[cfe-dev] [LLVMdev] SPIR Portability Discussion

[Eli Friedman]

On Wed, Sep 12, 2012 at 3:40 PM, Villmow, Micah <Micah.Villmow at amd.com> wrote:
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> 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
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> On Wed, Sep 12, 2012 at 3:26 PM, Villmow, Micah <Micah.Villmow at amd.com>
> wrote:
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>> -----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?
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> [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.
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> 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. I view this case as being well defined in SPIR. It can be
> produced with something like the following:
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> %0 = call %spir.size_t @__spir_sizet_convert_size_t(i32 0)
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> %1 = call %spir.size_t @__spir_sizet_convert_size_t(i32 4)
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> %2 = call %spir.size_t @__spir_sizet_convert_size_t(i64 8)
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> %3 = call %spir.size_t @__spir_sizet_neg(%spir.size_t %2)
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> %4 = call %spir.size_t @__spir_sizet_add(%spir.size_t %1, %spir.size_t %3)
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> %5 = call %spir.size_t @__spir_sizet_cmp(%spir.size_t %4, %spir.size_t %0)
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> %6 = call i1 %spir.size_t @__spir_size_t_convert_i1(%spir.size_t %5)

This conversion simply isn't correct: the type of the comparison is
not size_t if size_t is 32 bits.

-Eli