[cfe-commits] [OpenCL patch] Clang fails on nested vector literals

[Anton Lokhmotov]

Hi Eli,

> > This code is necessary for Clang to recognise that a vector literal
> > with a nested constant initializer is constant.
> 
> All of your tests pass without those changes... are they necessary for
> some other reason I'm missing?

Without those changes, Clang thinks that the right-hand side of:

  __constant int4 i_1_2_1 = (int4)(1,(int2)(2,3),4);

is not a compile-time constant.  This contradicts the OpenCL spec (rev44,
p187): 

"Variables in the program scope or the outermost scope of kernel functions
can be declared in the __constant address space. These variables are
required to be initialized and the values used to initialize these variables
must be a compile time constant."

The tests pass only because Clang currently does not enforce this behaviour
(but we will submit another patch for that).  We include the tests in this
patch because the problem occurs specifically on nested vector literals.

> Our code is more generic, as it allows nested vector initializers
> (see OpenCL 6.1.6).  The added tests cover all possible uses.
> 
> Your patch breaks the following:
> 
> typedef int i4 __attribute((vector_size(16)));
> i4 x = (i4){1,2};
> 
> It takes out the code that would otherwise pad the vector with zeros.

You are right.  But why was this behaviour used in the first place?

Both OpenCL 1.1 (http://www.khronos.org/registry/cl/specs/opencl-1.1.pdf,
p164) and AltiVec PIM
(http://www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf, p26)
say that the number of elements in the source must be equal to the number of
elements in the result vector or one.  GCC vector extensions
(http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html) are also silent
on the issue.

Could you please point to a specification that says that missing vector
elements must be filled with zeros?

Many thanks,
Anton.