Add an empty_mask_is_expensive hook

This patch adds a hook to control whether we avoid executing masked
(predicated) stores when the mask is all false.  We don't want to do
that by default for SVE.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* target.def (empty_mask_is_expensive): New hook.
	* doc/tm.texi.in (TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE): New hook.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_empty_mask_is_expensive): Declare.
	* targhooks.c (default_empty_mask_is_expensive): New function.
	* tree-vectorizer.c (vectorize_loops): Only call optimize_mask_stores
	if the target says that empty masks are expensive.
	* config/aarch64/aarch64.c (aarch64_empty_mask_is_expensive):
	New function.
	(TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE): Redefine.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256631

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 25b0a1b..faf7b8b 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -5924,6 +5924,12 @@ is @var{length} bytes long and that contains @var{nunits} elements,
 if such a mode exists.
 @end deftypefn
 
+@deftypefn {Target Hook} bool TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE (unsigned @var{ifn})
+This hook returns true if masked internal function @var{ifn} (really of
+type @code{internal_fn}) should be considered expensive when the mask is
+all zeros.  GCC can then try to branch around the instruction instead.
+@end deftypefn
+
 @deftypefn {Target Hook} {void *} TARGET_VECTORIZE_INIT_COST (struct loop *@var{loop_info})
 This hook should initialize target-specific data structures in preparation for modeling the costs of vectorizing a loop or basic block.  The default allocates three unsigned integers for accumulating costs for the prologue, body, and epilogue of the loop or basic block.  If @var{loop_info} is non-NULL, it identifies the loop being vectorized; otherwise a single block is being vectorized.
 @end deftypefn