mbox

[0/6] ira: Fix performance regression in exchange2 [PR98782]

Message ID mptr19k2a92.fsf@arm.com
Headers

Message

Richard Sandiford Jan. 6, 2022, 2:45 p.m. UTC 
  This series of patches recovers the exchange2 performance lost in the
GCC 11 timeframe (at least on aarch64 and Power9 -- thanks Pat for
testing the latter).

There are 6 patches, split into two groups of 3.  The first 3 are just
preparatory patches, although patch 2 does contain a minor bug fix.
The other 3 patches are the ones that together fix the regression.

I realise this is a bit invasive for stage 3.  However, the series is
fixing a large regression in an important benchmark and AFAIK there are
no known acceptable mitigations that we could apply instead.  I think
the series is also working with concepts that already exist in IRA:
it's really about tweaking the cost model used to control them.

We also still have at least 3 months (more realistically 4 months) of
testing before GCC 12 is released.  So perhaps one option would be to
apply any approved version of the series now, but with the understanding
that if there's significant fallout (more than a handful of small tweaks
or fixes), we would simply revert the patches rather than trying to
rework them in-situ.  The series is confined to IRA so reverting it
should be simple.  Would that be OK?

Each patch bootstrapped & regression-tested individually on
aarch64-linux-gnu.  Also tested as a series on aarch64_be-elf,
arm-linux-gnueabihf, powerpc64le-linux-gnu, and x86_64-linux-gnu.

Richard