On 7/29/25 7:41 AM, Richard Sandiford wrote:
This patch adds a new rule for distributing lowpart subregs through
ANDs, IORs, and XORs with a constant, in cases where one of the terms
then disappears. For example:
(lowart-subreg:QI (and:HI x 0x100))
simplifies to zero and
(lowart-subreg:QI (and:HI x 0xff))
simplifies to (lowart-subreg:QI x).
This would often be handled at some point using nonzero bits. However,
the specific case I want the optimisation for is SVE predicates,
where nonzero bit tracking isn't currently an option. Specifically:
the predicate modes VNx8BI, VNx4BI and VNx2BI have the same size as
VNx16BI, but treat only every second, fourth, or eighth bit as
significant. Thus if we have:
(subreg:VNx8BI (and:VNx16BI x C))
where C is the repeating constant { 1, 0, 1, 0, ... }, then the
AND only clears bits that are made insignificant by the subreg,
and so the result is equal to (subreg:VNx8BI x). Later patches
rely on this.
Tested on aarch64-linux-gnu. OK to install?
Richard
gcc/
* simplify-rtx.cc (simplify_context::simplify_subreg): Distribute
lowpart subregs through AND/IOR/XOR, if doing so eliminates one
of the terms.
(test_scalar_int_ext_ops): Add some tests of the above for integers.
* config/aarch64/aarch64.cc (aarch64_test_sve_folding): Likewise
add tests for predicate modes.
Funny. I was in this some code not terribly long ago to address a
missed simplification as well as part of another larger patch that I
haven't completed yet. Segher also pointed out the lack of distribution
of the subreg and subsequent simplifications in another thread recently.
Those issues may not be addressed by this patch (I'm pretty sure my case
wouldn't), but all point to the fact that we're probably missing all
kinds of simplifications once subregs get into play.
OK for the trunk. Thank!
jeff
