[PATCH] libcpp: suppress builtin macro redefined warnings for __LINE__

From 0821df518b264e754d698d399f98be1a62945e32 Mon Sep 17 00:00:00 2001
From: Longjun Luo <luolongjuna@gmail.com>
Date: Thu, 12 Jan 2023 23:59:54 +0800
Subject: [PATCH] libcpp: suppress builtin macro redefined warnings for
 __LINE__

As implied in
gcc.gnu.org/legacy-ml/gcc-patches/2008-09/msg00076.html,
gcc provides -Wno-builtin-macro-redefined to suppress warning when
redefining builtin macro. However, at that time, there was no
scenario for __LINE__ macro.

But, when we try to build a live-patch, we compare sections by using
-ffunction-sections. Some same functions are considered changed because
of __LINE__ macro.

At present, to detect such a changed caused by __LINE__ macro, we
have to analyse code and maintain a function list. For example,
in kpatch, check this commit
github.com/dynup/kpatch/commit/0e1b95edeafa36edb7bcf11da6d1c00f76d7e03d.

So, in this scenario, when we try to compared sections, it would
be better to support suppress builtin macro redefined warnings for
__LINE__ macro.

libcpp:
	* init.cc (builtin_array): Do not always warn for a redefinition
	of __LINE__.

gcc/testsuite

	* gcc.dg/builtin-redefine.c: Test for redefintion warnings
	for __LINE__.
	* gcc.dg/builtin-redefine-1.c: New test.

gcc/testsuite/gcc.dg/builtin-redefine-1.c

@@ -0,0 +1,49 @@
+/* Test default warnings for redefining builtin macros.  */
+
+/* { dg-do compile } */
+/* { dg-options "-D__TIMESTAMP__=x -D__TIME__=x -D__DATE__=x -D__FILE__=x -D__FILE_NAME__=x -D__BASE_FILE__=x -D__LINE__=0" } */
+
+/* Check default behavior for builtin macros redefinition.  */
+
+/* { dg-message "\"__TIMESTAMP__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __TIMESTAMP__
+#error "__TIMESTAMP__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+
+/* { dg-message "\"__TIME__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __TIME__
+#error "__TIME__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+
+/* { dg-message "\"__DATE__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __DATE__
+#error "__DATE__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+
+/* { dg-message "\"__FILE__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __FILE__
+#error "__FILE__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+
+/* { dg-message "\"__FILE_NAME__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __FILE_NAME__
+#error "__FILE_NAME__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+
+/* { dg-message "\"__BASE_FILE__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __BASE_FILE__
+#error "__BASE_FILE__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+
+/* { dg-message "\"__LINE__\" redefined" "" {target "*-*-*"} 0 } */
+#ifndef __LINE__
+#error "__LINE__ builtin is not defined"
+/* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
+#endif
+

gcc/testsuite/gcc.dg/builtin-redefine.c

@@ -1,9 +1,9 @@
 /* Test -Wno-builtin-macro-redefined warnings.  */
 
 /* { dg-do compile } */
-/* { dg-options "-Wno-builtin-macro-redefined -U__DATE__ -D__TIME__=X" } */
+/* { dg-options "-Wno-builtin-macro-redefined -U__DATE__ -D__TIME__=X -D__LINE__=0" } */
 
-/* Check date, time, and datestamp built-ins warnings may be suppressed.  */
+/* Check date, time, datestamp and line built-ins warnings may be suppressed.  */
 
 #if defined(__DATE__)
 #error "__DATE__ is defined, but should not be (-U command line error)"
@@ -15,6 +15,11 @@
 /* { dg-bogus "__TIME__ is not defined" "" { target *-*-* } .-1 } */
 #endif
 
+#if __LINE__ != 0
+#error "__LINE__ is not defined as expected (-D command line error)"
+/* { dg-bogus "__LINE__ is not defined" "" { target *-*-* } .-1 } */
+#endif
+
 #if !defined(__TIMESTAMP__)
 #error "__TIMESTAMP__ is not defined (built-in macro expectation error)"
 /* { dg-bogus "__TIMESTAMP__ is not defined" "" { target *-*-* } .-1 } */
@@ -53,6 +58,18 @@
 #undef __TIMESTAMP__         /* Undefine while defined.  */
 
 
+#undef __LINE__              /* Undefine while defined.  */
+#undef __LINE__              /* Undefine while already undefined.  */
+
+#define __LINE__ "1"         /* Define while undefined.  */
+#define __LINE__ "1"         /* Re-define while defined.  */ /* { dg-line line_prev } */
+
+#define __LINE__ "2"         /* { dg-warning "-:\"__LINE__\" redefined" } */
+/* { dg-message "-:previous definition" "" { target *-*-* } line_prev } */
+
+#undef __LINE__              /* Undefine while defined.  */
+
+
 /* Check other built-ins with warnings that may be suppressed.  */
 
 #if !defined(__FILE__) || !defined(__BASE_FILE__)
@@ -66,12 +83,11 @@
 
 /* Check selected built-ins not affected by warning suppression. */
 
-#if !defined(__LINE__) || !defined(__INCLUDE_LEVEL__) || !defined(__COUNTER__)
+#if !defined(__INCLUDE_LEVEL__) || !defined(__COUNTER__)
 #error "Expected built-in is not defined (built-in macro expectation error)"
 /* { dg-bogus "Expected built-in is not defined" "" { target *-*-* } .-1 } */
 #endif
 
-#define __LINE__ 0           /* { dg-warning "-:\"__LINE__\" redef" } */
 #define __INCLUDE_LEVEL__ 0  /* { dg-warning "-:\"__INCLUDE_LEVEL__\" redef" } */
 #define __COUNTER__ 0        /* { dg-warning "-:\"__COUNTER__\" redef" } */
 

libcpp/init.cc

@@ -421,7 +421,7 @@ static const struct builtin_macro builtin_array[] =
   B("__FILE__",		 BT_FILE,          false),
   B("__FILE_NAME__",	 BT_FILE_NAME,     false),
   B("__BASE_FILE__",	 BT_BASE_FILE,     false),
-  B("__LINE__",		 BT_SPECLINE,      true),
+  B("__LINE__",		 BT_SPECLINE,      false),
   B("__INCLUDE_LEVEL__", BT_INCLUDE_LEVEL, true),
   B("__COUNTER__",	 BT_COUNTER,       true),
   /* Make sure to update the list of built-in