[PATCH] -Wtautological-compare: fix comparison of macro expansions

[David Malcolm <dmalcolm at redhat dot com>]

On Wed, 2018-12-19 at 17:27 -0600, Aaron Sawdey wrote:
> Assuming you applied this as svn 267273, it causes bootstrap failure
> on powerpc64le-unknown-linux-gnu. Stage 2 fails with multiple
> instances
> of this error:
> 
> ../../trunk-base/gcc/c-family/c-pragma.c: In function ‘void
> handle_pragma_scalar_storage_order(cpp_reader*)’:
> ../../trunk-base/gcc/c-family/c-pragma.c:417:24: error: self-
> comparison always evaluates to false [-Werror=tautological-compare]
>   417 |   if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN)
>       |                        ^~
> ../../trunk-base/gcc/c-family/c-attribs.c: In function ‘tree_node*
> handle_scalar_storage_order_attribute(tree_node**, tree, tree, int,
> bool*)’:
> ../../trunk-base/gcc/c-family/c-attribs.c:1401:24: error: self-
> comparison always evaluates to false [-Werror=tautological-compare]
>  1401 |   if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN)
>       |                        ^~
> ../../trunk-base/gcc/builtins.c: In function ‘rtx_def*
> c_readstr(const char*, scalar_int_mode)’:
> ../../trunk-base/gcc/builtins.c:830:28: error: self-comparison always
> evaluates to false [-Werror=tautological-compare]
>   830 |       if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN
>       |                            ^~
> ../../trunk-base/gcc/combine.c: In function ‘int
> rtx_equal_for_field_assignment_p(rtx, rtx, bool)’:
> ../../trunk-base/gcc/combine.c:9668:28: error: self-comparison always
> evaluates to false [-Werror=tautological-compare]
>  9668 |       if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN)
>       |                            ^~
> 
> Aaron

Sorry about that.

Does the following patch help?  (testing in progress here)

gcc/c-family/ChangeLog:
	* c-warn.c (get_outermost_macro_expansion): New function.
	(spelled_the_same_p): Use it to unwind the macro expansions, and
	compare the outermost macro in each nested expansion, rather than
	the innermost.

gcc/testsuite/ChangeLog:
	* c-c++-common/Wtautological-compare-8.c: New test.
---
 gcc/c-family/c-warn.c                              | 26 +++++++++++++----
 .../c-c++-common/Wtautological-compare-8.c         | 33 ++++++++++++++++++++++
 2 files changed, 54 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/c-c++-common/Wtautological-compare-8.c

diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
index b0f6da0..6013202 100644
--- a/gcc/c-family/c-warn.c
+++ b/gcc/c-family/c-warn.c
@@ -399,6 +399,25 @@ warn_tautological_bitwise_comparison (const op_location_t &loc, tree_code code,
 		"bitwise comparison always evaluates to true");
 }
 
+/* Given LOC from a macro expansion, return the map for the outermost
+   macro in the nest of expansions.  */
+
+static const line_map_macro *
+get_outermost_macro_expansion (location_t loc)
+{
+  gcc_assert (from_macro_expansion_at (loc));
+
+  const line_map *map = linemap_lookup (line_table, loc);
+  const line_map_macro *macro_map;
+  do
+    {
+      macro_map = linemap_check_macro (map);
+      loc = linemap_unwind_toward_expansion (line_table, loc, &map);
+    } while (linemap_macro_expansion_map_p (map));
+
+  return macro_map;
+}
+
 /* Given LOC_A and LOC_B from macro expansions, return true if
    they are "spelled the same" i.e. if they are both directly from
    expansion of the same non-function-like macro.  */
@@ -409,11 +428,8 @@ spelled_the_same_p (location_t loc_a, location_t loc_b)
   gcc_assert (from_macro_expansion_at (loc_a));
   gcc_assert (from_macro_expansion_at (loc_b));
 
-  const line_map_macro *map_a
-    = linemap_check_macro (linemap_lookup (line_table, loc_a));
-
-  const line_map_macro *map_b
-    = linemap_check_macro (linemap_lookup (line_table, loc_b));
+  const line_map_macro *map_a = get_outermost_macro_expansion (loc_a);
+  const line_map_macro *map_b = get_outermost_macro_expansion (loc_b);
 
   if (map_a->macro == map_b->macro)
     if (!cpp_fun_like_macro_p (map_a->macro))
diff --git a/gcc/testsuite/c-c++-common/Wtautological-compare-8.c b/gcc/testsuite/c-c++-common/Wtautological-compare-8.c
new file mode 100644
index 0000000..1adedad
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wtautological-compare-8.c
@@ -0,0 +1,33 @@
+/* { dg-options "-Wtautological-compare" } */
+
+int foo;
+#define INCOMING_FRAME_SP_OFFSET foo
+#define DEFAULT_INCOMING_FRAME_SP_OFFSET INCOMING_FRAME_SP_OFFSET
+
+int test (void)
+{
+  if (DEFAULT_INCOMING_FRAME_SP_OFFSET != INCOMING_FRAME_SP_OFFSET) /* { dg-warning "self-comparison" "" { target c } } */
+    return 1;
+  else
+    return 0;
+}
+
+#define BYTES_BIG_ENDIAN foo
+#define WORDS_BIG_ENDIAN foo
+
+int test_2 (void)
+{
+  if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN) /* { dg-warning "self-comparison" "" { target c } } */
+    return 1;
+  else
+    return 0;
+}
+
+#define COND DEFAULT_INCOMING_FRAME_SP_OFFSET != INCOMING_FRAME_SP_OFFSET
+int test_3 (void)
+{
+  if (COND)
+    return 1;
+  else
+    return 0;
+}
-- 
1.8.5.3