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Date:      Thu, 29 Nov 2018 06:44:20 +0000 (UTC)
From:      Andrew Rybchenko <arybchik@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r341198 - head/sys/dev/sfxge/common
Message-ID:  <201811290644.wAT6iKSj025313@repo.freebsd.org>
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Author: arybchik
Date: Thu Nov 29 06:44:20 2018
New Revision: 341198
URL: https://svnweb.freebsd.org/changeset/base/341198

Log:
  sfxge(4): cope with clang warning on negative shift
  
  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.
  
  Sponsored by:   Solarflare Communications, Inc.
  Differential Revision:  https://reviews.freebsd.org/D18229

Modified:
  head/sys/dev/sfxge/common/efx_types.h

Modified: head/sys/dev/sfxge/common/efx_types.h
==============================================================================
--- head/sys/dev/sfxge/common/efx_types.h	Thu Nov 29 06:44:09 2018	(r341197)
+++ head/sys/dev/sfxge/common/efx_types.h	Thu Nov 29 06:44:20 2018	(r341198)
@@ -357,6 +357,16 @@ extern int fix_lint;
 #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).
  *
@@ -375,8 +385,8 @@ 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
@@ -565,29 +575,29 @@ 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
@@ -1316,22 +1326,22 @@ 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)				\

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