Re: [PATCH #3/4] warn on cast of pointer to packed plus constant

[Alexandre Oliva <oliva@adacore.com>]

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.

Regstrapped on x86_64-linux-gnu, also tested on arm-eabi with default
cpu on trunk, and with tms570 on gcc-13.  Ok to install?


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.
---
 gcc/c-family/c-warn.cc |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
index d2938b91043d3..2ef73d7088f22 100644
--- a/gcc/c-family/c-warn.cc
+++ b/gcc/c-family/c-warn.cc
@@ -3108,10 +3108,20 @@ 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);
+      while (TREE_CODE (rhs) == VIEW_CONVERT_EXPR)
+	rhs = TREE_OPERAND (rhs, 0);
       nop_p |= orig_rhs != rhs;
     }
   while (orig_rhs != rhs);

-- 
Alexandre Oliva, happy hacker            https://FSFLA.org/blogs/lxo/
   Free Software Activist                   GNU Toolchain Engineer
More tolerance and less prejudice are key for inclusion and diversity
Excluding neuro-others for not behaving ""normal"" is *not* inclusive