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From: Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org>
To: Jojo R <rjiejie@linux.alibaba.com>, Richard Sandiford <richard.sandiford@arm.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] Adding target hook allows to reject initialization of register
Date: Tue, 10 Aug 2021 13:03:09 +0200
Message-ID: <CAFiYyc2W5bL93-SsMXo3Yz_eYzHkyGVTR8GE24_p_n=zWVbW+g@mail.gmail.com>

On Tue, Aug 10, 2021 at 10:33 AM Jojo R via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Some target like RISC-V allow to group vector register as a whole,
> and only operate part of it in fact, but the 'init-regs' pass will add initialization
> for uninitialized registers. Add this hook to reject this action for reducing instruction.

Are these groups "visible"?  That is, are the pseudos multi-reg
pseudos?  I wonder
if there's a more generic way to tame down initregs w/o introducing a new target
hook.

Btw, initregs is a red herring - it ideally should go away.  See PR61810.

So instead of adding to it can you see whether disabling the pass for RISC-V
works w/o fallout (and add a comment to the PR)?  Maybe some more RTL
literate (in particular DF literate) can look at the remaining issue.
Richard, did you
ever have a look into the "issue" that initregs covers up (whatever
that exactly is)?

Thanks,
Richard.

>         gcc/
>         * init-regs.c (initialize_uninitialized_regs): Call register_reject_init_p.
>         * target.def (register_reject_init_p): New hook.
>         * doc/tm.texi.in: Add TARGET_REGISTER_REJECT_INIT_P.
>         * doc/tm.texi: Regenerated.
> ---
>  gcc/doc/tm.texi    | 6 ++++++
>  gcc/doc/tm.texi.in | 2 ++
>  gcc/init-regs.c    | 5 +++++
>  gcc/target.def     | 8 ++++++++
>  4 files changed, 21 insertions(+)
>
> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
> index a30fdcbbf3d6..83fd5496ca3f 100644
> --- a/gcc/doc/tm.texi
> +++ b/gcc/doc/tm.texi
> @@ -12588,3 +12588,9 @@ Return an RTX representing @var{tagged_pointer} with its tag set to zero.
>  Store the result in @var{target} if convenient.
>  The default clears the top byte of the original pointer.
>  @end deftypefn
> +
> +@deftypefn {Target Hook} bool TARGET_REGISTER_REJECT_INIT_P (rtx @var{reg})
> +This target hook should return @code{true} if reject initialization for a uninitialized @var{reg}.
> +
> +The default value of this hook is @code{NULL}.
> +@end deftypefn
> diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
> index 611fc500ac86..13174ce66d59 100644
> --- a/gcc/doc/tm.texi.in
> +++ b/gcc/doc/tm.texi.in
> @@ -8180,3 +8180,5 @@ maintainer is familiar with.
>  @hook TARGET_MEMTAG_EXTRACT_TAG
>
>  @hook TARGET_MEMTAG_UNTAGGED_POINTER
> +
> +@hook TARGET_REGISTER_REJECT_INIT_P
> diff --git a/gcc/init-regs.c b/gcc/init-regs.c
> index 72e898f3e334..51c0d669d30b 100644
> --- a/gcc/init-regs.c
> +++ b/gcc/init-regs.c
> @@ -21,6 +21,7 @@ along with GCC; see the file COPYING3.  If not see
>  #include "system.h"
>  #include "coretypes.h"
>  #include "backend.h"
> +#include "target.h"
>  #include "rtl.h"
>  #include "tree.h"
>  #include "df.h"
> @@ -101,6 +102,10 @@ initialize_uninitialized_regs (void)
>                   rtx_insn *move_insn;
>                   rtx reg = DF_REF_REAL_REG (use);
>
> +                 if (targetm.register_reject_init_p
> +                     && targetm.register_reject_init_p (reg))
> +                   continue;
> +
>                   bitmap_set_bit (already_genned, regno);
>
>                   start_sequence ();
> diff --git a/gcc/target.def b/gcc/target.def
> index 7676d5e626e3..c2b54421618d 100644
> --- a/gcc/target.def
> +++ b/gcc/target.def
> @@ -4545,6 +4545,14 @@ by a subtarget.",
>   unsigned HOST_WIDE_INT, (void),
>   NULL)
>
> +/* Return true if reject initialization for a uninitialized register.  */
> +DEFHOOK
> +(register_reject_init_p,
> + "This target hook should return @code{true} if reject initialization for a uninitialized @var{reg}.\n\
> +\n\
> +The default value of this hook is @code{NULL}.",
> + bool, (rtx reg), NULL)
> +
>  /* Functions relating to calls - argument passing, returns, etc.  */
>  /* Members of struct call have no special macro prefix.  */
>  HOOK_VECTOR (TARGET_CALLS, calls)
> --
> 2.24.3 (Apple Git-128)
>


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