From: Jeff Law <law@redhat.com>
To: Richard Biener <richard.guenther@gmail.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Richard Sandiford <richard.sandiford@linaro.org>
Subject: Re: [14/nn] Add helpers for shift count modes
Date: Mon, 30 Oct 2017 15:03:00 -0000 [thread overview]
Message-ID: <ec016515-8e20-c882-c81f-142137a6fa71@redhat.com> (raw)
In-Reply-To: <CAFiYyc3JGZMWtjtdWa-CDPw_Gc6R1ex7KFfnGkQ4r9Bcduu-Og@mail.gmail.com>
On 10/26/2017 06:06 AM, Richard Biener wrote:
> On Mon, Oct 23, 2017 at 1:25 PM, Richard Sandiford
> <richard.sandiford@linaro.org> wrote:
>> This patch adds a stub helper routine to provide the mode
>> of a scalar shift amount, given the mode of the values
>> being shifted.
>>
>> One long-standing problem has been to decide what this mode
>> should be for arbitrary rtxes (as opposed to those directly
>> tied to a target pattern). Is it the mode of the shifted
>> elements? Is it word_mode? Or maybe QImode? Is it whatever
>> the corresponding target pattern says? (In which case what
>> should the mode be when the target doesn't have a pattern?)
>>
>> For now the patch picks word_mode, which should be safe on
>> all targets but could perhaps become suboptimal if the helper
>> routine is used more often than it is in this patch. As it
>> stands the patch does not change the generated code.
>>
>> The patch also adds a helper function that constructs rtxes
>> for constant shift amounts, again given the mode of the value
>> being shifted. As well as helping with the SVE patches, this
>> is one step towards allowing CONST_INTs to have a real mode.
>
> I think gen_shift_amount_mode is flawed and while encapsulating
> constant shift amount RTX generation into a gen_int_shift_amount
> looks good to me I'd rather have that ??? in this function (and
> I'd use the mode of the RTX shifted, not word_mode...).
>
> In the end it's up to insn recognizing to convert the op to the
> expected mode and for generic RTL it's us that should decide
> on the mode -- on GENERIC the shift amount has to be an
> integer so why not simply use a mode that is large enough to
> make the constant fit?
>
> Just throwing in some comments here, RTL isn't my primary
> expertise.
I wonder if encapsulation + a target hook to specify the mode would be
better? We'd then have to argue over word_mode, vs QImode vs something
else for the default, but at least we'd have a way for the target to
specify the mode is generally best when working on shift counts.
In the end I doubt there's a single definition that is overall better.
Largely because I suspect there are times when the narrowest mode is
best, or the mode of the operand being shifted.
So thoughts on doing the encapsulation with a target hook to specify the
desired mode? Does that get us what we need for SVE and does it provide
us a path forward on this issue if we were to try to move towards
CONST_INTs with modes?
jeff
next prev parent reply other threads:[~2017-10-30 15:02 UTC|newest]
Thread overview: 90+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-23 11:16 [00/nn] Patches preparing for runtime offsets and sizes Richard Sandiford
2017-10-23 11:17 ` [01/nn] Add gen_(const_)vec_duplicate helpers Richard Sandiford
2017-10-25 16:29 ` Jeff Law
2017-10-27 16:12 ` Richard Sandiford
2017-10-23 11:19 ` [03/nn] Allow vector CONSTs Richard Sandiford
2017-10-25 16:59 ` Jeff Law
2017-10-27 16:19 ` Richard Sandiford
2017-10-23 11:19 ` [02/nn] Add more vec_duplicate simplifications Richard Sandiford
2017-10-25 16:35 ` Jeff Law
2017-11-10 9:42 ` Christophe Lyon
2017-10-23 11:20 ` [04/nn] Add a VEC_SERIES rtl code Richard Sandiford
