[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:9d7a231e7496533834e194131a0971a533534d3b

commit 9d7a231e7496533834e194131a0971a533534d3b
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Sun Nov 19 01:25:01 2023 -0300

    warn on cast of pointer to packed plus constant
    
    c-c++-common/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.
    
    This should improve the warning even in C, if the packed enum is at a
    nonzero offset into the containing struct.
    
    
    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..39f5ddc577c 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
+		 && TREE_CONSTANT (TREE_OPERAND (rhs, 1)))
+	    rhs = TREE_OPERAND (rhs, 0);
+	}
+      while (orig_rhs != rhs);
       STRIP_NOPS (rhs);
       nop_p |= orig_rhs != rhs;
     }