[PATCH] avoid -Wrestrict on sprintf %p with destination as argument (PR 84919)

[Martin Sebor]

The early front-end only implementation of -Wrestrict that's still
present in GCC 10 issues a false postive for %p arguments that are
the same as the destination.  Bug 84919 reports an instance of this
false positive in the Linux kernel.

That attached patch suppresses the front-end warning for the sprintf
family of functions, letting the sprintf pass that was in GCC 10
extended to also handle -Wrestrict for these functions, handle
them instead.

Tested on x86_64-linux.

Since this is a regression I'd like to commit the fix to GCC 10.

Martin

PR c/84919 - bogus -Wrestrict on sprintf %p with destination as argument

gcc/c-family/ChangeLog:

	PR c/84919
	* c-common.c (check_function_arguments): Avoid overlap checking
	of sprintf functions.

gcc/testsuite/ChangeLog:

	PR c/84919
	* gcc.dg/Wrestrict-20.c: New test.

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 4a7f3a52335..4bb21772c64 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -5732,8 +5732,26 @@ check_function_arguments (location_t loc, const_tree fndecl, const_tree fntype,
   if (warn_format)
     check_function_sentinel (fntype, nargs, argarray);
 
-  if (warn_restrict)
-    warned_p |= check_function_restrict (fndecl, fntype, nargs, argarray);
+  if (fndecl && fndecl_built_in_p (fndecl, BUILT_IN_NORMAL))
+    {
+      switch (DECL_FUNCTION_CODE (fndecl))
+	{
+	case BUILT_IN_SPRINTF:
+	case BUILT_IN_SPRINTF_CHK:
+	case BUILT_IN_SNPRINTF:
+	case BUILT_IN_SNPRINTF_CHK:
+	  /* Let the sprintf pass handle these.  */
+	  return warned_p;
+
+	default:
+	  break;
+	}
+    }
+
+  /* check_function_restrict sets the DECL_READ_P for arguments
+     so it must be called unconditionally.  */
+  warned_p |= check_function_restrict (fndecl, fntype, nargs, argarray);
+
   return warned_p;
 }
 
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-20.c b/gcc/testsuite/gcc.dg/Wrestrict-20.c
new file mode 100644
index 00000000000..9826e7f4503
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wrestrict-20.c
@@ -0,0 +1,41 @@
+/* PR c/84919 - bogus -Wrestrict on sprintf %p with destination as argument
+   { dg-do compile }
+   -O2 isn't strictly necessary but setting also verifies that the sprintf/
+   strlen pass doesn't warn with non-constant arguments.
+   { dg-options "-O2 -Wall" } */
+
+extern int sprintf (char* restrict, const char* restrict, ...);
+extern int snprintf (char* restrict, __SIZE_TYPE__, const char* restrict, ...);
+
+char a[32];
+
+void test_warn (char *p)
+{
+  a[0] = 0;
+  sprintf (a, "a=%s", a);     /* { dg-warning "-Wrestrict" } */
+
+  p = a;
+  char *q = p + 1;
+  sprintf (p, "a=%s", q);     /* { dg-warning "-Wrestrict" } */
+}
+
+void test_nowarn_front_end (char *d)
+{
+  sprintf (d, "%p", d);
+  snprintf (d, 32, "%p", d);
+
+  sprintf (a, "p=%p", a);
+  snprintf (a, sizeof a, "%p", a);
+}
+
+void test_nowarn_sprintf_pass (char *d)
+{
+  char *q = d;
+  
+  sprintf (d, "p=%p", q);
+  snprintf (d, 32, "p=%p", q);
+
+  q = a;
+  sprintf (a, "a=%p", q);
+  snprintf (a, sizeof a, "a=%p", q);
+}