Question

OpenCL 1.1, using Cloo 0.9.1.0.

For a certain CL module I get a crash at the following line:

program.Build(null, null, null, IntPtr.Zero);

Visual Studio 2010 tells me this:

An unhandled exception of type ‘System.AccessViolationException’
occurred in Cloo.dll

Additional information: Attempted to read or write protected memory.
This is often an indication that other memory is corrupt.

I have trace the error to a certain line of code.

int offset = 1000000 * (input == 0); // This is the culprit!
const sampler_t smp = CLK_NORMALIZED_COORDS_FALSE | CLK_ADDRESS_CLAMP | CLK_FILTER_NEAREST;
int4 pixel = read_imagei(image, smp, (int2)(offset + x, y));

I’m using the trick above to avoid branching. It works perfectly in all but one situation where the above yields an access violation at compile-time. If I remove the conditional multiplication it works. E.g.

int offset = 1000000;
// -or-
int offset = (input != 0 ? 0 : 1000000);

Am I facing a compiler bug here? How do I work around this error?

Some details about my Graphics card/driver:

Driver Packaging Version: 8.85.7.2-110901a1-125827C-Fujitsu Technology

Solutions Provider: ATI Technologies Inc. 2D Driver Version: 8.01.01.1152

Answer 1

It’s a compiler bug. Hopefully it will get fixed some day. For now, I will just avoid that specific optimization trick.