[PATCH] D27334: [OpenCL] Ambiguous function call.

[Bader, Alexey via cfe-commits]

Actually OpenCL specification has the following text in “Built-in Functions” chapter (OpenCL C 2.0 rev. 33):

“ User defined OpenCL C functions, behave per C standard rules for functions (C99, TC2, Section
6.9.1). On entry to the function, the size of each variably modified parameter is evaluated and
the value of each argument expression is converted to the type of the corresponding parameter as
per usual arithmetic conversion rules described in section 6.2.6. Built-in functions described in
this section behave similarly, except that in order to avoid ambiguity between multiple forms of
the same built-in function, implicit scalar widening shall not occur. Note that some built-in
functions described in this section do have forms that operate on mixed scalar and vector types,
however.”

If I understand it correctly this text can help to resolve the overloading, since “usual arithmetic conversion rules” from section 6.2.6 rank floating point and integer data types including vector flavors. On the other hand I don’t think it’s good idea to adopt this rule, since it doesn’t follow C++ overloading rules and definitely will deviate OpenCL behavior from C or C++.

Thanks,
Alexey

From: Anastasia Stulova [mailto:Anastasia.Stulova at arm.com]
Sent: Monday, December 5, 2016 9:21 PM
To: Richard Smith <richard at metafoo.co.uk>; Bruno Cardoso Lopes via Phabricator <reviews at reviews.llvm.org>; reviews+D27334+public+f2c5a66032c4c02e at reviews.llvm.org
Cc: Bader, Alexey <alexey.bader at intel.com>; egor.churaev at gmail.com; yaxun.liu at amd.com; cfe-commits at lists.llvm.org; nd <nd at arm.com>
Subject: RE: [PATCH] D27334: [OpenCL] Ambiguous function call.

> Perhaps that is the problem (that there are two modes that do different things)? Could we make the double overload be present but unselectable to diagnose this problem in that mode too?

If we could resolve the overload candidate to prefer ‘int -> float’ than ‘int->double’, it would work best.

From: metafoo at gmail.com<mailto:metafoo at gmail.com> [mailto:metafoo at gmail.com] On Behalf Of Richard Smith
Sent: 05 December 2016 17:53
To: Bruno Cardoso Lopes via Phabricator; reviews+D27334+public+f2c5a66032c4c02e at reviews.llvm.org<mailto:reviews+D27334+public+f2c5a66032c4c02e at reviews.llvm.org>
Cc: alexey.bader at intel.com<mailto:alexey.bader at intel.com>; egor.churaev at gmail.com<mailto:egor.churaev at gmail.com>; yaxun.liu at amd.com<mailto:yaxun.liu at amd.com>; Anastasia Stulova; cfe-commits at lists.llvm.org<mailto:cfe-commits at lists.llvm.org>
Subject: Re: [PATCH] D27334: [OpenCL] Ambiguous function call.

On 5 Dec 2016 9:42 am, "Anastasia Stulova via Phabricator via cfe-commits" <cfe-commits at lists.llvm.org<mailto:cfe-commits at lists.llvm.org>> wrote:
Anastasia added a comment.

In https://reviews.llvm.org/D27334#612858, @bader wrote:

> In https://reviews.llvm.org/D27334#611703, @Anastasia wrote:
>
> > This change seems to modify normal C behavior again. Is there any strong motivation for doing this and if yes could it be done generically with C?
>
>
> Motivation:
>
>   // Non-portable OpenCL 1.2 code
>   __kernel void foo(global float* out) {
>     out[get_global_id(0)] = sin(get_global_id(0));
>   }
>
>
> This program compiles fine on OpenCL platform w/o doubles support

Perhaps that is the problem (that there are two modes that do different things)? Could we make the double overload be present but unselectable to diagnose this problem in that mode too?

and fails otherwise.
>  If OpenCL driver supports doubles it provides at least two versions of 'sin' built-in math function and compiler will not be able to choose the right one for 'size_t' argument.

Do you have a real example? If someone passes an integer to 'sin', I think there's a better warning we can give them :)

>  The goal of this warning is to let OpenCL developer know about potential issues with code portability.
I would argue this improves the portability much as it can also be misleading in some situations (because it refers to a potentially hypothetical problem). For example there can be builtin functions that only have a float parameter (without a double version of it). This is for example the case with read_image functions that take a float coordinate value between 0 and 1. Unfortunately this warning won't be triggered on read_image functions because there is an overload candidate with an int type of the same parameter too. But we can't exclude this situations to appear in the future or from some vendor extensions or even custom OpenCL code.


https://reviews.llvm.org/D27334



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