[dpdk-dev] [PATCH v3 1/8] net/sfc/base: cope with clang warning on negative shift

[Andrew Rybchenko <arybchenko@solarflare.com>]

clang 4.0.1-6 on Ubuntu generates false positive warning that shift
is negative.  It is done regardless of the fact that the branch is
not taken because of previous check.

The warning is generate in EFX_INSERT_NATIVE32 used by
EFX_INSERT_FIELD_NATIVE32. All similar cases are fixed as well.

It is undesirable to suppress the warning completely.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
---
 drivers/net/sfc/base/efx_types.h | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/drivers/net/sfc/base/efx_types.h b/drivers/net/sfc/base/efx_types.h
index 0581f67f4..65168ab79 100644
--- a/drivers/net/sfc/base/efx_types.h
+++ b/drivers/net/sfc/base/efx_types.h
@@ -328,6 +328,16 @@ extern int fix_lint;
 #define	FIX_LINT(_x)	(_x)
 #endif
 
+/*
+ * Saturation arithmetic subtract with minimum equal to zero.
+ *
+ * Use saturating arithmetic to ensure a non-negative result. This
+ * avoids undefined behaviour (and compiler warnings) when used as a
+ * shift count.
+ */
+#define	EFX_SSUB(_val, _sub) \
+	((_val) > (_sub) ? ((_val) - (_sub)) : 0)
+
 /*
  * Extract bit field portion [low,high) from the native-endian element
  * which contains bits [min,max).
@@ -347,8 +357,8 @@ extern int fix_lint;
 	((FIX_LINT(_low > _max) || FIX_LINT(_high < _min)) ?		\
 		0U :							\
 		((_low > _min) ?					\
-			((_element) >> (_low - _min)) :			\
-			((_element) << (_min - _low))))
+			((_element) >> EFX_SSUB(_low, _min)) :		\
+			((_element) << EFX_SSUB(_min, _low))))
 
 /*
  * Extract bit field portion [low,high) from the 64-bit little-endian
@@ -537,29 +547,29 @@ extern int fix_lint;
 	(((_low > _max) || (_high < _min)) ?				\
 		0U :							\
 		((_low > _min) ?					\
-			(((uint64_t)(_value)) << (_low - _min)) :	\
-			(((uint64_t)(_value)) >> (_min - _low))))
+			(((uint64_t)(_value)) << EFX_SSUB(_low, _min)) :\
+			(((uint64_t)(_value)) >> EFX_SSUB(_min, _low))))
 
 #define	EFX_INSERT_NATIVE32(_min, _max, _low, _high, _value)		\
 	(((_low > _max) || (_high < _min)) ?				\
 		0U :							\
 		((_low > _min) ?					\
-			(((uint32_t)(_value)) << (_low - _min)) :	\
-			(((uint32_t)(_value)) >> (_min - _low))))
+			(((uint32_t)(_value)) << EFX_SSUB(_low, _min)) :\
+			(((uint32_t)(_value)) >> EFX_SSUB(_min, _low))))
 
 #define	EFX_INSERT_NATIVE16(_min, _max, _low, _high, _value)		\
 	(((_low > _max) || (_high < _min)) ?				\
 		0U :							\
 		(uint16_t)((_low > _min) ?				\
-				((_value) << (_low - _min)) :		\
-				((_value) >> (_min - _low))))
+				((_value) << EFX_SSUB(_low, _min)) :	\
+				((_value) >> EFX_SSUB(_min, _low))))
 
 #define	EFX_INSERT_NATIVE8(_min, _max, _low, _high, _value)		\
 	(((_low > _max) || (_high < _min)) ?				\
 		0U :							\
 		(uint8_t)((_low > _min) ?				\
-				((_value) << (_low - _min)) :	\
-				((_value) >> (_min - _low))))
+				((_value) << EFX_SSUB(_low, _min)) :	\
+				((_value) >> EFX_SSUB(_min, _low))))
 
 /*
  * Construct bit field portion
@@ -1288,22 +1298,22 @@ extern int fix_lint;
 
 #define	EFX_SHIFT64(_bit, _base)					\
 	(((_bit) >= (_base) && (_bit) < (_base) + 64) ?			\
-		((uint64_t)1 << ((_bit) - (_base))) :			\
+		((uint64_t)1 << EFX_SSUB((_bit), (_base))) :		\
 		0U)
 
 #define	EFX_SHIFT32(_bit, _base)					\
 	(((_bit) >= (_base) && (_bit) < (_base) + 32) ?			\
-		((uint32_t)1 << ((_bit) - (_base))) :			\
+		((uint32_t)1 << EFX_SSUB((_bit),(_base))) :		\
 		0U)
 
 #define	EFX_SHIFT16(_bit, _base)					\
 	(((_bit) >= (_base) && (_bit) < (_base) + 16) ?			\
-		(uint16_t)(1 << ((_bit) - (_base))) :			\
+		(uint16_t)(1 << EFX_SSUB((_bit), (_base))) :		\
 		0U)
 
 #define	EFX_SHIFT8(_bit, _base)						\
 	(((_bit) >= (_base) && (_bit) < (_base) + 8) ?			\
-		(uint8_t)(1 << ((_bit) - (_base))) :			\
+		(uint8_t)(1 << EFX_SSUB((_bit), (_base))) :		\
 		0U)
 
 #define	EFX_SET_OWORD_BIT64(_oword, _bit)				\
-- 
2.14.1