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

[Gao, Yunzhong]

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