On 12/7/22 13:55, Raphael Moreira Zinsly wrote:
Due to RISC-V limitations on operations with big constants combine
is failing to match such operations and is not being able to
produce optimal code as it keeps splitting them. By pretending we
can do those operations we can get more opportunities for
simplification of surrounding instructions.
2022-12-06 Raphael Moreira Zinsly <rzin...@ventanamicro.com>
Jeff Law <j...@ventanamicro.com>
gcc/Changelog:
PR target/95632
PR target/106602
* config/riscv/riscv.md: New pattern to simulate complex
const_int loads.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/pr95632.c: New test.
* gcc.target/riscv/pr106602.c: Likewise.
So to give a little background to others.
The core issue is that when we break down constants early, it can make
it difficult for combine to reconstruct the constant and simplify code
using the reconstructed constant -- you end up trying to do 4->3 or
worse combination sequences which aren't supported by the combiner.
Usually this kind of scenario is handled with a "bridge" pattern. Those
are generally defined as patterns that exist solely for combine and may
not correspond to any real instruction on the target. "bridge" patterns
are typically 2->1 or 3->1 combinations and are intermediate steps for
4->N or even larger combination opportunities. Obviously if the bridge
doesn't allow subsequent simplifications, then the bridge pattern must
generate correct code (either by generating suitable assembly or
splitting later).
Raphael's patch introduces a bridge pattern that pretends we can load up
splittable constants in a single insn. We restrict the bridge pattern
to be active from the point when CSE is no longer expected through the
combiner up to the first splitter pass (where we'll break it down again
if it's still in the IL).
So we get most of the benefit of splitting constants early (CSE, LICM,
etc) while also getting the benefits of splitting late (combine
simplifications).
Given I was working with Raphael on the patch, it's probably best for
someone else to do the review rather than me approving it :-)
Jeff
