From 81fd62d1378b7ddc1fa0967cbddcdcdcdd2d8d8c Mon Sep 17 00:00:00 2001
From: Tamar Christina <tamar.christina@arm.com>
Date: Sun, 12 Mar 2023 18:42:04 +0000
Subject: middle-end: Implement preferred_div_as_shifts_over_mult [PR108583]

This now implements a hook
preferred_div_as_shifts_over_mult that indicates whether a target prefers that
the vectorizer decomposes division as shifts rather than multiplication when
possible.

In order to be able to use this we need to check whether the current precision
has enough bits to do the operation without any of the additions overflowing.

We use range information to determine this and only do the operation if we're
sure am overflow won't occur. This now uses ranger to do this range check.

This seems to work better than vect_get_range_info which uses range_query, but I
have not switched the interface of vect_get_range_info over in this PR fix.

As Andy said before initializing a ranger instance is cheap but not free, and if
the intention is to call it often during a pass it should be instantiated at
pass startup and passed along to the places that need it.  This is a big
refactoring and doesn't seem right to do in this PR.  But we should in GCC 14.

Currently we only instantiate it after a long series of much cheaper checks.

gcc/ChangeLog:

	PR target/108583
	* target.def (preferred_div_as_shifts_over_mult): New.
	* doc/tm.texi.in: Document it.
	* doc/tm.texi: Regenerate.
	* targhooks.cc (default_preferred_div_as_shifts_over_mult): New.
	* targhooks.h (default_preferred_div_as_shifts_over_mult): New.
	* tree-vect-patterns.cc (vect_recog_divmod_pattern): Use it.

gcc/testsuite/ChangeLog:

	PR target/108583
	* gcc.dg/vect/vect-div-bitmask-4.c: New test.
	* gcc.dg/vect/vect-div-bitmask-5.c: New test.
---
 gcc/target.def | 12 ++++++++++++
 1 file changed, 12 insertions(+)

(limited to 'gcc/target.def')

diff --git a/gcc/target.def b/gcc/target.def
index 049dab6..f401fe1 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -1868,6 +1868,18 @@ correct for most targets.",
  poly_uint64, (const_tree type),
  default_preferred_vector_alignment)
 
+/* Returns whether the target has a preference for decomposing divisions using
+   shifts rather than multiplies.  */
+DEFHOOK
+(preferred_div_as_shifts_over_mult,
+ "Sometimes it is possible to implement a vector division using a sequence\n\
+of two addition-shift pairs, giving four instructions in total.\n\
+Return true if taking this approach for @var{vectype} is likely\n\
+to be better than using a sequence involving highpart multiplication.\n\
+Default is false if @code{can_mult_highpart_p}, otherwise true.",
+ bool, (const_tree type),
+ default_preferred_div_as_shifts_over_mult)
+
 /* Return true if vector alignment is reachable (by peeling N
    iterations) for the given scalar type.  */
 DEFHOOK
-- 
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