From 7e8ae1e611bdd07b74a07bb24d8d38c1ac698853 Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Tue, 14 May 2019 10:49:37 -0400
Subject: [PATCH] Silence compiler "always false comparison" warning

Compiling the bitfield test with gcc -Wextra generates those warnings:

 ../../include/babeltrace/bitfield-internal.h:38:45: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
 #define _bt_is_signed_type(type) ((type) -1 < (type) 0)

This is the intent of the macro. Disable compiler warnings around use of
that macro.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com
---
 include/lttng/bitfield.h | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/include/lttng/bitfield.h b/include/lttng/bitfield.h
index fe07824d..eb79156f 100644
--- a/include/lttng/bitfield.h
+++ b/include/lttng/bitfield.h
@@ -25,6 +25,7 @@
 
 #include <stdint.h>	/* C99 5.2.4.2 Numerical limits */
 #include <limits.h>	/* C99 5.2.4.2 Numerical limits */
+#include <stdbool.h>	/* C99 7.16 bool type */
 #include <lttng/ust-endian.h>	/* Non-standard BIG_ENDIAN, LITTLE_ENDIAN, BYTE_ORDER */
 
 /*
@@ -40,6 +41,26 @@
 #error "bitfield.h requires the compiler representation of signed integers to be two's complement."
 #endif
 
+/*
+ * _bt_is_signed_type() willingly generates comparison of unsigned
+ * expression < 0, which is always false. Silence compiler warnings.
+ */
+#ifdef __GNUC__
+# define _BT_DIAG_PUSH			_Pragma("GCC diagnostic push")
+# define _BT_DIAG_POP			_Pragma("GCC diagnostic pop")
+
+# define _BT_DIAG_STRINGIFY_1(x)	#x
+# define _BT_DIAG_STRINGIFY(x)		_BT_DIAG_STRINGIFY_1(x)
+
+# define _BT_DIAG_IGNORE(option)	\
+	_Pragma(_BT_DIAG_STRINGIFY(GCC diagnostic ignored option))
+# define _BT_DIAG_IGNORE_TYPE_LIMITS	_BT_DIAG_IGNORE("-Wtype-limits")
+#else
+# define _BT_DIAG_PUSH
+# define _BT_DIAG_POP
+# define _BT_DIAG_IGNORE
+#endif
+
 #define _bt_is_signed_type(type)	((type) -1 < (type) 0)
 
 /*
@@ -359,6 +380,7 @@ do {									\
 	unsigned long ts = sizeof(type) * CHAR_BIT; /* type size */	\
 	unsigned long start_unit, end_unit, this_unit;			\
 	unsigned long end, cshift; /* cshift is "complement shift" */	\
+	bool is_signed_type;						\
 									\
 	if (!__length) {						\
 		*__vptr = 0;						\
@@ -370,7 +392,11 @@ do {									\
 	end_unit = (end + (ts - 1)) / ts;				\
 									\
 	this_unit = end_unit - 1;					\
-	if (_bt_is_signed_type(__typeof__(__v))				\
+	_BT_DIAG_PUSH							\
+	_BT_DIAG_IGNORE_TYPE_LIMITS					\
+	is_signed_type = _bt_is_signed_type(__typeof__(__v));		\
+	_BT_DIAG_POP							\
+	if (is_signed_type						\
 	    && (__ptr[this_unit] & _bt_lshift((type) 1, (end % ts ? end % ts : ts) - 1))) \
 		__v = ~(__typeof__(__v)) 0;				\
 	else								\
@@ -426,6 +452,7 @@ do {									\
 	unsigned long ts = sizeof(type) * CHAR_BIT; /* type size */	\
 	unsigned long start_unit, end_unit, this_unit;			\
 	unsigned long end, cshift; /* cshift is "complement shift" */	\
+	bool is_signed_type;						\
 									\
 	if (!__length) {						\
 		*__vptr = 0;						\
@@ -437,7 +464,11 @@ do {									\
 	end_unit = (end + (ts - 1)) / ts;				\
 									\
 	this_unit = start_unit;						\
-	if (_bt_is_signed_type(__typeof__(__v))				\
+	_BT_DIAG_PUSH							\
+	_BT_DIAG_IGNORE_TYPE_LIMITS					\
+	is_signed_type = _bt_is_signed_type(__typeof__(__v));		\
+	_BT_DIAG_POP							\
+	if (is_signed_type						\
 	    && (__ptr[this_unit] & _bt_lshift((type) 1, ts - (__start % ts) - 1))) \
 		__v = ~(__typeof__(__v)) 0;				\
 	else								\
-- 
2.34.1