cmd/asm: process forward jump to PCALIGN

The forward jump target are not processed when the target is PCALIGN, so
process it before emit nops for PCALIGN.

Fixes golang#74648

Change-Id: I690fbfacf79e26d7a37628a2551729b2381616c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/696915
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: zhouguangyuan0718

src/cmd/internal/obj/x86/asm6.go

@@ -2120,19 +2120,6 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			c0 := c
 			c = pjc.padJump(ctxt, s, p, c)
 
-			if p.As == obj.APCALIGN || p.As == obj.APCALIGNMAX {
-				v := obj.AlignmentPadding(c, p, ctxt, s)
-				if v > 0 {
-					s.Grow(int64(c) + int64(v))
-					fillnop(s.P[c:], int(v))
-				}
-				p.Pc = int64(c)
-				c += int32(v)
-				pPrev = p
-				continue
-
-			}
-
 			if maxLoopPad > 0 && p.Back&branchLoopHead != 0 && c&(loopAlign-1) != 0 {
 				// pad with NOPs
 				v := -c & (loopAlign - 1)
@@ -2165,6 +2152,18 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 				}
 			}
 
+			if p.As == obj.APCALIGN || p.As == obj.APCALIGNMAX {
+				v := obj.AlignmentPadding(c, p, ctxt, s)
+				if v > 0 {
+					s.Grow(int64(c) + int64(v))
+					fillnop(s.P[c:], int(v))
+				}
+				p.Pc = int64(c)
+				c += int32(v)
+				pPrev = p
+				continue
+			}
+
 			p.Rel = nil
 
 			p.Pc = int64(c)

test/fixedbugs/issue74648.dir/a.s

@@ -0,0 +1,11 @@
+// Copyright 2025 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.
+
+TEXT ·F(SB), $0
+	JMP prealigned
+	INT $3 // should never be reached
+prealigned:
+	PCALIGN $0x10
+aligned:
+	RET

test/fixedbugs/issue74648.dir/x.go

@@ -0,0 +1,13 @@
+// Copyright 2025 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.
+
+// Issue 74648: wrong jump target when using PCALIGN.
+
+package main
+
+func F()
+
+func main() {
+	F()
+}

test/fixedbugs/issue74648.go

@@ -0,0 +1,9 @@
+// runindir
+
+//go:build amd64
+
+// Copyright 2025 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 ignored