PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

Segher Boessenkool
Fri Sep 11 17:18:09 GMT 2020

On Thu, Sep 10, 2020 at 05:50:40PM -0500, Qing Zhao wrote:
> >>>> Shrink-wrapped stuff.  Quite important for performance.  Not something
> >>>> you can throw away.

^^^ !!! ^^^

> > Start looking at handle_simple_exit()?  targetm.gen_simple_return()…
> Yes, I have been looking at this since this morning. 
> You are right, we also need to insert zeroing sequence before  this simple_return which the current patch missed.

Please run the performance loss numbers again after you have something
more realistic :-(

> I am currently try to resolve this issue with the following idea:
> In the routine “thread_prologue_and_epilogue_insns”,  After both “make_epilogue_seq” and “try_shrink_wrapping” finished, 
> Scan every exit block to see whether the last insn is a ANY_RETURN_P(insn), 
> If YES, generate the zero sequence before this RETURN insn. 
> Then we should take care all the exit path that returns.
> Do you see any issue from this idea? 

You need to let the backend decide what to do, for this as well as for
all other cases.  I do not know how often I will have to repeat that.

There also is separate shrink-wrapping, which you haven't touched on at
all yet.  Joy.


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