cmd/compile: revert CL/316890

This is a revert of https://go-review.googlesource.com/c/go/+/316890,
which has positive effects on debugging + DWARF variable locations
for register parameters when the reg abi is in effect, but also
turns out to interact badly with the register allocator.

Fixes #46304.

Change-Id: I624bd980493411a9cde45d44fcd3c46cad796909
Reviewed-on: https://go-review.googlesource.com/c/go/+/321830
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 76 insertions, 0 deletions

diff --git a/test/fixedbugs/issue46304.go b/test/fixedbugs/issue46304.go
new file mode 100644
index 0000000000..b8ecfc93a5
--- /dev/null
+++ b/test/fixedbugs/issue46304.go
@@ -0,0 +1,76 @@
+// run
+
+// Copyright 2021 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.
+
+// This testcase caused a crash when the register ABI was in effect,
+// on amd64 (problem with register allocation).
+
+package main
+
+type Op struct {
+	tag   string
+	_x    []string
+	_q    [20]uint64
+	plist []P
+}
+
+type P struct {
+	tag string
+	_x  [10]uint64
+	b   bool
+}
+
+type M int
+
+//go:noinline
+func (w *M) walkP(p *P) *P {
+	np := &P{}
+	*np = *p
+	np.tag += "new"
+	return np
+}
+
+func (w *M) walkOp(op *Op) *Op {
+	if op == nil {
+		return nil
+	}
+
+	orig := op
+	cloned := false
+	clone := func() {
+		if !cloned {
+			cloned = true
+			op = &Op{}
+			*op = *orig
+		}
+	}
+
+	pCloned := false
+	for i := range op.plist {
+		if s := w.walkP(&op.plist[i]); s != &op.plist[i] {
+			if !pCloned {
+				pCloned = true
+				clone()
+				op.plist = make([]P, len(orig.plist))
+				copy(op.plist, orig.plist)
+			}
+			op.plist[i] = *s
+		}
+	}
+
+	return op
+}
+
+func main() {
+	var ww M
+	w := &ww
+	p1 := P{tag: "a"}
+	p1._x[1] = 9
+	o := Op{tag: "old", plist: []P{p1}}
+	no := w.walkOp(&o)
+	if no.plist[0].tag != "anew" {
+		panic("bad")
+	}
+}