RE: PING**4 - [PATCH] middle-end: Support ABIs that pass FP values as wider integers.

[Richard Biener <rguenther@suse.de>]

On Mon, 14 Mar 2022, Roger Sayle wrote:

> 
> I thought I'd add a few comments that might help reviewers of this patch.
> Most importantly, this patch should be completely safe, as both changes
> only trigger with FLOAT vs INT size mismatches which currently both ICE in
> the compiler a few lines later.  Admittedly, this indicates something odd 
> about a target's choice of ABI, but one alternative might be to issue a
> "sorry, unimplemented" error message rather than ICE, perhaps with a
> TODO or FIXME comment that support for mixed size FP/integer ABIs be
> added in future.
> 
> The only (other?) possible point of contention is the (arbitrary) decision that
> when passing floating point values in a larger integer register, the code is
> hardwired to use zero-extension.  This in theory could be turned into a target
> hook to support sign-extension, but given these are padding bits, zero seems
> appropriate. [On x86_64, if passing DFmode argument in a V2DFmode vector,
> say, it seems reasonable to use movq and zero the high bits].
> 
> The final point is that at the moment, the only affected target is nvptx-none,
> as I don't believe any other backend specifies an ABI that requires passing 
> floating point values in wider integer registers.  Having said that, most targets
> don't yet support HFmode, and their ABI specifications haven't yet been
> updated as what to do in that eventuality.  If they choose to treat these like
> HImode, they'll run into the same issues, as most ABIs pass HImode in 
> wider word_mode registers.
> 
> I hope this helps.  If folks could air their concerns out loud, I can try my best
> to address those worries.

First of all I'm not familiar with the ABI related code as all, so I
refrained from commenting.  But now I've looked closer (still unfamiliar 
code).

I suppose there's targets passing SFmode in a SImode GPR and DFmode
in a DImode GPR (all soft-float targets?), so that already works somehow.
Why does nvptx choose DImode for SFmode passing then?  Can't it choose
(subreg:SI di:..) or so?  Does it require zero-extending to DImode
on the caller side?  It seems your expand_expr_real_1 code does
not rely on that?  So, why does nvptx function_arg hook (?) insist
on returning a DImode reg for an SFmode argument rather than
an SImode subreg of that?

Richard.

> 
> Many thanks in advance (and thanks to Tobias and Tom for pushing for this).
> Roger
> --
> 
> > -----Original Message-----
> > From: Tom de Vries <tdevries@suse.de>
> > Sent: 14 March 2022 08:06
> > To: Jeff Law <jeffreyalaw@gmail.com>; Richard Biener <rguenther@suse.de>;
> > Tobias Burnus <tobias@codesourcery.com>
> > Cc: richard.sandiford@arm.com; Jeff Law <law@redhat.com>; gcc-
> > patches@gcc.gnu.org; Roger Sayle <roger@nextmovesoftware.com>
> > Subject: PING**4 - [PATCH] middle-end: Support ABIs that pass FP values as
> > wider integers.
> > 
> > On 3/2/22 20:18, Jeff Law via Gcc-patches wrote:
> > >
> > >
> > > On 2/28/2022 5:54 AM, Richard Biener via Gcc-patches wrote:
> > >> On Mon, 28 Feb 2022, Tobias Burnus wrote:
> > >>
> > >>> Ping**3
> > >>>
> > >>> On 23.02.22 09:42, Tobias Burnus wrote:
> > >>>> PING**2 for the ME review or at least comments to that patch, which
> > >>>> fixes a build issue/ICE with nvptx
> > >>>>
> > >>>> Patch:
> > >>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-February/590139.html
> > >>>> (for gcc/cfgexpand.cc + gcc/expr.cc)
> > >>>>
> > >>>> (There is some discussion by Tom and Roger about the BE in the
> > >>>> patch thread, which only not relate to the ME patch. But there is
> > >>>> no ME-patch comment so far.)
> > >>> The related BE patch has been already committed, but to be
> > >>> effective, it needs the ME patch.
> > >> I'm not sure I'm qualified to review this - maybe Richard is.
> > > I'd initially ignored the patch as it didn't seem a good fit for
> > > stage4, subsequent messages changed my mind about it, but I never went
> > > back to take a deeper look at Roger's patch.
> > 
> > Ping.
> > 
> > [ FWIW, I'd appreciate it if a response came before the end of stage 4, such that
> > I have some time left to deal with fallout in case the patch is not approved. ]
> > 
> > Thanks,
> > - Tom
> 

-- 
Richard Biener <rguenther@suse.de>
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