c: Try the type with the previous function attributes

When there are 2 conflicting function declarations, try the new type
with the previous TYPE_ATTRIBUTES if the current declaration has no
TYPE_ATTRIBUTES to support

extern void foo (void) __attribute__((nocf_check));

void
foo (void)
{
}

instead of issuing an error:

$ gcc -O2 -fcf-protection -S x.c
x.c:4:1: error: conflicting types for ‘foo’; have ‘void(void)’
    4 | foo (void)
      | ^~~
x.c:1:13: note: previous declaration of ‘foo’ with type ‘void(void)’
    1 | extern void foo (void) __attribute__((nocf_check));
      |             ^~~

The resulting function definition is compatible with the previous
declaration.

gcc/c/

	PR c/122427
	* c-decl.cc (diagnose_mismatched_decls): For FUNCTION_DECL, if
	OLDDECL has TYPE_ATTRIBUTES and NEWDECL doesn't, try the type
	with the OLDDECL attributes.

gcc/testsuite/

	PR c/122427
	* g++.target/i386/cf_check-1.C: New test.
	* g++.target/i386/cf_check-2.C: Likewise.
	* g++.target/i386/cf_check-3.C: Likewise.
	* g++.target/i386/cf_check-4.C: Likewise.
	* gcc.target/i386/cf_check-7.c: Likewise.
	* gcc.target/i386/cf_check-8.c: Likewise.
	* gcc.target/i386/cf_check-9.c: Likewise.
	* gcc.target/i386/cf_check-10.c: Likewise.
	* gcc.target/i386/cf_check-11.c: Likewise.
	* gcc.target/i386/no-callee-saved-12.c: Remove dg-error.
	* gcc.target/i386/preserve-none-17.c: Likewise.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>

gcc/c/c-decl.cc

@@ -2337,10 +2337,40 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 		error ("conflicting type qualifiers for %q+D", newdecl);
 	    }
 	  else
-	    error ("conflicting types for %q+D; have %qT", newdecl, newtype);
-	  diagnose_arglist_conflict (newdecl, olddecl, newtype, oldtype);
-	  locate_old_decl (olddecl);
-	  return false;
+	    {
+	      if (TREE_CODE (olddecl) == FUNCTION_DECL)
+		{
+		  tree attrs = TYPE_ATTRIBUTES (TREE_TYPE (olddecl));
+		  if (attrs && !TYPE_ATTRIBUTES (TREE_TYPE (newdecl)))
+		    {
+		      /* Similar to the C++ front-end, for FUNCTION_DECL,
+			 if OLDDECL has attributes and NEWDECL doesn't,
+			 try the type with OLDDECL attributes.  */
+		      tree rettype = TREE_TYPE (newtype);
+		      tree tryargs = TYPE_ARG_TYPES (newtype);
+		      tree trytype = c_build_function_type (rettype,
+							    tryargs);
+		      trytype = c_build_type_attribute_variant (trytype,
+								attrs);
+		      if (comptypes (oldtype, trytype))
+			{
+			  *newtypep = newtype = trytype;
+			  comptypes_result = 1;
+			}
+		    }
+		}
+
+	      if (!comptypes_result)
+		error ("conflicting types for %q+D; have %qT", newdecl,
+		       newtype);
+	    }
+	  if (!comptypes_result)
+	    {
+	      diagnose_arglist_conflict (newdecl, olddecl, newtype,
+					 oldtype);
+	      locate_old_decl (olddecl);
+	      return false;
+	    }
 	}
     }
   /* Warn about enum/integer type mismatches.  They are compatible types

gcc/testsuite/g++.target/i386/cf_check-1.C

@@ -0,0 +1,18 @@
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+
+extern void foo (void) __attribute__((nocf_check));
+extern void foo (void);
+
+void
+foo (void)
+{
+}
+
+extern void bar (void);
+extern void bar (void) __attribute__((nocf_check)); /* { dg-error "ambiguating new declaration" } */
+
+void
+bar (void)
+{
+}

gcc/testsuite/g++.target/i386/cf_check-2.C

@@ -0,0 +1,14 @@
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+
+extern void bar (void);
+extern void bar (void) __attribute__((nocf_check)); /* { dg-error "ambiguating new declaration" } */
+extern void foo (void) __attribute__((nocf_check));
+extern void foo (void);
+
+void
+func (void)
+{
+  bar ();
+  foo ();
+}

gcc/testsuite/g++.target/i386/cf_check-3.C

@@ -0,0 +1,19 @@
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+/* Keep labels and directives ('.cfi_startproc', '.cfi_endproc').  */
+/* { dg-final { check-function-bodies "**" "" "" { target "*-*-*" } {^\t?\.} } } */
+
+/*
+**_Z3foov:
+**.LFB[0-9]+:
+**	.cfi_startproc
+**	ret
+**...
+*/
+
+extern void foo (void) __attribute__((nocf_check));
+
+void
+foo (void)
+{
+}

