[cfe-dev] [LLVMdev] SPIR Portability Discussion

[Richard Smith]

On Wed, Sep 12, 2012 at 3:40 PM, Villmow, Micah <Micah.Villmow at amd.com>wrote:

>  ** **
>
> ** **
>
> *From:* metafoo at gmail.com [mailto:metafoo at gmail.com] *On Behalf Of *Richard
> Smith
> *Sent:* Wednesday, September 12, 2012 3:30 PM
> *To:* Villmow, Micah
> *Cc:* Eli Friedman; cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
>
> *Subject:* Re: [cfe-dev] [LLVMdev] SPIR Portability Discussion****
>
>  ** **
>
> On Wed, Sep 12, 2012 at 3:26 PM, Villmow, Micah <Micah.Villmow at amd.com>
> wrote:****
>
>
>
> > -----Original Message-----
> > From: Eli Friedman [mailto:eli.friedman at gmail.com]
> > Sent: Wednesday, September 12, 2012 3:22 PM
> > To: Villmow, Micah****
>
> > Cc: Richard Smith; cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> > Subject: Re: [cfe-dev] [LLVMdev] SPIR Portability Discussion
> >
> > On Wed, Sep 12, 2012 at 2:58 PM, Villmow, Micah <Micah.Villmow at amd.com>
> > wrote:
> > > Another factor to consider, with size_t etc as defined in SPIR, is
> > the usual
> > > arithmetic conversions. For instance (assuming a 64-bit long long),
> > > sizeof(int) + 1LL would be signed if size_t is 32 bits wide, and
> > would be
> > > unsigned if size_t is 64 bits wide. How is this handled?
> > >
> > > [Villmow, Micah] OpenCL C defines 'int' to be 32bits irrespective of
> > the
> > > host/device bitness. So this would follow the normal integer
> > promotion
> > > rules.
> >
> > I think you're misunderstanding the issue: the point is, is
> > "sizeof(int) + -8LL < 0" true or false?****
>
> [Villmow, Micah] Yep, I don't see why this is any different than "4 + -8LL
> < 0".  OpenCL C, and in turn SPIR, defines sizeof(int) == 4. While this
> might be a problem in C, this isn't an issue in OpenCL since there is no
> variance in the sizeof(int) across devices.****
>
>  ** **
>
> I think you're still misunderstanding. If size_t is 32 bits, sizeof(int) +
> -8LL is -4LL, so the comparison produces true. If it's 64 bits, the -8LL
> promotes to an unsigned long long, sizeof(int) + -8LL
> is 18446744073709551612ULL, the 0 promotes to 0ULL, and the comparison
> produces false.****
>
> *[Villmow, Micah] I see now, I think you had a type-o in the previous
> email, “sizeof(sizeof(int))” should have been size_t(sizeof(int)), which
> was throwing me off.*
>

What I wrote was what I meant. The *value* of sizeof(int) is not relevant
here, what matters is the precision of its type (or more specifically, its
integer conversion rank).

*Though I’m curious where it states we have to promote -8LL to unsigned
> long and not signed long, I would have thought it would be signed.\*
>

C99 6.3.1.8/1.
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