2 files changed, 29 insertions, 1 deletions

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 8404707..0fe7c50 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -2884,13 +2884,16 @@ quotient or remainder and generate the appropriate instruction.
 @item @samp{udivmod@var{m}4}
 Similar, but does unsigned division.
 
+@anchor{shift patterns}
 @cindex @code{ashl@var{m}3} instruction pattern
 @item @samp{ashl@var{m}3}
 Arithmetic-shift operand 1 left by a number of bits specified by operand
 2, and store the result in operand 0.  Here @var{m} is the mode of
 operand 0 and operand 1; operand 2's mode is specified by the
 instruction pattern, and the compiler will convert the operand to that
-mode before generating the instruction.
+mode before generating the instruction.  The meaning of out-of-range shift
+counts can optionally be specified by @code{TARGET_SHIFT_TRUNCATION_MASK}.
+@xref{TARGET_SHIFT_TRUNCATION_MASK}.
 
 @cindex @code{ashr@var{m}3} instruction pattern
 @cindex @code{lshr@var{m}3} instruction pattern
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 2672485..e21f727 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -8749,6 +8749,31 @@ the implied truncation of the shift instructions.
 You need not define this macro if it would always have the value of zero.
 @end defmac
 
+@anchor{TARGET_SHIFT_TRUNCATION_MASK}
+@deftypefn {Target Hook} int TARGET_SHIFT_TRUNCATION_MASK (enum machine_mode @var{mode})
+This function describes how the standard shift patterns for @var{mode}
+deal with shifts by negative amounts or by more than the width of the mode.
+@xref{shift patterns}.
+
+On many machines, the shift patterns will apply a mask @var{m} to the
+shift count, meaning that a fixed-width shift of @var{x} by @var{y} is
+equivalent to an arbitrary-width shift of @var{x} by @var{y & m}.  If
+this is true for mode @var{mode}, the function should return @var{m},
+otherwise it should return 0.  A return value of 0 indicates that no
+particular behavior is guaranteed.
+
+Note that, unlike @code{SHIFT_COUNT_TRUNCATED}, this function does
+@emph{not} apply to general shift rtxes; it applies only to instructions
+that are generated by the named shift patterns.
+
+The default implementation of this function returns
+@code{GET_MODE_BITSIZE (@var{mode}) - 1} if @code{SHIFT_COUNT_TRUNCATED}
+and 0 otherwise.  This definition is always safe, but if
+@code{SHIFT_COUNT_TRUNCATED} is false, and some shift patterns
+nevertheless truncate the shift count, you may get better code
+by overriding it.
+@end deftypefn
+
 @defmac TRULY_NOOP_TRUNCATION (@var{outprec}, @var{inprec})
 A C expression which is nonzero if on this machine it is safe to
 ``convert'' an integer of @var{inprec} bits to one of @var{outprec}