libgo patch committed: Check for preemption in fast syscall return

[Ian Lance Taylor]

This libgo patch checks for preemption in the fast return path from a
system call.  This helps with tight loops that make system calls, as
in BenchmarkSyscallExcessWork.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian

Index: gcc/go/gofrontend/MERGE
===================================================================
--- gcc/go/gofrontend/MERGE     (revision 257845)
+++ gcc/go/gofrontend/MERGE     (working copy)
@@ -1,4 +1,4 @@
-459a8a94e04a19bde7173ef7cf2db369c2e62e2d
+c6e0970f75508e209a10a7db5164d6ea3f9b28bf
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
Index: libgo/go/runtime/proc.go
===================================================================
--- libgo/go/runtime/proc.go    (revision 257743)
+++ libgo/go/runtime/proc.go    (working copy)
@@ -2794,6 +2794,13 @@ func exitsyscall(dummy int32) {
                exitsyscallclear(_g_)
                _g_.m.locks--
                _g_.throwsplit = false
+
+               // Check preemption, since unlike gc we don't check on
+               // every call.
+               if getg().preempt {
+                       checkPreempt()
+               }
+
                return
        }