rsh64Ux32 <t> x y) && !shiftIsBounded(v) => (CSEL [OpARM64LessThanU] (SRL <t> x y) (Const64 <t> [0]) (CMPWconst [64] y))
 (Rsh64Ux(16|8) <t> [bounded] x y) && !shiftIsBounded(v) => (Rsh64Ux32 <t> [bounded] x (ZeroExt(16|8)to32 y))
 
-(Rsh64x64 <t> x y) && !shiftIsBounded(v) => (SRA x (CSEL [OpARM64LessThanU] <y.Type> y (Const64 <y.Type> [63]) (CMPconst [64] y)))
-(Rsh64x32 <t> x y) && !shiftIsBounded(v) => (SRA x (CSEL [OpARM64LessThanU] <y.Type> y (Const64 <y.Type> [63]) (CMPconst [64] y)))
+(Rsh64x64 <t> x y) && !shiftIsBounded(v) => (SRA x (CSEL [OpARM64LessThanU] <y.Type> y (Const64 <y.Type> [63]) (CMPconst  [64] y)))
+(Rsh64x32 <t> x y) && !shiftIsBounded(v) => (SRA x (CSEL [OpARM64LessThanU] <y.Type> y (Const64 <y.Type> [63]) (CMPWconst [64] y)))
 (Rsh64x(16|8) <t> [bounded] x y) && !shiftIsBounded(v) => (Rsh64x32 <t> [bounded] x (ZeroExt(16|8)to32 y))
 
 // constants

src/cmd/compile/internal/ssa/rewriteARM64.go

diff --git a/src/cmd/compile/internal/ssa/rewriteARM64.go b/src/cmd/compile/internal/ssa/rewriteARM64.go
index 1a985c4..d93a204 100644
--- a/src/cmd/compile/internal/ssa/rewriteARM64.go
+++ b/src/cmd/compile/internal/ssa/rewriteARM64.go
@@ -22941,7 +22941,7 @@
 	}
 	// match: (Rsh64x32 <t> x y)
 	// cond: !shiftIsBounded(v)
-	// result: (SRA x (CSEL [OpARM64LessThanU] <y.Type> y (Const64 <y.Type> [63]) (CMPconst [64] y)))
+	// result: (SRA x (CSEL [OpARM64LessThanU] <y.Type> y (Const64 <y.Type> [63]) (CMPWconst [64] y)))
 	for {
 		x := v_0
 		y := v_1
@@ -22953,8 +22953,8 @@
 		v0.AuxInt = opToAuxInt(OpARM64LessThanU)
 		v1 := b.NewValue0(v.Pos, OpConst64, y.Type)
 		v1.AuxInt = int64ToAuxInt(63)
-		v2 := b.NewValue0(v.Pos, OpARM64CMPconst, types.TypeFlags)
-		v2.AuxInt = int64ToAuxInt(64)
+		v2 := b.NewValue0(v.Pos, OpARM64CMPWconst, types.TypeFlags)
+		v2.AuxInt = int32ToAuxInt(64)
 		v2.AddArg(y)
 		v0.AddArg3(y, v1, v2)
 		v.AddArg2(x, v0)

test/fixedbugs/issue79886.go

diff --git a/test/fixedbugs/issue79886.go b/test/fixedbugs/issue79886.go
new file mode 100644
index 0000000..405d892
--- /dev/null
+++ b/test/fixedbugs/issue79886.go
@@ -0,0 +1,25 @@
+// run
+
+// Copyright 2026 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 main
+
+import (
+	"fmt"
+)
+
+//go:noinline
+func shifted(x int64, y uint64) int64 {
+	return x >> uint32(y)
+}
+
+func main() {
+	x := int64(-0x4000000000000000)
+	got := shifted(x, ^uint64(0)&^(1<<32-2))
+	want := x >> 1
+	if got != want {
+		panic(fmt.Sprintf("want %d; got %d", want, got))
+	}
+}