[gcc r16-3836] Fix latent LRA bug

[Jeff Law]

https://gcc.gnu.org/g:0c6ad3f5dfbd45150eeef2474899ba7ef0d8e592

commit r16-3836-g0c6ad3f5dfbd45150eeef2474899ba7ef0d8e592
Author: Jeff Law <jlaw@ventanamicro.com>
Date:   Fri Sep 12 16:08:38 2025 -0600

    Fix latent LRA bug
    
    Shreya's work to add the addptr pattern on the RISC-V port exposed a latent bug
    in LRA.
    
    We lazily allocate/reallocate the ira_reg_equiv structure and when we do
    (re)allocation we'll over-allocate and zero-fill so that we don't have to
    actually allocate and relocate the data so often.
    
    In the case exposed by Shreya's work we had N requested entries at the last
    rellocation step.  We actually allocate N+M entries.  During LRA we allocate
    enough new pseudos and thus have N+M+1 pseudos.
    
    In get_equiv we read ira_reg_equiv[regno] without bounds checking so we read
    past the allocated part of the array and get back junk which we use and
    depending on the precise contents we fault in various fun and interesting ways.
    
    We could either arrange to re-allocate ira_reg_equiv again on some path through
    LRA (possibly in get_equiv itself).  We could also just insert the bounds check
    in get_equiv like is done elsewhere in LRA.  Vlad indicated no strong
    preference in an email last week.
    
    So this just adds the bounds check in a manner similar to what's done elsewhere
    in LRA.  Bootstrapped and regression tested on x86_64 as well as RISC-V with
    Shreya's work enabled and regtested across the various embedded targets.
    
    gcc/
            * lra-constraints.cc (get_equiv): Bounds check before accessing
            data in ira_reg_equiv.

Diff:
---
 gcc/lra-constraints.cc | 1 +
 1 file changed, 1 insertion(+)

diff --git a/gcc/lra-constraints.cc b/gcc/lra-constraints.cc
index 83f8fda3b525..aa3962bb567c 100644
--- a/gcc/lra-constraints.cc
+++ b/gcc/lra-constraints.cc
@@ -542,6 +542,7 @@ get_equiv (rtx x)
   rtx res;
 
   if (! REG_P (x) || (regno = REGNO (x)) < FIRST_PSEUDO_REGISTER
+      || regno >= ira_reg_equiv_len
       || ! ira_reg_equiv[regno].defined_p
       || ! ira_reg_equiv[regno].profitable_p
       || lra_get_regno_hard_regno (regno) >= 0)