From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Sat, 7 Apr 2018 04:45:55 +0000 (-0700)
Subject: runtime: avoid unnecessary scanblock calls
X-Git-Tag: go1.11beta1~590
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=d91e9705f87da8b939a6e074ebaa0de6b0340541;p=gostls13.git

runtime: avoid unnecessary scanblock calls

This is the scanstack analog of CL 104737,
which made a similar change for copystack.

name         old time/op  new time/op  delta
ScanStack-8  41.1ms ± 6%  38.9ms ± 5%  -5.52%  (p=0.000 n=50+48)

Change-Id: I7427151dea2895ed3934f8a0f61d96b568019217
Reviewed-on: https://go-review.googlesource.com/105536
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
---

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 7be2820909..ce3f63883e 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -839,9 +839,11 @@ func scanframeworker(frame *stkframe, cache *pcvalueCache, gcw *gcWork) {
 			print("runtime: pcdata is ", pcdata, " and ", stkmap.n, " locals stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n")
 			throw("scanframe: bad symbol table")
 		}
-		bv := stackmapdata(stkmap, pcdata)
-		size = uintptr(bv.n) * sys.PtrSize
-		scanblock(frame.varp-size, size, bv.bytedata, gcw)
+		if stkmap.nbit > 0 {
+			bv := stackmapdata(stkmap, pcdata)
+			size = uintptr(bv.n) * sys.PtrSize
+			scanblock(frame.varp-size, size, bv.bytedata, gcw)
+		}
 	}
 
 	// Scan arguments.
@@ -862,7 +864,9 @@ func scanframeworker(frame *stkframe, cache *pcvalueCache, gcw *gcWork) {
 			}
 			bv = stackmapdata(stkmap, pcdata)
 		}
-		scanblock(frame.argp, uintptr(bv.n)*sys.PtrSize, bv.bytedata, gcw)
+		if bv.n > 0 {
+			scanblock(frame.argp, uintptr(bv.n)*sys.PtrSize, bv.bytedata, gcw)
+		}
 	}
 }