gcc/testsuite/g++.target/i386/cf_check-4.C

@@ -0,0 +1,23 @@
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-require-weak "" } */
+/* { dg-options "-O2 -fcf-protection" } */
+/* Keep labels and directives ('.cfi_startproc', '.cfi_endproc').  */
+/* { dg-final { check-function-bodies "**" "" "" { target "*-*-*" } {^\t?\.} } } */
+
+/*
+**_Z3foov:
+**.LFB[0-9]+:
+**	.cfi_startproc
+**	ret
+**...
+*/
+
+extern void foo (void) __attribute__((nocf_check));
+
+__attribute__((weak))
+void
+foo (void)
+{
+}
+
+/* { dg-final { scan-assembler ".weak\[ \t\]_?_Z3foov" } } */

gcc/testsuite/gcc.target/i386/cf_check-10.c

@@ -0,0 +1,19 @@
+/* PR c/122427 */
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+
+extern void foo (void) __attribute__((nocf_check));
+extern void foo (void);
+
+void
+foo (void)
+{
+}
+
+extern void bar (void);
+extern void bar (void) __attribute__((nocf_check)); /* { dg-error "conflicting types" } */
+
+void
+bar (void)
+{
+}

gcc/testsuite/gcc.target/i386/cf_check-11.c

@@ -0,0 +1,24 @@
+/* PR c/122427 */
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-require-weak "" } */
+/* { dg-options "-O2 -fcf-protection" } */
+/* Keep labels and directives ('.cfi_startproc', '.cfi_endproc').  */
+/* { dg-final { check-function-bodies "**" "" "" { target "*-*-*" } {^\t?\.} } } */
+
+/*
+**foo:
+**.LFB[0-9]+:
+**	.cfi_startproc
+**	ret
+**...
+*/
+
+extern void foo (void) __attribute__((nocf_check));
+
+__attribute__((weak))
+void
+foo (void)
+{
+}
+
+/* { dg-final { scan-assembler ".weak\[ \t\]_?foo" } } */

gcc/testsuite/gcc.target/i386/cf_check-7.c

@@ -0,0 +1,20 @@
+/* PR c/122427 */
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+/* Keep labels and directives ('.cfi_startproc', '.cfi_endproc').  */
+/* { dg-final { check-function-bodies "**" "" "" { target "*-*-*" } {^\t?\.} } } */
+
+/*
+**foo:
+**.LFB[0-9]+:
+**	.cfi_startproc
+**	ret
+**...
+*/
+
+extern void foo (void) __attribute__((nocf_check));
+
+void
+foo (void)
+{
+}

gcc/testsuite/gcc.target/i386/cf_check-8.c

@@ -0,0 +1,11 @@
+/* PR c/122427 */
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+
+extern void foo (void);
+
+__attribute__((nocf_check))
+void
+foo (void) /* { dg-error "conflicting types" } */
+{
+}

gcc/testsuite/gcc.target/i386/cf_check-9.c

@@ -0,0 +1,15 @@
+/* PR c/122427 */
+/* { dg-do compile { target { "i?86-*-* x86_64-*-*" } } } */
+/* { dg-options "-O2 -fcf-protection" } */
+
+extern void bar (void);
+extern void bar (void) __attribute__((nocf_check)); /* { dg-error "conflicting types" } */
+extern void foo (void) __attribute__((nocf_check));
+extern void foo (void);
+
+void
+func (void)
+{
+  bar ();
+  foo ();
+}

gcc/testsuite/gcc.target/i386/no-callee-saved-12.c

@@ -1,10 +1,9 @@
 /* { dg-do compile } */
 /* { dg-options "-O2" } */
 
-extern void foo (void) __attribute__ ((no_callee_saved_registers)); /* { dg-note "previous declaration" } */
+extern void foo (void) __attribute__ ((no_callee_saved_registers));
 
 void
-foo (void) /* { dg-error "conflicting types" } */
+foo (void)
 {
 }
-

gcc/testsuite/gcc.target/i386/preserve-none-17.c

@@ -1,10 +1,9 @@
 /* { dg-do compile } */
 /* { dg-options "-O2" } */
 
-extern void foo (void) __attribute__ ((preserve_none)); /* { dg-note "previous declaration" } */
+extern void foo (void) __attribute__ ((preserve_none));
 
 void
-foo (void) /* { dg-error "conflicting types" } */
+foo (void)
 {
 }
-