[PATCH][1/3][ARM] Keep ctz expressions together until after reload

[PATCH][1/3][ARM] Keep ctz expressions together until after reload

[Kyrill Tkachov]

[-- Attachment #1: Type: text/plain, Size: 1131 bytes --]

Hi all,

On arm we don't have a dedicated instruction that corresponds to a CTZ rtx but we synthesise it
with an RBIT instruction followed by a CLZ. This is currently done at expand time.
However, I'd like to push that step until after reload and keep the CTZ rtx as a single whole in
the early RTL optimisers.  This better expresses the semantics of the operation as a whole, since
the RBIT operation is represented as an UNSPEC anyway and so will not see the benefits of combine,
and a CTZ-specific optimisation that is implemented in patch 3/3 of this series won't be triggered
if the expression is broken up into an UNSPEC and a CLZ.

Therefore this patch changes the expander to expand to a CTZ rtx and split it after reload into
an RBIT + CLZ to allow sched2 to schedule them apart if it deems necessary.
This patch enables the optimisation in patch 3/3 where the appropriate test is added.

Bootstrapped and tested on arm-none-linux-gnueabihf.

Ok for trunk?

Thanks,
Kyrill

2016-05-26  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR middle-end/37780
     * config/arm/arm.md (ctzsi2): Convert to define_insn_and_split.

[-- Attachment #2: arm-ctz.patch --]
[-- Type: text/x-patch, Size: 1245 bytes --]

diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
index 0d491f7ea41e4fb5fb58bbb3047294abda541a73..fcb07e7629dc14b7cf8a0cd3d4d1a57ff33efe07 100644
--- a/gcc/config/arm/arm.md
+++ b/gcc/config/arm/arm.md
@@ -10784,19 +10784,22 @@ (define_insn "rbitsi2"
    (set_attr "predicable_short_it" "no")
    (set_attr "type" "clz")])
 
-(define_expand "ctzsi2"
- [(set (match_operand:SI           0 "s_register_operand" "")
-       (ctz:SI (match_operand:SI  1 "s_register_operand" "")))]
+;; Keep this as a CTZ expression until after reload and then split
+;; into RBIT + CLZ.  Since RBIT is represented as an UNSPEC it is unlikely
+;; to fold with any other expression.
+
+(define_insn_and_split "ctzsi2"
+ [(set (match_operand:SI           0 "s_register_operand" "=r")
+       (ctz:SI (match_operand:SI  1 "s_register_operand" "r")))]
   "TARGET_32BIT && arm_arch_thumb2"
+  "#"
+  "&& reload_completed"
+  [(const_int 0)]
   "
-   {
-     rtx tmp = gen_reg_rtx (SImode); 
-     emit_insn (gen_rbitsi2 (tmp, operands[1]));
-     emit_insn (gen_clzsi2 (operands[0], tmp));
-   }
-   DONE;
-  "
-)
+  emit_insn (gen_rbitsi2 (operands[0], operands[1]));
+  emit_insn (gen_clzsi2 (operands[0], operands[0]));
+  DONE;
+")
 
 ;; V5E instructions.
 

Re: [PATCH][1/3][ARM] Keep ctz expressions together until after reload

[Joseph Myers]

On Thu, 26 May 2016, Kyrill Tkachov wrote:

> the early RTL optimisers.  This better expresses the semantics of the 
> operation as a whole, since the RBIT operation is represented as an 
> UNSPEC anyway and so will not see the benefits of combine,

This doesn't affect your patch, but I think it would make sense for RBIT 
not to be an UNSPEC but to have architecture-independent RTL and built-in 
function - see bug 50481.

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: [PATCH][1/3][ARM] Keep ctz expressions together until after reload

[Kyrill Tkachov]

Ping.
https://gcc.gnu.org/ml/gcc-patches/2016-05/msg02078.html

Patches 1 and 3 have been approved.
Thanks,
Kyrill

On 26/05/16 10:52, Kyrill Tkachov wrote:
> Hi all,
>
> On arm we don't have a dedicated instruction that corresponds to a CTZ rtx but we synthesise it
> with an RBIT instruction followed by a CLZ. This is currently done at expand time.
> However, I'd like to push that step until after reload and keep the CTZ rtx as a single whole in
> the early RTL optimisers.  This better expresses the semantics of the operation as a whole, since
> the RBIT operation is represented as an UNSPEC anyway and so will not see the benefits of combine,
> and a CTZ-specific optimisation that is implemented in patch 3/3 of this series won't be triggered
> if the expression is broken up into an UNSPEC and a CLZ.
>
> Therefore this patch changes the expander to expand to a CTZ rtx and split it after reload into
> an RBIT + CLZ to allow sched2 to schedule them apart if it deems necessary.
> This patch enables the optimisation in patch 3/3 where the appropriate test is added.
>
> Bootstrapped and tested on arm-none-linux-gnueabihf.
>
> Ok for trunk?
>
> Thanks,
> Kyrill
>
> 2016-05-26  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     PR middle-end/37780
>     * config/arm/arm.md (ctzsi2): Convert to define_insn_and_split.

Re: [PATCH][1/3][ARM] Keep ctz expressions together until after reload

[Ramana Radhakrishnan]

On Thu, May 26, 2016 at 10:52 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
> Hi all,
>
> On arm we don't have a dedicated instruction that corresponds to a CTZ rtx
> but we synthesise it
> with an RBIT instruction followed by a CLZ. This is currently done at expand
> time.
> However, I'd like to push that step until after reload and keep the CTZ rtx
> as a single whole in
> the early RTL optimisers.  This better expresses the semantics of the
> operation as a whole, since
> the RBIT operation is represented as an UNSPEC anyway and so will not see
> the benefits of combine,
> and a CTZ-specific optimisation that is implemented in patch 3/3 of this
> series won't be triggered
> if the expression is broken up into an UNSPEC and a CLZ.
>
> Therefore this patch changes the expander to expand to a CTZ rtx and split
> it after reload into
> an RBIT + CLZ to allow sched2 to schedule them apart if it deems necessary.
> This patch enables the optimisation in patch 3/3 where the appropriate test
> is added.
>
> Bootstrapped and tested on arm-none-linux-gnueabihf.
>
> Ok for trunk?
>
> Thanks,
> Kyrill
>
> 2016-05-26  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     PR middle-end/37780
>     * config/arm/arm.md (ctzsi2): Convert to define_insn_and_split.

OK.

Ramana