https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115144
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
Last reconfirmed| |2024-05-22
Status|UNCONFIRMED |ASSIGNED
--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Hans-Peter Nilsson from comment #7)
> (In reply to Richard Biener from comment #6)
[...]
> > I do want to play with
> > sinking to the start of the else {, but without doing any lifetime analysis
> > I fear that's going to be worse in the average as the current location
> > at least ensures we're close to the first use of the DEF we sink.
>
> Thank you in advance and for the look this far! I haven't looked closer at
> what happens with later passes in main, but looking at the generated
> assembly code, the "sinking" of a division has the eventual effect of
> increasing register pressure; see the previously attached dumps.
Indeed, we have originally
_38 = _36 / _37;
_39 = _36 % _37;
r2_78 = (signed char) _39;
where both _36 and _37 die (but _39 and _38 are live for a lot longer). We
sink the _38 def across
<bb 28> [local count: 173045540]:
# iftmp.10_49 = PHI <iftmp.10_79(27), _31(51)>
if (_41 >= iftmp.10_49)
goto <bb 42>; [0.00%]
else
goto <bb 29>; [100.00%]
<bb 29> [local count: 173045540]:
r1.13_43 = (unsigned char) _38;
which the original profile check avoided. I'll note the above is a more
sensible case where to avoid such sinking but I'll also note that sinking
does not look at register pressure (or basically whether a sinking
increases or decreases register pressure) at all and generally GIMPLE
passes are not supposed to do this (it's also not an easy feat).
