On Thu, Feb 14, 2019 at 2:21 AM Ian Lance Taylor <i...@golang.org> wrote:
>
> Nikhil Benesch noticed that changes in the GCC backend were making the
> use of defer functions that call recover less efficient. A defer
> thunk is a generated function that looks like this (this is the entire
> function body):
>
> if !runtime.setdeferretaddr(&L) {
> deferredFunction()
> }
> L:
>
> The idea is that the address of the label passed to setdeferretaddr is
> the address to which deferredFunction returns. The code in canrecover
> compares the return address of the function to this saved address to
> see whether the recover function can return non-nil. This is
> explained in marginally more detail at
> https://www.airs.com/blog/archives/376 .
>
> When the return address does not match, the canrecover code does a
> more costly check that requires unwinding the stack. What Nikhil
> Benesch noticed is that we were always taking that fallback.
>
> It turned out that the label address passed to setdeferretaddr was not
> the label to which the deferred function would return. And that was
> because the epilogue was being duplicated by the bb-reorder pass, and
> the label was moved to one copy of the epilogue while the deferred
> function returned to the other epilogue.
>
> Of course there is no reason to duplicate the epilogue in such a small
> function. One easy way to disable that epilogue duplication is to
> compile the function with -Os. That is what this patch does. This
> patch compiles all thunks, not just defer thunks, with -Os, but since
> they are all small that does no harm.
>
> Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu. Committed
> to mainline.
I think an easier way would have been to mark it with the cold attribute?
> Ian
>
> 2019-02-13 Ian Lance Taylor <i...@golang.org>
>
> * go-gcc.cc: #include "opts.h".
> (Gcc_backend::function): Compile thunks with -Os.
