implement-c.texi (Integers Implementation): Make GCC's promises about signed…

implement-c.texi (Integers Implementation): Make GCC's promises about signed left shift stronger and clarify the cases when...

2015-11-26  Paolo Bonzini <bonzini@gnu.org>

	* doc/implement-c.texi (Integers Implementation): Make GCC's promises
	about signed left shift stronger and clarify the cases when they're
	broken.

From-SVN: r230931

gcc/ChangeLog

+2015-11-26  Paolo Bonzini <bonzini@gnu.org>
+
+	* doc/implement-c.texi (Integers Implementation): Make GCC's promises
+	about signed left shift stronger and clarify the cases when they're
+	broken.
+
 2015-11-26  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
             Bernd Schmidt  <bschmidt@redhat.com>
 
@@ -7682,6 +7688,9 @@
 
 	* fold-const.c (fold_binary_loc) : Move (-A) * (-B) -> A * B
 	to match.pd.
+* doc/implement-c.texi (Integers Implementation): Make GCC's promises
+about signed left shift stronger and clarify the cases when they're
+broken.
 	Move (a * (1 << b)) is (a << b) to match.pd.
 	Move convert (C1/X)*C2 into (C1*C2)/X to match.pd.
 	Move ~X & X, (X == 0) & X, and !X & X are zero to match.pd.

gcc/doc/implement-c.texi

@@ -266,9 +266,11 @@ both the sign and value bits, where the sign bit is considered
 immediately above the highest-value value bit.  Signed @samp{>>} acts
 on negative numbers by sign extension.
 
-GCC does not use the latitude given in C99 and C11 only to treat certain
-aspects of signed @samp{<<} as undefined, but this is subject to
-change.
+As an extension to the C language, GCC does not use the latitude given in
+C99 and C11 only to treat certain aspects of signed @samp{<<} as undefined.
+However, @option{-fsanitize=shift} (and @option{-fsanitize=undefined}) will
+diagnose such cases.  They are also diagnosed where constant
+expressions are required.
 
 @item
 @cite{The sign of the remainder on integer division (C90 6.3.5).}