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

gcc/ChangeLog

@@ -2,6 +2,21 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* 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.
+
+2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* tree-vectorizer.h (_loop_vec_info::mask_skip_niters): New field.
 	(LOOP_VINFO_MASK_SKIP_NITERS): New macro.
 	(vect_use_loop_mask_for_alignment_p): New function.

gcc/config/aarch64/aarch64.c

@@ -16875,6 +16875,16 @@ aarch64_gen_adjusted_ldpstp (rtx *operands, bool load,
   return true;
 }
 
+/* Implement TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE.  Assume for now that
+   it isn't worth branching around empty masked ops (including masked
+   stores).  */
+
+static bool
+aarch64_empty_mask_is_expensive (unsigned)
+{
+  return false;
+}
+
 /* Return 1 if pseudo register should be created and used to hold
    GOT address for PIC code.  */
 
@@ -17499,6 +17509,9 @@ aarch64_libgcc_floating_mode_supported_p
 
 #undef TARGET_VECTORIZE_GET_MASK_MODE
 #define TARGET_VECTORIZE_GET_MASK_MODE aarch64_get_mask_mode
+#undef TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE
+#define TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE \
+  aarch64_empty_mask_is_expensive
 
 #undef TARGET_INIT_LIBFUNCS
 #define TARGET_INIT_LIBFUNCS aarch64_init_libfuncs

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

gcc/doc/tm.texi.in

@@ -4108,6 +4108,8 @@ address;  but often a machine-dependent strategy can generate better code.
 
 @hook TARGET_VECTORIZE_GET_MASK_MODE
 
+@hook TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE
+
 @hook TARGET_VECTORIZE_INIT_COST
 
 @hook TARGET_VECTORIZE_ADD_STMT_COST

gcc/target.def

@@ -1933,6 +1933,17 @@ if such a mode exists.",
  (poly_uint64 nunits, poly_uint64 length),
  default_get_mask_mode)
 
+/* Function to say whether a masked operation is expensive when the
+   mask is all zeros.  */
+DEFHOOK
+(empty_mask_is_expensive,
+ "This hook returns true if masked internal function @var{ifn} (really of\n\
+type @code{internal_fn}) should be considered expensive when the mask is\n\
+all zeros.  GCC can then try to branch around the instruction instead.",
+ bool,
+ (unsigned ifn),
+ default_empty_mask_is_expensive)
+
 /* Target builtin that implements vector gather operation.  */
 DEFHOOK
 (builtin_gather,

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.  */

gcc/targhooks.h

@@ -111,6 +111,7 @@ extern machine_mode default_preferred_simd_mode (scalar_mode mode);
 extern machine_mode default_split_reduction (machine_mode);
 extern void default_autovectorize_vector_sizes (vector_sizes *);
 extern opt_machine_mode default_get_mask_mode (poly_uint64, poly_uint64);
+extern bool default_empty_mask_is_expensive (unsigned);
 extern void *default_init_cost (struct loop *);
 extern unsigned default_add_stmt_cost (void *, int, enum vect_cost_for_stmt,
 				       struct _stmt_vec_info *, int,

gcc/tree-vectorizer.c

@@ -826,7 +826,8 @@ vectorize_loops (void)
       if (loop_vinfo)
 	has_mask_store = LOOP_VINFO_HAS_MASK_STORE (loop_vinfo);
       delete loop_vinfo;
-      if (has_mask_store)
+      if (has_mask_store
+	  && targetm.vectorize.empty_mask_is_expensive (IFN_MASK_STORE))
 	optimize_mask_stores (loop);
       loop->aux = NULL;
     }