runtime: avoid calling adjustpointers unnecessarily

adjustpointers loops over a bitmap.
If the length of that bitmap is zero,
we can skip making the call entirely.
This speeds up stack copying when there are
no pointers present in either args or locals.

name                old time/op  new time/op  delta
StackCopyPtr-8       101ms ± 4%    90ms ± 4%  -10.95%  (p=0.000 n=87+93)
StackCopy-8         80.1ms ± 4%  72.6ms ± 4%   -9.41%  (p=0.000 n=98+100)
StackCopyNoCache-8   121ms ± 3%   113ms ± 3%   -6.57%  (p=0.000 n=98+97)

Change-Id: I7a272e19bc9a14fa3e3318771ebd082dc6247d25
Reviewed-on: https://go-review.googlesource.com/104737
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/stack.go

@@ -643,24 +643,28 @@ func adjustframe(frame *stkframe, arg unsafe.Pointer) bool {
 		minsize = sys.MinFrameSize
 	}
 	if size > minsize {
-		var bv bitvector
 		stackmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
 		if stackmap == nil || stackmap.n <= 0 {
 			print("runtime: frame ", funcname(f), " untyped locals ", hex(frame.varp-size), "+", hex(size), "\n")
 			throw("missing stackmap")
 		}
+		// If nbit == 0, there's no work to do.
+		if stackmap.nbit > 0 {
 			// Locals bitmap information, scan just the pointers in locals.
 			if pcdata < 0 || pcdata >= stackmap.n {
 				// don't know where we are
 				print("runtime: pcdata is ", pcdata, " and ", stackmap.n, " locals stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n")
 				throw("bad symbol table")
 			}
-		bv = stackmapdata(stackmap, pcdata)
+			bv := stackmapdata(stackmap, pcdata)
 			size = uintptr(bv.n) * sys.PtrSize
 			if stackDebug >= 3 {
 				print("      locals ", pcdata, "/", stackmap.n, " ", size/sys.PtrSize, " words ", bv.bytedata, "\n")
 			}
 			adjustpointers(unsafe.Pointer(frame.varp-size), &bv, adjinfo, f)
+		} else if stackDebug >= 3 {
+			print("      no locals to adjust\n")
+		}
 	}
 
 	// Adjust saved base pointer if there is one.
@@ -707,8 +711,10 @@ func adjustframe(frame *stkframe, arg unsafe.Pointer) bool {
 		if stackDebug >= 3 {
 			print("      args\n")
 		}
+		if bv.n > 0 {
 			adjustpointers(unsafe.Pointer(frame.argp), &bv, adjinfo, funcInfo{})
 		}
+	}
 	return true
 }