runtime: skip stack barrier copy when there are no pointers

methane · randall77 · commit 5372257e6009 · 2019-01-03T18:48:52.000Z
After CL 31455, "go fun(n)" may put "n" to write barrier buffer when there are no pointers in fun's arguments. Fixes #29362 Change-Id: Icfa42b8759ce8ad9267dcb3859c626feb6fda381 Reviewed-on: https://go-review.googlesource.com/c/155779 Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
@@ -3303,9 +3303,11 @@ func newproc1(fn *funcval, argp *uint8, narg int32, callergp *g, callerpc uintpt
 		if writeBarrier.needed && !_g_.m.curg.gcscandone {
 			f := findfunc(fn.fn)
 			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps))
-			// We're in the prologue, so it's always stack map index 0.
-			bv := stackmapdata(stkmap, 0)
-			bulkBarrierBitmap(spArg, spArg, uintptr(narg), 0, bv.bytedata)
+			if stkmap.nbit > 0 {
+				// We're in the prologue, so it's always stack map index 0.
+				bv := stackmapdata(stkmap, 0)
+				bulkBarrierBitmap(spArg, spArg, uintptr(narg), 0, bv.bytedata)
+			}
 		}
 	}
 

Original file line number	Diff line number	Diff line change
`@@ -3303,9 +3303,11 @@ func newproc1(fn funcval, argp uint8, narg int32, callergp *g, callerpc uintpt`
`3303`	`3303`	`if writeBarrier.needed && !_g_.m.curg.gcscandone {`
`3304`	`3304`	`f := findfunc(fn.fn)`
`3305`	`3305`	`stkmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps))`
`3306`		`- // We're in the prologue, so it's always stack map index 0.`
`3307`		`- bv := stackmapdata(stkmap, 0)`
`3308`		`- bulkBarrierBitmap(spArg, spArg, uintptr(narg), 0, bv.bytedata)`
	`3306`	`+ if stkmap.nbit > 0 {`
	`3307`	`+ // We're in the prologue, so it's always stack map index 0.`
	`3308`	`+ bv := stackmapdata(stkmap, 0)`
	`3309`	`+ bulkBarrierBitmap(spArg, spArg, uintptr(narg), 0, bv.bytedata)`
	`3310`	`+ }`
`3309`	`3311`	`}`
`3310`	`3312`	`}`
`3311`	`3313`