PR tree-optimization/90879 - fold zero-equality of strcmp between a longer string and a smaller array

gcc/c-family/ChangeLog:

	PR tree-optimization/90879
	* c.opt (-Wstring-compare): New option.

gcc/testsuite/ChangeLog:

	PR tree-optimization/90879
	* gcc.dg/Wstring-compare-2.c: New test.
	* gcc.dg/Wstring-compare.c: New test.
	* gcc.dg/strcmpopt_3.c: Scan the optmized dump instead of strlen.
	* gcc.dg/strcmpopt_6.c: New test.
	* gcc.dg/strlenopt-65.c: Remove uinnecessary declarations, add
	test cases.
	* gcc.dg/strlenopt-66.c: Run it.
	* gcc.dg/strlenopt-68.c: New test.

gcc/ChangeLog:

	PR tree-optimization/90879
	* builtins.c (check_access): Avoid using maxbound when null.
	* calls.c (maybe_warn_nonstring_arg): Adjust to get_range_strlen change.
	* doc/invoke.texi (-Wstring-compare): Document new warning option.
	* gimple-fold.c (get_range_strlen_tree): Make setting maxbound
	conditional.
	(get_range_strlen): Overwrite initial maxbound when non-null.
	* gimple-ssa-sprintf.c (get_string_length): Adjust to get_range_strlen
	changes.
	* tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Same.
	(used_only_for_zero_equality): New function.
	(handle_builtin_memcmp): Call it.
	(determine_min_objsize): Return an integer instead of tree.
	(get_len_or_size, strxcmp_eqz_result): New functions.
	(maybe_warn_pointless_strcmp): New function.
	(handle_builtin_string_cmp): Call it.  Fold zero-equality of strcmp
	between a longer string and a smaller array.
	(get_range_strlen_dynamic): Overwrite initial maxbound when non-null.

From-SVN: r276773

1 files changed, 26 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index dc9a3e5..9247603 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -349,6 +349,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wsizeof-pointer-memaccess  -Wsizeof-array-argument @gol
 -Wstack-protector  -Wstack-usage=@var{byte-size}  -Wstrict-aliasing @gol
 -Wstrict-aliasing=n  -Wstrict-overflow  -Wstrict-overflow=@var{n} @gol
+-Wstring-compare @gol
 -Wstringop-overflow=@var{n}  -Wstringop-truncation  -Wsubobject-linkage @gol
 -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{|}malloc@r{]} @gol
 -Wsuggest-final-types @gol  -Wsuggest-final-methods  -Wsuggest-override @gol
@@ -4485,6 +4486,7 @@ name is still supported, but the newer name is more descriptive.)
 -Wold-style-declaration @r{(C only)}  @gol
 -Woverride-init  @gol
 -Wsign-compare @r{(C only)} @gol
+-Wstring-compare @gol
 -Wredundant-move @r{(only for C++)}  @gol
 -Wtype-limits  @gol
 -Wuninitialized  @gol
@@ -5800,6 +5802,30 @@ comparisons, so this warning level gives a very large number of
 false positives.
 @end table
 
+@item -Wstring-compare
+@opindex Wstring-compare
+@opindex Wno-string-compare
+Warn for calls to @code{strcmp} and @code{strncmp} whose result is
+determined to be either zero or non-zero in tests for such equality
+owing to the length of one argument being greater than the size of
+the array the other argument is stored in (or the bound in the case
+of @code{strncmp}).  Such calls could be mistakes.  For example,
+the call to @code{strcmp} below is diagnosed because its result is
+necessarily non-zero irrespective of the contents of the array @code{a}.
+
+@smallexample
+extern char a[4];
+void f (char *d)
+@{
+  strcpy (d, "string");
+  @dots{}
+  if (0 == strcmp (a, d))   // cannot be true
+    puts ("a and d are the same");
+@}
+@end smallexample
+
+@option{-Wstring-compare} is enabled by @option{-Wextra}.
+
 @item -Wstringop-overflow
 @itemx -Wstringop-overflow=@var{type}
 @opindex Wstringop-overflow