[cxx-abi-dev] Mangling for OpenCL and GCC vectors

Wed Feb 24 04:39:16 UTC 2016

Actually, I might have been confused with how SPIR works. I thought OpenCL can target CPU codes, so when compiling an OpenCL program and when compiling a C++ program, they could be targeting the same CPU architecture, so linking is possible. But looking at github.com/KhronosGroup/SPIR, they talk about the SPIR triples as spir-unknown-unknown or spir64-unknown-unknown. So SPIR is being treated as a virtual backend architecture.

Sent from my iPhone

> On Feb 23, 2016, at 7:48 PM, Gao, Yunzhong <yunzhong_gao at playstation.sony.com> wrote:
> 
> GCC 4.8.2 follows an older version of GNU ABI, so given,
>  typedef float float4 __attribute__((__vector_size__(16)));
>  void foo(float4 f) { }
> G++ mangles a <4 x float> vector into U8_floatf. And a <2 x double> vector is mangled into U8_floatd.
> I do not have GCC 5.2 or 5.3 but presumably they changed their mangling scheme there.
> 
> Clang has always mangled these types into Dv4_f and Dv2_d respectively.
> 
> If I use the ext_vector_size attribute instead, which is for OpenCL compatibility,
>  typedef float float4 __attribute__((__ext_vector_size__(4)));
>  void foo(float4 f) { }
> GCC 4.8.2 does not support this attribute.
> Clang always mangles this type into Dv4_f.
> 
> I thought about whether the delicate mechanism of swapping mangling would work as you
> described, and I think it depends on whether anyone would want to link OpenCL program
> and C++ programs together, and if so, you may want the OpenCL vector types to have
> matching mangled names. Maybe.
> 
> - Gao
> 
> 
>> -----Original Message-----
>> From: metafoo at gmail.com [mailto:metafoo at gmail.com] On Behalf Of
>> Richard Smith
>> Sent: Friday, February 19, 2016 6:21 PM
>> To: Gao, Yunzhong
>> Cc: rjmccall at apple.com
>> Subject: Re: Mangling for OpenCL and GCC vectors
>> 
>> On Fri, Feb 19, 2016 at 5:48 PM, Gao, Yunzhong
>> <yunzhong_gao at playstation.sony.com> wrote:
>>> Hey Richard,
>>> Thanks! My understanding is that the GCC vector type is being mangled
>>> according to the Itanium ABI
>> 
>> The Itanium C++ ABI does not appear to specify any mangling for vector
>> types. This Dv <number> _ <type> mangling is a GNU extension.
>> 
>>> and the OpenCL vector type is being mangled according to the SPIR 1.2
>>> spec.
>> 
>> SPIR presumably inherited their mangling from whatever Clang did at the
>> time. :-(
>> 
>>> So does your answer imply that one of the ABIs need be modified to
>>> reflect a different mangling?
>> 
>> There's an alternative: we can do the same dance we do for similar messes,
>> like long double vs __float128 on PPC (the Itanium ABI has both a long double
>> mangling, e, and a __float128 mangling, g, but because of Reasons, the
>> mangling 'g' is used for long double and the mangling u10__float128 is used
>> for __float128, sometimes).
>> 
>> So, when targeting SPIR, we'd mangle ext_vector_type as Dv... and mangle
>> vector_size as something else.
>> When targeting anything else, we'd mangle vector_size as Dv... and mangle
>> ext_vector_type as something else.
>> 
>>> Any advice on how
>>> to initiate a discussion to change either of the ABIs?
>>> - Gao
>>> 
>>> 
>>>> -----Original Message-----
>>>> From: metafoo at gmail.com [mailto:metafoo at gmail.com] On Behalf Of
>>>> Richard Smith
>>>> Sent: Friday, February 19, 2016 5:29 PM
>>>> To: Gao, Yunzhong
>>>> Cc: rjmccall at apple.com
>>>> Subject: Re: Mangling for OpenCL and GCC vectors
>>>> 
>>>> I'm not seeing it show up on the list either. FWIW, I think these
>>>> types should have different manglings.
>>>> 
>>>> On Fri, Feb 19, 2016 at 4:47 PM, Gao, Yunzhong
>>>> <yunzhong_gao at playstation.sony.com> wrote:
