libgo patch committed: Avoid write barriers when calling deferred function

[Ian Lance Taylor <iant at golang dot org>]

Calling a deferred function in libgo currently requires changing from
a uintptr to the function code to a Go function value. That is done by
setting the value of a func local variable using unsafe.Pointer. The
local variable will always be on the stack. Adjust the code that sets
the local variable to avoid generating a write barrier.

A write barrier is never needed here. Also, for deferreturn, we must
avoid write barriers entirely when called from a cgo function; that
requires more than just this, but this is a start.

The test for this is runtime tests that use the go tool; these are not
currently run, but they will be in the future.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian

Attachment: patch.txt
Description: Text document