2017-10-26 11:49 ` Richard Biener
2017-10-23 11:21 ` [05/nn] Add VEC_DUPLICATE_{CST,EXPR} and associated optab Richard Sandiford
2017-10-26 11:53 ` Richard Biener
2017-11-06 15:09 ` Richard Sandiford
2017-11-07 10:37 ` Richard Biener
2017-12-15 0:29 ` Richard Sandiford
2017-12-15 8:58 ` Richard Biener
2017-12-15 12:52 ` Richard Sandiford
2017-12-15 13:20 ` Richard Biener
2017-10-23 11:22 ` [07/nn] Add unique CONSTs Richard Sandiford
2017-10-27 15:51 ` Jeff Law
2017-10-27 15:58 ` Richard Sandiford
2017-10-30 14:49 ` Jeff Law
2017-10-23 11:22 ` [06/nn] Add VEC_SERIES_{CST,EXPR} and associated optab Richard Sandiford
2017-10-26 12:26 ` Richard Biener
2017-10-26 12:43 ` Richard Biener
2017-11-06 15:21 ` Richard Sandiford
2017-11-07 10:38 ` Richard Biener
2017-12-15 0:34 ` Richard Sandiford
2017-12-15 9:03 ` Richard Biener
2017-10-23 11:22 ` [08/nn] Add a fixed_size_mode class Richard Sandiford
2017-10-26 11:57 ` Richard Biener
2017-10-23 11:23 ` [09/nn] Add a fixed_size_mode_pod class Richard Sandiford
2017-10-26 11:59 ` Richard Biener
2017-10-26 12:18 ` Richard Sandiford
2017-10-26 12:46 ` Richard Biener
2017-10-26 19:42 ` Eric Botcazou
2017-10-27 8:34 ` Richard Biener
2017-10-27 9:28 ` Eric Botcazou
2017-10-30 3:14 ` Trevor Saunders
2017-10-30 8:52 ` Richard Sandiford
2017-10-30 10:13 ` Eric Botcazou
2017-10-31 10:39 ` Trevor Saunders
2017-10-31 17:29 ` Eric Botcazou
2017-10-31 17:57 ` Jeff Law
2017-11-01 2:50 ` Trevor Saunders
2017-11-01 16:30 ` Jeff Law
2017-11-02 4:28 ` Trevor Saunders
2017-10-26 19:44 ` Richard Sandiford
2017-10-26 19:45 ` Jakub Jelinek
2017-10-27 8:43 ` Richard Biener
2017-10-27 8:45 ` Jakub Jelinek
2017-10-27 10:19 ` Pedro Alves
2017-10-27 15:23 ` Jeff Law
2017-10-23 11:24 ` [10/nn] Widening optab cleanup Richard Sandiford
2017-10-30 18:32 ` Jeff Law
2017-10-23 11:24 ` [11/nn] Add narrower_subreg_mode helper function Richard Sandiford
2017-10-30 15:06 ` Jeff Law
2017-10-23 11:25 ` [13/nn] More is_a <scalar_int_mode> Richard Sandiford
2017-10-26 12:03 ` Richard Biener
2017-10-23 11:25 ` [12/nn] Add an is_narrower_int_mode helper function Richard Sandiford
2017-10-26 11:59 ` Richard Biener
2017-10-23 11:26 ` [14/nn] Add helpers for shift count modes Richard Sandiford
2017-10-26 12:07 ` Richard Biener
2017-10-26 12:07 ` Richard Biener
2017-11-20 21:04 ` Richard Sandiford
2017-11-21 15:00 ` Richard Biener
2017-12-15 0:48 ` Richard Sandiford
2017-12-15 9:06 ` Richard Biener
2017-12-15 15:17 ` Richard Sandiford
2017-12-19 19:13 ` Richard Sandiford
2017-12-20 0:27 ` Jeff Law
2017-10-30 15:03 ` Jeff Law [this message]
2017-10-23 11:27 ` [16/nn] Factor out the mode handling in lower-subreg.c Richard Sandiford
2017-10-26 12:09 ` Richard Biener
2017-10-23 11:27 ` [15/nn] Use more specific hash functions in rtlhash.c Richard Sandiford
2017-10-26 12:08 ` Richard Biener
2017-10-23 11:28 ` [17/nn] Turn var-tracking.c:INT_MEM_OFFSET into a function Richard Sandiford
2017-10-26 12:10 ` Richard Biener
2017-10-23 11:29 ` [18/nn] Use (CONST_VECTOR|GET_MODE)_NUNITS in simplify-rtx.c Richard Sandiford
2017-10-26 12:13 ` Richard Biener
2017-10-23 11:29 ` [19/nn] Don't treat zero-sized ranges as overlapping Richard Sandiford
2017-10-26 12:14 ` Richard Biener
2017-10-23 11:30 ` [20/nn] Make tree-ssa-dse.c:normalize_ref return a bool Richard Sandiford
2017-10-30 17:49 ` Jeff Law
2017-10-23 11:31 ` [21/nn] Minor vn_reference_lookup_3 tweak Richard Sandiford
2017-10-26 12:18 ` Richard Biener
2017-10-23 11:45 ` [22/nn] Make dse.c use offset/width instead of start/end Richard Sandiford
2017-10-26 12:18 ` Richard Biener
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