>>>>> Ping?
>>>>> 
>>>>> Hmm, still not seeing the mail on the mailing list after two weeks.
>>>>> It’s strange.
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> From: Gao, Yunzhong
>>>>> Sent: Monday, February 08, 2016 9:55 PM
>>>>> To: Richard Smith; rjmccall at apple.com
>>>>> Subject: Fwd: Mangling for OpenCL and GCC vectors
>>>>> 
>>>>> 
>>>>> 
>>>>> Hi John, Richard,
>>>>> 
>>>>> I did not see the following email appear on the online cxx-abi-dev
>>>>> archive after the weekend, so I thought to just forward it to you
>>>>> directly. Sorry for the spam.
>>>>> 
>>>>> 
>>>>> 
>>>>> The core issue is that we have two types which clang would consider
>>>>> distinct but which are mangled the same. In this case one could
>>>>> argue that since the linker cannot tell the two types apart (if
>>>>> they came from different translation units) based on their
>>>>> mangling, then the compiler should also treat them the same coming
>>>>> from the same translation unit. But one could also argue that if
>>>>> they are meant to be distinct types, they should have different
>>>>> mangling, and the ABIs need be changed. We cannot decide which is
>>>>> the intention in giving them the same mangling in the first place.
>>>>> Are there any precedent cases with conflicting symbol mangling? How
>> were they resolved?
>>>>> 
>>>>> - Gao
>>>>> 
>>>>> 
>>>>> Sent from my iPhone
>>>>> 
>>>>> 
>>>>> Begin forwarded message:
>>>>> 
>>>>> From: "Yung, Douglas" <douglas_yung at playstation.sony.com>
>>>>> Date: February 3, 2016 at 5:06:15 PM PST
>>>>> To: "cxx-abi-dev at codesourcery.com" <cxx-abi-
>> dev at codesourcery.com>
>>>>> Cc: "Robinson, Paul" <Paul_Robinson at playstation.sony.com>, "Gao,
>>>> Yunzhong"
>>>>> <yunzhong_gao at playstation.sony.com>
>>>>> Subject: Mangling for OpenCL and GCC vectors
>>>>> 
>>>>> Hi,
>>>>> 
>>>>> 
>>>>> 
>>>>> Recently in our testing we encountered an abi issue that we were
>>>>> hoping to get clarified. Specifically, this concerns the mangling
>>>>> of OpenCL and GCC vectors which seems to be identical when
>>>>> comparing the results of GCC vectors compiled by GCC5 and OpenCL
>>>>> vectors when mangled according to the SPIR 1.2 spec.
>>>>> 
>>>>> 
>>>>> 
>>>>> First a little background. Clang supports using both OpenCL and GCC
>>>>> vectors in a program. When used, clang permits certain operations
>>>>> to be performed on each as detailed in the table found at
>>>>> http://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-
>>>> extended-vectors.
>>>>> From the differences there, it seems the compiler treats the types
>>>>> as unique which makes sense as they support different operations.
>>>>> 
>>>>> 
>>>>> 
>>>>> The problem we found arises when you try to use both in the same
>>>> program.
>>>>> Consider the following example:
>>>>> 
>>>>> 
>>>>> 
>>>>> template <typename alpha>
>>>>> 
>>>>> static void foo() {}
>>>>> 
>>>>> 
>>>>> 
>>>>> void bar() {
>>>>> 
>>>>>  foo<float __attribute__((__vector_size__(16)))>();
>>>>> 
>>>>>  foo<float __attribute__((ext_vector_type(4)))>();
>>>>> 
>>>>> }
>>>>> 
>>>>> 
>>>>> 
>>>>> In this example, the compiler creates two copies of the function
>>>>> foo, once using a GCC vector, and once using an OpenCL vector.
>>>>> These functions mangle to the same string ” _ZL3fooIDv4_fEvv” which
>>>>> the compiler detects and issues an error. Is the fact that the two
>>>>> vectors mangle identically by design or just a coincidence? If it
>>>>> is by coincidence, should the compiler merge the two type
>>>>> definitions when it encounters them, or was there some other
>>>>> intention here that we are
>>>> unaware of?
>>>>> 
>>>>> 
>>>>> 
>>>>> Thoughts?
>>>>> 
>>>>> 
>>>>> 
>>>>> Douglas Yung