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, 8 insertions, 0 deletions

diff --git a/gcc/targhooks.c b/gcc/targhooks.c
index 5b60944..d83076f 100644
--- a/gcc/targhooks.c
+++ b/gcc/targhooks.c
@@ -1319,6 +1319,14 @@ default_get_mask_mode (poly_uint64 nunits, poly_uint64 vector_size)
   return opt_machine_mode ();
 }
 
+/* By default consider masked stores to be expensive.  */
+
+bool
+default_empty_mask_is_expensive (unsigned ifn)
+{
+  return ifn == IFN_MASK_STORE;
+}
+
 /* By default, the cost model accumulates three separate costs (prologue,
    loop body, and epilogue) for a vectorized loop or block.  So allocate an
    array of three unsigned ints, set it to zero, and return its address.  */