[x86 PATCH] Allow SCmode and DImode to be tieable on TARGET_64BIT.

[x86 PATCH] Allow SCmode and DImode to be tieable on TARGET_64BIT.

[Roger Sayle]

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This patch is a form of insurance policy in case my patch for PR 7061 runs
into problems on non-x86 targets; the middle-end can add an extra check
that the backend is happy placing SCmode and DImode values in the same
register, before creating a SUBREG.  Unfortunately, ix86_modes_tieable_p
currently claims this is not allowed(?), even though the default target
hook for modes_tieable_p is to always return true [i.e. false can be
used to specifically prohibit bad combinations], and the x86_64 ABI
passes SCmode values in DImode registers!.  This makes the backend's
modes_tiable_p hook a little more forgiving, and additionally enables
interconversion between SCmode and V2SFmode, and between DCmode and
VD2Fmode, which opens interesting opportunities in the future.

I believe there should currently be no code generation differences
with this change.  This patch has been tested on x86_64-pc-linux-gnu
with make bootstrap and make -k check, both with and without
--target_board=unix{-m32}, with no new failures.  Ok for mainline?


2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * config/i386/i386.cc (ix86_modes_tieable_p): Allow SCmode to be
        tieable with DImode on TARGET_64BIT, and SCmode tieable with
        V2SFmode, and DCmode with V2DFmode.


Thanks in advance,
Roger
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diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index daa60ac..df5c80d 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -20141,6 +20141,18 @@ ix86_modes_tieable_p (machine_mode mode1, machine_mode mode2)
     return (GET_MODE_SIZE (mode1) == 8
 	    && ix86_hard_regno_mode_ok (FIRST_MMX_REG, mode1));
 
+  /* SCmode and DImode can be tied.  */
+  if ((mode1 == E_SCmode && mode2 == E_DImode)
+      || (mode1 == E_DImode && mode2 == E_SCmode))
+    return TARGET_64BIT;
+
+  /* [SD]Cmode and V2[SD]Fmode modes can be tied.  */
+  if ((mode1 == E_SCmode && mode2 == E_V2SFmode)
+      || (mode1 == E_V2SFmode && mode2 == E_SCmode)
+      || (mode1 == E_DCmode && mode2 == E_V2DFmode)
+      || (mode1 == E_V2DFmode && mode2 == E_DCmode))
+    return true;
+
   return false;
 }
 

Re: [x86 PATCH] Allow SCmode and DImode to be tieable on TARGET_64BIT.

[Uros Bizjak]

On Mon, May 30, 2022 at 3:22 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch is a form of insurance policy in case my patch for PR 7061 runs
> into problems on non-x86 targets; the middle-end can add an extra check
> that the backend is happy placing SCmode and DImode values in the same
> register, before creating a SUBREG.  Unfortunately, ix86_modes_tieable_p
> currently claims this is not allowed(?), even though the default target
> hook for modes_tieable_p is to always return true [i.e. false can be
> used to specifically prohibit bad combinations], and the x86_64 ABI
> passes SCmode values in DImode registers!.  This makes the backend's
> modes_tiable_p hook a little more forgiving, and additionally enables
> interconversion between SCmode and V2SFmode, and between DCmode and
> VD2Fmode, which opens interesting opportunities in the future.
>
> I believe there should currently be no code generation differences
> with this change.  This patch has been tested on x86_64-pc-linux-gnu
> with make bootstrap and make -k check, both with and without
> --target_board=unix{-m32}, with no new failures.  Ok for mainline?
>
>
> 2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * config/i386/i386.cc (ix86_modes_tieable_p): Allow SCmode to be
>         tieable with DImode on TARGET_64BIT, and SCmode tieable with
>         V2SFmode, and DCmode with V2DFmode.

I *think* this is OK, but hard to say for sure without some testcases.
Please note that x86_64 ABI passes SDmode in two separate XMM
registers.

Uros.

Re: [x86 PATCH] Allow SCmode and DImode to be tieable on TARGET_64BIT.

[Uros Bizjak]

On Mon, May 30, 2022 at 10:12 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Mon, May 30, 2022 at 3:22 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
> >
> >
> > This patch is a form of insurance policy in case my patch for PR 7061 runs
> > into problems on non-x86 targets; the middle-end can add an extra check
> > that the backend is happy placing SCmode and DImode values in the same
> > register, before creating a SUBREG.  Unfortunately, ix86_modes_tieable_p
> > currently claims this is not allowed(?), even though the default target
> > hook for modes_tieable_p is to always return true [i.e. false can be
> > used to specifically prohibit bad combinations], and the x86_64 ABI
> > passes SCmode values in DImode registers!.  This makes the backend's
> > modes_tiable_p hook a little more forgiving, and additionally enables
> > interconversion between SCmode and V2SFmode, and between DCmode and
> > VD2Fmode, which opens interesting opportunities in the future.
> >
> > I believe there should currently be no code generation differences
> > with this change.  This patch has been tested on x86_64-pc-linux-gnu
> > with make bootstrap and make -k check, both with and without
> > --target_board=unix{-m32}, with no new failures.  Ok for mainline?
> >
> >
> > 2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * config/i386/i386.cc (ix86_modes_tieable_p): Allow SCmode to be
> >         tieable with DImode on TARGET_64BIT, and SCmode tieable with
> >         V2SFmode, and DCmode with V2DFmode.
>
> I *think* this is OK, but hard to say for sure without some testcases.
> Please note that x86_64 ABI passes SDmode in two separate XMM
> registers.

I meant DCmode here.

Uros.