On 12/14/25 2:04 AM, Andrew Pinski wrote:
So it turns out creating a forwarder block in some cases causes a regression.
The rtl ifcvt does like the case were the middle bb does not have a single
predecessor.
So in some cases ifcvt does not happen any more. So the way to fix this
is not to create a forwarder block for those edges which causes out of ssa to
create a middle bb which has the constant or the copy in it.
I tried to do a simple ifcvt on the gimple level with the last phiopt but
that introduces regressions as then `v += cmp - 43;` is not optimized
and produces an extra subtraction. This is the best workaround I could come up
with until that is fixed. I filed PR 123116 for that issue.
Bootstrapped and tested on x86_64-linux-gnu.
gcc/ChangeLog:
* tree-cfg.cc (ifconvertable_edge): New function.
(make_forwarders_with_degenerate_phis): Add skip_ifcvtable argument,
check ifconvertable_edge if skip_ifcvtable is true.
* tree-cfg.h (make_forwarders_with_degenerate_phis): New argument
with default of false.
* tree-cfgcleanup.cc (execute_cleanup_cfg_post_optimizing): Update
argument to make_forwarders_with_degenerate_phis.
Not really working today, so not a review. Just a note that this fixes
the SFB regressions on the RISC-V port as well.
jeff
