Re: [PATCH] [lra] force reg update after spilling to memory [PR120424]

[Alexandre Oliva]

On Jun  6, 2025, Alexandre Oliva <ol...@adacore.com> wrote:

> Now, since lra_update_fp2sp_elimination checks that
> !elimination_fp2sp_occured_p, we *could* disable the fp2sp elimination,
> if it's selected, right away, so that it is not applied after we've
> disabled it, and then we don't have to worry about disabling it half-way
> or reversing it later.

[lra] inactivate disabled fp2sp elimination

Even after we disable the fp2sp elimination when it is the active
elimination for the fp, spilling might use it before
update_reg_eliminate runs and inactivates it for good.  If it is used,
update_reg_eliminate will fail the check that fp2sp was not used.

Since we keep track of uses of this specific elimination, and
lra_update_fp2sp_elimination checks it before disabling it, we know it
hasn't been used, so we can inactivate it without any ill effects.

This fixes the pr118591-1.c avr-none regression exposed by the
PR120424 fix.

Regstrapped on x86_64-linux-gnu.  Also testing with gcc-14 on
arm-vx7r2.  Ok to install?


for  gcc/ChangeLog

        * lra-eliminations.cc (lra_update_fp2sp_elimination):
        Inactivate the unused fp2sp elimination right away.
---
 gcc/lra-eliminations.cc |   15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/gcc/lra-eliminations.cc b/gcc/lra-eliminations.cc
index bb708b007a4ee..6c8c91086f323 100644
--- a/gcc/lra-eliminations.cc
+++ b/gcc/lra-eliminations.cc
@@ -1415,6 +1415,14 @@ lra_update_fp2sp_elimination (int *spilled_pseudos)
   if (frame_pointer_needed || !targetm.frame_pointer_required ())
     return 0;
   gcc_assert (!elimination_fp2sp_occured_p);
+  ep = elimination_map[FRAME_POINTER_REGNUM];
+  if (ep->to == STACK_POINTER_REGNUM)
+    {
+      elimination_map[FRAME_POINTER_REGNUM] = NULL;
+      setup_can_eliminate (ep, false);
+    }
+  else
+    ep = NULL;
   if (lra_dump_file != NULL)
     fprintf (lra_dump_file,
             "     Frame pointer can not be eliminated anymore\n");
@@ -1422,9 +1430,10 @@ lra_update_fp2sp_elimination (int *spilled_pseudos)
   CLEAR_HARD_REG_SET (set);
   add_to_hard_reg_set (&set, Pmode, HARD_FRAME_POINTER_REGNUM);
   n = spill_pseudos (set, spilled_pseudos);
-  for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
-    if (ep->from == FRAME_POINTER_REGNUM && ep->to == STACK_POINTER_REGNUM)
-      setup_can_eliminate (ep, false);
+  if (!ep)
+    for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
+      if (ep->from == FRAME_POINTER_REGNUM && ep->to == STACK_POINTER_REGNUM)
+       setup_can_eliminate (ep, false);
   return n;
 }
 


-- 
Alexandre Oliva, happy hacker            https://blog.lx.oliva.nom.br/
Free Software Activist     FSFLA co-founder     GNU Toolchain Engineer
More tolerance and less prejudice are key for inclusion and diversity.
Excluding neuro-others for not behaving ""normal"" is *not* inclusive!