[gcc(refs/vendors/ARM/heads/morello)] cfgexpand: Fix uninitialized read in defer_stack_allocation

[gcc(refs/vendors/ARM/heads/morello)] cfgexpand: Fix uninitialized read in defer_stack_allocation

[Alex Coplan]

https://gcc.gnu.org/g:f8c1a0100575f373db07a45d81344f3983461546

commit f8c1a0100575f373db07a45d81344f3983461546
Author: Alex Coplan <alex.coplan@arm.com>
Date:   Wed May 31 10:29:05 2023 +0100

    cfgexpand: Fix uninitialized read in defer_stack_allocation
    
    When introducing stack bounds narrowing we'd introduced the potential
    for an uninitialized read since we unconditionally used the size
    computed by poly_int_tree_p (to determine the CHERI alignment
    requirement), and this triggered a bootstrap failure on x86_64 due to
    the compiler diagnosing this.
    
    On further inspection, it appears that poly_int_tree_p shouldn't ever
    return false here, so in this patch we assert that this is the case.
    This restores bootstrap on x86_64.

Diff:
---
 gcc/cfgexpand.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 4aafeb22f36..ca68d1949eb 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -1573,14 +1573,15 @@ defer_stack_allocation (tree var, bool toplevel)
   tree size_unit = TREE_CODE (var) == SSA_NAME
     ? TYPE_SIZE_UNIT (TREE_TYPE (var))
     : DECL_SIZE_UNIT (var);
+
   poly_uint64 size;
+  if (!poly_int_tree_p (size_unit, &size))
+    gcc_unreachable ();
 
   /* Whether the variable is small enough for immediate allocation not to be
      a problem with regard to the frame size.  */
   bool smallish
-    = (poly_int_tree_p (size_unit, &size)
-       && (estimated_poly_value (size)
-	   < param_min_size_for_stack_sharing));
+    = (estimated_poly_value (size) < param_min_size_for_stack_sharing);
 
   /* If stack protection is enabled, *all* stack variables must be deferred,
      so that we can re-order the strings to the top of the frame.