runtime: add fast version of getArgInfo

getArgInfo is called a lot during stack copying.
In the common case it doesn't do much work,
but it cannot be inlined.

This change works around that.

name                old time/op  new time/op  delta
StackCopyPtr-8       108ms ± 5%    96ms ± 4%  -10.40%  (p=0.000 n=20+20)
StackCopy-8         82.6ms ± 3%  78.4ms ± 6%   -5.15%  (p=0.000 n=19+20)
StackCopyNoCache-8   130ms ± 3%   122ms ± 3%   -6.44%  (p=0.000 n=20+20)

Change-Id: If7d8a08c50a4e2e76e4331b399396c5dbe88c2ce
Reviewed-on: https://go-review.googlesource.com/108945
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

Author: josharian

src/cmd/compile/internal/gc/inl_test.go

@@ -49,6 +49,7 @@ func TestIntendedInlining(t *testing.T) {
 			"fastrand",
 			"float64bits",
 			"funcPC",
+			"getArgInfoFast",
 			"getm",
 			"isDirectIface",
 			"itabHashFunc",

src/runtime/traceback.go

@@ -67,7 +67,11 @@ func tracebackdefers(gp *g, callback func(*stkframe, unsafe.Pointer) bool, v uns
 			}
 			frame.fn = f
 			frame.argp = uintptr(deferArgs(d))
-			frame.arglen, frame.argmap = getArgInfo(&frame, f, true, fn)
+			var ok bool
+			frame.arglen, frame.argmap, ok = getArgInfoFast(f, true)
+			if !ok {
+				frame.arglen, frame.argmap = getArgInfo(&frame, f, true, fn)
+			}
 		}
 		frame.continpc = frame.pc
 		if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
@@ -279,7 +283,11 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		// metadata recorded by f's caller.
 		if callback != nil || printing {
 			frame.argp = frame.fp + sys.MinFrameSize
-			frame.arglen, frame.argmap = getArgInfo(&frame, f, callback != nil, nil)
+			var ok bool
+			frame.arglen, frame.argmap, ok = getArgInfoFast(f, callback != nil)
+			if !ok {
+				frame.arglen, frame.argmap = getArgInfo(&frame, f, callback != nil, nil)
+			}
 		}
 
 		// Determine frame's 'continuation PC', where it can continue.
@@ -546,6 +554,15 @@ type reflectMethodValue struct {
 	stack *bitvector // args bitmap
 }
 
+// getArgInfoFast returns the argument frame information for a call to f.
+// It is short and inlineable. However, it does not handle all functions.
+// If ok reports false, you must call getArgInfo instead.
+// TODO(josharian): once we do mid-stack inlining,
+// call getArgInfo directly from getArgInfoFast and stop returning an ok bool.
+func getArgInfoFast(f funcInfo, needArgMap bool) (arglen uintptr, argmap *bitvector, ok bool) {
+	return uintptr(f.args), nil, !(needArgMap && f.args == _ArgsSizeUnknown)
+}
+
 // getArgInfo returns the argument frame information for a call to f
 // with call frame frame.
 //