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Re: Questions about LTO infrastructure and pragma omp target
- From: Ilya Verbin <iverbin at gmail dot com>
- To: Jakub Jelinek <jakub at redhat dot com>
- Cc: Richard Biener <richard dot guenther at gmail dot com>, Uday Khedker <uday at cse dot iitb dot ac dot in>, Jan Hubicka <hubicka at ucw dot cz>, Richard Henderson <rth at redhat dot com>, Kirill Yukhin <kirill dot yukhin at gmail dot com>, "Michael V. Zolotukhin" <michael dot v dot zolotukhin at gmail dot com>, GCC Development <gcc at gcc dot gnu dot org>
- Date: Thu, 19 Sep 2013 14:44:30 +0400
- Subject: Re: Questions about LTO infrastructure and pragma omp target
- Authentication-results: sourceware.org; auth=none
- References: <20130815133639 dot GA42355 at msticlxl57 dot ims dot intel dot com> <20130815134437 dot GB1814 at tucnak dot redhat dot com> <4df844f6-f385-4e63-9413-8ea341992b77 at email dot android dot com> <20130823105527 dot GA6976 at msticlxl57 dot ims dot intel dot com> <85e37f42-69fe-4bbf-bf1d-f73194e7c444 at email dot android dot com> <20130916171405 dot GA18788 at msticlxl57 dot ims dot intel dot com> <CAFiYyc3nGuvf-=H1k8DvrCp9zLkEm-WP7kpq7MywzU6pMG4+nA at mail dot gmail dot com> <20130917113010 dot GA22722 at msticlxl57 dot ims dot intel dot com> <CAFiYyc1Y1Pr00hwm9Auu3DX5Z-B7P9vAQBOFywKLpaHegswaOg at mail dot gmail dot com> <20130917121228 dot GX1817 at tucnak dot redhat dot com>
On 17 Sep 14:12, Jakub Jelinek wrote:
> On Tue, Sep 17, 2013 at 01:56:39PM +0200, Richard Biener wrote:
> >
> > Are you sure we have the same IL for all targets and the same targets
> > for all functions? That would certainly simplify things, but you still need
> > a way to tell the target compiler which symbol to emit the function on
> > as the compile-stage will already necessarily refer to all target
> > variant symbols.
>
> This has been discussed to some extent during Cauldron.
> Yes, there are various target dependencies in the GIMPLE IL, many of them
> very early.
> Some of the dependencies are there already during preprocessing, there is
> nothing to do about those.
> For some things we will just rely on the host and target having the same
> properties, stuff like BITS_PER_UNIT, type layout/alignment, endianity,
> the OpenMP (and I believe OpenACC too) model effectively requires that,
> while you don't need to have shared address space between host and target
> (but can have that), for the mapping/unmapping it is assumed that you can
> simply take host portions of memory and copy them over to the target device
> or back, as sequence of bytes, there is no form of RPC or similar that would
> tweak endianity, differently sized types, padding, etc.
> While you can say have 64-bit host and 32-bit target or vice versa, the
> target IL will simply contain precision info, alignment, structure layout
> etc. and just will have to generate right code for that (something that is
> native long on the host can be native long long on the target or vice versa
> etc.).
> Then there are dependencies we'd ideally get rid of, at least pre-IPA,
> stuff like BRANCH_COST, but generally that is just an optimization issue and
> thus not that big deal.
> Bigger issue are target specific builtins, I guess we'll either have to just
> sorry on those, or have some helper targhook that will translate a subset of
> md builtins from selected hosts to selected targets.
> Preferrably, before IPA we'd introduce as few target dependencies into the
> IL as possible, and gradually towards RTL can add more dependencies (e.g.
> the vectorizer adds so many target dependencies that at that point trying to
> use the IL for a different target is practically impossible).
>
> Jakub
Do I understand correctly that GIMPLE IL is target dependent, but we will emit
the same IL for all targets?
-- Ilya