On Mon, Jan 23, 2017 at 05:25:45PM +0100, Bernd Schmidt wrote:
> There's still a a few details that need addressing, and some questions I
> have. Also Ccing Jakub to have another pair of eyes on the name of the
> section - I don't know if we want some sort of .gnu.something name.
So Jakub, Richard E., what's the outcome?
Is this all the feedback the "gcc developers" will provide? Bernd, don't
get me wrong, I also consider it important to watch the coding style in a
project, I've seen cases were nobody cared ... not good.
But OTOH some of the code you rightfully criticised in the previous run
just went away after some smart feedback from Richard Biener (thanks!);
I'd rather sort out the design issues first before I polish the code.
So, can I please have a statement from a responsible maintainer (if such
a person really exists for this area) about whether this feature would be
acceptable at all?
Thank you very much in advance.
> >diff --git a/gcc/testsuite/c-c++-common/attribute-prolog_pad-1.c
> >b/gcc/testsuite/c-c++-common/attribute-prolog_pad-1.c
> >new file mode 100644
> >index 0000000..2236aa8
> >--- /dev/null
> >+++ b/gcc/testsuite/c-c++-common/attribute-prolog_pad-1.c
> >@@ -0,0 +1,34 @@
> >+/* { dg-do compile } */
> >+/* { dg-options "-fprolog-pad=3,1" } */
>
> This test does not actually seem to verify that the padding has the expected
> size.
Honestly, I do not know how to complete that test case, this is a mere sketch.
Shame on me I'm really not familiar with that framework; suggestions welcome.
> > /* Do not use IPA optimizations for register allocation if profiler is
> > active
> >+ or prolog pads are inserted for run-time instrumentation
> > or port does not emit prologue and epilogue as RTL. */
> >- if (profile_flag || !targetm.have_prologue () || !targetm.have_epilogue
> >())
> >+ if (profile_flag || prolog_nop_pad_size
> >+ || !targetm.have_prologue () || !targetm.have_epilogue ())
> > flag_ipa_ra = 0;
>
> Was this explained? Why would ipa-ra be a problem?
I thought that was obvious, sorry. Instrumentation / profiling will need
some registers, and live patching (the current use case) may allocate
different registers for the replacement functions. You will probably want
to replace the NOPs with a standardised code snippet etc. Conclusion: do not
make any assumptions about callee's register utilisation and strictly stick
to the ABI! I remember to have discussed this, maybe not here.
Torsten
