cmd/asm: fix encoding for arm right shift by constant 0

Right shifts, for some odd reasons, can encode shifts of constant
1-32 instead of 0-31. Left shifts, however, can encode shifts 0-31.
When the shift amount is 0, arm recommends encoding right shifts
using left shifts.

Fixes #64715

Change-Id: Id3825349aa7195028037893dfe01fa0e405eaa51
Reviewed-on: https://go-review.googlesource.com/c/go/+/549955
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/asm/internal/asm/testdata/arm.s

@@ -870,10 +870,13 @@ jmp_label_3:
 	BIC.S	R0@>R1, R2           // 7021d2e1
 
 // SRL
+	SRL	$0, R5, R6           // 0560a0e1
+	SRL	$1, R5, R6           // a560a0e1
 	SRL	$14, R5, R6          // 2567a0e1
 	SRL	$15, R5, R6          // a567a0e1
 	SRL	$30, R5, R6          // 256fa0e1
 	SRL	$31, R5, R6          // a56fa0e1
+	SRL	$32, R5, R6          // 2560a0e1
 	SRL.S	$14, R5, R6          // 2567b0e1
 	SRL.S	$15, R5, R6          // a567b0e1
 	SRL.S	$30, R5, R6          // 256fb0e1
@@ -892,10 +895,13 @@ jmp_label_3:
 	SRL.S	R5, R7               // 3775b0e1
 
 // SRA
+	SRA	$0, R5, R6           // 0560a0e1
+	SRA	$1, R5, R6           // c560a0e1
 	SRA	$14, R5, R6          // 4567a0e1
 	SRA	$15, R5, R6          // c567a0e1
 	SRA	$30, R5, R6          // 456fa0e1
 	SRA	$31, R5, R6          // c56fa0e1
+	SRA	$32, R5, R6          // 4560a0e1
 	SRA.S	$14, R5, R6          // 4567b0e1
 	SRA.S	$15, R5, R6          // c567b0e1
 	SRA.S	$30, R5, R6          // 456fb0e1
@@ -914,6 +920,8 @@ jmp_label_3:
 	SRA.S	R5, R7               // 5775b0e1
 
 // SLL
+	SLL	$0, R5, R6           // 0560a0e1
+	SLL	$1, R5, R6           // 8560a0e1
 	SLL	$14, R5, R6          // 0567a0e1
 	SLL	$15, R5, R6          // 8567a0e1
 	SLL	$30, R5, R6          // 056fa0e1

src/cmd/internal/obj/arm/asm5.go

@@ -1099,6 +1099,14 @@ func (c *ctxt5) oplook(p *obj.Prog) *Optab {
 		fmt.Printf("\t\t%d %d\n", p.From.Type, p.To.Type)
 	}
 
+	if (p.As == ASRL || p.As == ASRA) && p.From.Type == obj.TYPE_CONST && p.From.Offset == 0 {
+		// Right shifts are weird - a shift that looks like "shift by constant 0" actually
+		// means "shift by constant 32". Use left shift in this situation instead.
+		// See issue 64715.
+		// TODO: rotate by 0? Not currently supported, but if we ever do then include it here.
+		p.As = ASLL
+	}
+
 	ops := oprange[p.As&obj.AMask]
 	c1 := &xcmp[a1]
 	c3 := &xcmp[a3]

test/fixedbugs/issue64715.go

@@ -0,0 +1,25 @@
+// run
+
+// Copyright 2023 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
+
+func boolInt32(b bool) int32 {
+	if b {
+		return 1
+	}
+
+	return 0
+}
+
+func f(left uint16, right int32) (r uint16) {
+	return left >> right
+}
+
+var n = uint16(65535)
+
+func main() {
+	println(f(n, boolInt32(int64(n^n) > 1)))
+}

test/fixedbugs/issue64715.out

@@ -0,0 +1 @@
+65535