cmd/compile,runtime: allocate defer records on the stack

When a defer is executed at most once in a function body,
we can allocate the defer record for it on the stack instead
of on the heap.

This should make defers like this (which are very common) faster.

This optimization applies to 363 out of the 370 static defer sites
in the cmd/go binary.

name     old time/op  new time/op  delta
Defer-4  52.2ns ± 5%  36.2ns ± 3%  -30.70%  (p=0.000 n=10+10)

Fixes #6980
Update #14939

Change-Id: I697109dd7aeef9e97a9eeba2ef65ff53d3ee1004
Reviewed-on: https://go-review.googlesource.com/c/go/+/171758
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/stack.go

@@ -719,16 +719,21 @@ func adjustctxt(gp *g, adjinfo *adjustinfo) {
 }
 
 func adjustdefers(gp *g, adjinfo *adjustinfo) {
-	// Adjust defer argument blocks the same way we adjust active stack frames.
-	tracebackdefers(gp, adjustframe, noescape(unsafe.Pointer(adjinfo)))
-
 	// Adjust pointers in the Defer structs.
-	// Defer structs themselves are never on the stack.
+	// We need to do this first because we need to adjust the
+	// defer.link fields so we always work on the new stack.
+	adjustpointer(adjinfo, unsafe.Pointer(&gp._defer))
 	for d := gp._defer; d != nil; d = d.link {
 		adjustpointer(adjinfo, unsafe.Pointer(&d.fn))
 		adjustpointer(adjinfo, unsafe.Pointer(&d.sp))
 		adjustpointer(adjinfo, unsafe.Pointer(&d._panic))
+		adjustpointer(adjinfo, unsafe.Pointer(&d.link))
 	}
+
+	// Adjust defer argument blocks the same way we adjust active stack frames.
+	// Note: this code is after the loop above, so that if a defer record is
+	// stack allocated, we work on the copy in the new stack.
+	tracebackdefers(gp, adjustframe, noescape(unsafe.Pointer(adjinfo)))
 }
 
 func adjustpanics(gp *g, adjinfo *adjustinfo) {