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[ifcvt,3/3] Create a new target hook for deciding profitability of noce if-conversion

Message ID 1443193479-10830-4-git-send-email-james.greenhalgh@arm.com
State New
Headers show

Commit Message

James Greenhalgh Sept. 25, 2015, 3:04 p.m. UTC 
Hi,

This patch introduces a new costs hook for deciding on the profitability
of an if-conversion candidate. We defer as much as possible for this
decision to the target, permitting the target to vary the outcome based
on the specific behaviours of a branch predictor in addition to any other
target-specific knowledge that might be available.

I had hoped to keep more of this generic, using rtx_costs and an additional
branch weighting factor to come up with a common formula, but that
proves troublesome for AArch64 where the expansion of multiple conditional
moves generates multiple redundant comparisons, which we know will be
later cleaned up.

As a target would have to make a judgement on how much of the new sequence
to cost, and can probably only do that reasonably with the old sequence as
context, I just expose both parts to the target and allow them to implement
whatever they feel best.

Bootstrapped on aarch64-none-linux-gnu, arm-none-linux-gnueabihf and
x86_64-none-linux-gnu with no issues, and checked code generation on these
platforms to ensure it has not changed.

OK?

Thanks,
James

---
2015-09-26  James Greenhalgh  <james.greenhalgh@arm.com>

	* target.def (costs): New hook vector.
	(ifcvt_noce_profitable_p): New hook.
	* doc/tm.texi.in: Document it.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_ifcvt_noce_profitable_p): New.
	* targhooks.c (default_ifcvt_noce_profitable_p): New.
	* ifcvt.c (noce_profitable_p): Use new target hook.
diff mbox

Patch

diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index eb495a8..b169d7c 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -6190,6 +6190,12 @@  true for well-predicted branches. On many architectures the
 @code{BRANCH_COST} can be reduced then.
 @end defmac
 
+@deftypefn {Target Hook} bool TARGET_COSTS_IFCVT_NOCE_PROFITABLE_P (rtx_insn *@var{seq}, struct noce_if_info *@var{info})
+This hook should return TRUE if converting the IF-THEN-ELSE blocks
+  described in INFO with the if-converted sequence SEQ is expected to
+  be profitable.
+@end deftypefn
+
 Here are additional macros which do not specify precise relative costs,
 but only that certain actions are more expensive than GCC would
 ordinarily expect.
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 92835c1..4765ec9 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -4575,6 +4575,8 @@  true for well-predicted branches. On many architectures the
 @code{BRANCH_COST} can be reduced then.
 @end defmac
 
+@hook TARGET_COSTS_IFCVT_NOCE_PROFITABLE_P
+
 Here are additional macros which do not specify precise relative costs,
 but only that certain actions are more expensive than GCC would
 ordinarily expect.
diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index d7fc523..e5e76bc 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -794,7 +794,7 @@  static bool
 noce_is_profitable_p (rtx_insn *seq ATTRIBUTE_UNUSED,
 		      struct noce_if_info *if_info)
 {
-  return (if_info->branch_cost >= if_info->magic_number);
+  return targetm.costs.ifcvt_noce_profitable_p (seq, if_info);
 }
 
 /* Helper function for noce_try_store_flag*.  */
diff --git a/gcc/target.def b/gcc/target.def
index f330709..996f31d 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -5876,6 +5876,21 @@  DEFHOOK
 
 HOOK_VECTOR_END (mode_switching)
 
+/* Cost functions.  */
+#undef HOOK_PREFIX
+#define HOOK_PREFIX "TARGET_COSTS_"
+HOOK_VECTOR (TARGET_COSTS_, costs)
+
+DEFHOOK
+(ifcvt_noce_profitable_p,
+ "This hook should return TRUE if converting the IF-THEN-ELSE blocks\n\
+  described in INFO with the if-converted sequence SEQ is expected to\n\
+  be profitable.",
+ bool, (rtx_insn *seq, struct noce_if_info *info),
+ default_ifcvt_noce_profitable_p)
+
+HOOK_VECTOR_END (costs)
+
 #undef HOOK_PREFIX
 #define HOOK_PREFIX "TARGET_"
 
diff --git a/gcc/targhooks.c b/gcc/targhooks.c
index 7238c8f..7b6dbe8 100644
--- a/gcc/targhooks.c
+++ b/gcc/targhooks.c
@@ -82,6 +82,7 @@  along with GCC; see the file COPYING3.  If not see
 #include "gimplify.h"
 #include "stringpool.h"
 #include "tree-ssanames.h"
+#include "ifcvt.h"
 
 
 bool
@@ -1922,4 +1923,14 @@  can_use_doloop_if_innermost (const widest_int &, const widest_int &,
   return loop_depth == 1;
 }
 
+/* For the default implementation, match the legacy logic by simply
+   comparing the estimated branch cost against a magic number.  */
+
+bool
+default_ifcvt_noce_profitable_p (rtx_insn *seq ATTRIBUTE_UNUSED,
+				 struct noce_if_info *if_info)
+{
+  return (if_info->branch_cost >= if_info->magic_number);
+}
+
 #include "gt-targhooks.h"
diff --git a/gcc/targhooks.h b/gcc/targhooks.h
index 5ae991d..076d513 100644
--- a/gcc/targhooks.h
+++ b/gcc/targhooks.h
@@ -240,4 +240,7 @@  extern void default_setup_incoming_vararg_bounds (cumulative_args_t ca ATTRIBUTE
 						  tree type ATTRIBUTE_UNUSED,
 						  int *pretend_arg_size ATTRIBUTE_UNUSED,
 						  int second_time ATTRIBUTE_UNUSED);
+
+extern bool default_ifcvt_noce_profitable_p (rtx_insn *,
+					     struct noce_if_info *);
 #endif /* GCC_TARGHOOKS_H */