Re: [PATCH zero-call-used-regs] Add leafy mode for zero-call-used-regs

[Qing Zhao <qing.zhao@oracle.com>]

Hi, Alexandre,

> On Oct 24, 2022, at 10:48 PM, Alexandre Oliva <oliva@adacore.com> wrote:
> 
> Hello, Qing,
> 
> It was a pleasure to meet you at the Cauldron.

Me too!
> 
> 
> On Oct 21, 2022, Qing Zhao <qing.zhao@oracle.com> wrote:
> 
>> Hi, Alexandre,
>> Could you please explain a little bit on the motivation of this patch first?
> 
> It was a suggestion I got after the Cauldron presentation.
> It made sense to me, and was easy enough to implement.
> 
> 'all' for leaf functions is likely wasteful.  If no other functions are
> called, one can determine exactly which registers might carry
> information out and thus need zeroing, and 'used' is thus likely enough,
> depending on the purpose of register scrubbing.  (In some scenarios, it
> might make sense to want scrubbing of all registers, even unused ones
> that carry incoming values)

Under what kinds of situations, we should clear the un-used registers for leaf functions?


> 
> Though some functions are coded as leaf functions, others may become
> leaf functions because of inlining or other optimizations.  It's hard
> for users to predict, so it makes sense to have a mode that tells the
> compiler to figure it out.

Yes, make sense.

Now I am wondering whether we should make “leafy” mode by default then?


Another thing is, do you have any information on how much this new mode can save the 
code size and run-time compared to mode “all”?


> 
> 
> There's room for a follow-up improvement, to save on a little more
> potentially-wasteful anti-leaking scrubbing even in non-leaf functions:
> for this purpose, they need not scrub registers that they don't use
> themselves, if all potential callees are known to have scrubbed them.

Yes, looks like another potential improvement we might add. 
> 
> I have not (yet?) implemented this variant; I haven't even found a name
> I'm happy with for it.  (seal?  plug?  cork?  another leak antonym?)

For this improvement, I am still thinking no need to add a new mode, just add it by default?

Qing
> 
> I'm not entirely happy with leafy either, FWIW.  Bikeshedding anyone? :-)
> 
> https://gcc.gnu.org/pipermail/gcc-patches/2022-October/604083.html
> 
> -- 
> Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
>   Free Software Activist                       GNU Toolchain Engineer
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