On 2/27/2026 7:47 AM, Richard Biener wrote:
On Fri, Feb 27, 2026 at 3:21 PM Jeffrey Law
<[email protected]> wrote:
This is the first of a few patches to fix pr90036.
I've gone back and forth about whether or not to fix this for gcc-16 or
queue for gcc-17. Ultimately I don't think these opportunities are that
common, so I don't expect widespread code generation changes.
I'm going to drop the changes in a small series as the changes stand on
their own. This gives us better bisectability.
--
The first patch allows refinement of existing equivalences in a case
where we'd missed it before. In particular say we have <res> = <expr>
in the expression hash table. We later use <expr> in a way that creates
a temporary expression equivalence. We'll fail to record that temporary
expression equivalence because of the pre-existing entry in the hash table.
And just to be clear, the old equivalence will be restored when we leave
the domwalk scope of the newer, more precise, hash table entry.
This matters for pr90036 as we initially enter a simple equivalence in
the table with the result being an SSA_NAME. Later we have a
conditional that allows us to refine the result to a constant. And
we're going to need that constant result to trigger additional
simplifications and equivalence discovery.
Bootstrapped and regression tested on x86_64, aarch64, riscv64 and
probably a couple others as well. It's also been tested across the
embedded targets in my tester. Pushing to the trunk. More to come.
I'll note that DOM should go away, so I'm not sure it's great to add
sth to it that we cannot do elsewhere?
This fits into one space where DOM may have long term value --
particular context sensitive optimizations. THe next best place I see
is FRE, but I don't really see how to handle the path specific nature of
this stuff in FRE.
Jeff
