[gcc(refs/users/aoliva/heads/testme)] warn on cast of pointer to packed plus constant

[Alexandre Oliva <aoliva@gcc.gnu.org>]

https://gcc.gnu.org/g:c50d78d2eb189e6cdcf5b2c341f9cf76e3fb7dc1

commit c50d78d2eb189e6cdcf5b2c341f9cf76e3fb7dc1
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Sun Nov 19 01:25:01 2023 -0300

    warn on cast of pointer to packed plus constant
    
    gcc.dg/analyzer/null-deref-pr108251-smp-fetch_ssl_fc_has_early.c gets
    an unaligned pointer value warning on -fshort-enums targets in C, but
    not in C++.  The former simplifies the offset-and-cast expression
    enough that check_and_warn_address_or_pointer_of_packed_member finds
    no more than a type cast of the base pointer, but in C++, the entire
    expression, with cast, constant offsetting, and cast again, is
    retained, and that's too much for the warning code.
    
    Or rather it was.  It's easy enough to take the base pointer from
    PLUS_POINTER_EXPR, and a constant offset can't possibly increase
    alignment for just any pointer of laxer alignment, so we can safely
    disregard the offset.
    
    
    for  gcc/c-family/ChangeLog
    
            * c-warn.cc
            (check_and_warn_address_or_pointer_of_packed_member): Take the
            base pointer from PLUS_POINTER_EXPR when the addend is
            constant.

Diff:
---
 gcc/c-family/c-warn.cc | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
index d2938b91043..c705f43e5c3 100644
--- a/gcc/c-family/c-warn.cc
+++ b/gcc/c-family/c-warn.cc
@@ -3108,9 +3108,17 @@ check_and_warn_address_or_pointer_of_packed_member (tree type, tree rhs)
 
   do
     {
-      while (TREE_CODE (rhs) == COMPOUND_EXPR)
-	rhs = TREE_OPERAND (rhs, 1);
-      orig_rhs = rhs;
+      do
+	{
+	  orig_rhs = rhs;
+	  while (TREE_CODE (rhs) == COMPOUND_EXPR)
+	    rhs = TREE_OPERAND (rhs, 1);
+	  /* Constants can't increase the alignment.  */
+	  while (TREE_CODE (rhs) == POINTER_PLUS_EXPR
+		 && CONSTANT_CLASS_P (TREE_OPERAND (rhs, 1)))
+	    rhs = TREE_OPERAND (rhs, 0);
+	}
+      while (orig_rhs != rhs);
       STRIP_NOPS (rhs);
       nop_p |= orig_rhs != rhs;
     }