cmd/compile, runtime: use PC of deferreturn for panic transfer

this removes the old conditional-on-register-value
handshake from the deferproc/deferprocstack logic.

The "line" for the recovery-exit frame itself (not the defers
that it runs) is the closing brace of the function.

Reduces code size slightly (e.g. go command is 0.2% smaller)

Sample output showing effect of this change, also what sort of
code it requires to observe the effect:
```
package main

import "os"

func main() {
	g(len(os.Args) - 1)           // stack[0]
}

var gi int
var pi *int = &gi

//go:noinline
func g(i int) {
	switch i {
	case 0:
		defer func() {
			println("g0", i)
			q()                  // stack[2] if i == 0
		}()
		for j := *pi; j < 1; j++ {
			defer func() {
				println("recover0", recover().(string))
			}()
		}
	default:
		for j := *pi; j < 1; j++ {
			defer func() {
				println("g1", i)
				q()              // stack[2] if i == 1
			}()
		}
		defer func() {
			println("recover1", recover().(string))
		}()
	}
	p()
}                                // stack[1] (deferreturn)

//go:noinline
func p() {
	panic("p()")
}

//go:noinline
func q() {
	panic("q()")                 // stack[3]
}

/* Sample output for "./foo foo":
recover1 p()
g1 1
panic: q()

goroutine 1 [running]:
main.q()
	.../main.go:46 +0x2c
main.g.func3()
	.../main.go:29 +0x48
main.g(0x1?)
	.../main.go:37 +0x68
main.main()
	.../main.go:6 +0x28
*/
```

Change-Id: Ie39ea62ecc244213500380ea06d44024cadc2317
Reviewed-on: https://go-review.googlesource.com/c/go/+/650795
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/cmd/internal/obj/x86/obj6.go b/src/cmd/internal/obj/x86/obj6.go
index e6ea8985e4..53c0918254 100644
--- a/src/cmd/internal/obj/x86/obj6.go
+++ b/src/cmd/internal/obj/x86/obj6.go
@@ -448,6 +448,9 @@ func rewriteToUseGot(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 		//
 		// We disable open-coded defers in buildssa() on 386 ONLY with shared
 		// libraries because of this extra code added before deferreturn calls.
+		//
+		// computeDeferReturn in cmd/link/internal/ld/pcln.go depends
+		// on the size of these instructions.
 		if ctxt.Arch.Family == sys.AMD64 || (p.To.Sym != nil && p.To.Sym.Local()) || p.RegTo2 != 0 {
 			return
 		}