On Thu, Jan 28, 2016 at 12:36:19 +0100, Thomas Schwinge wrote:
> I made an attempt to capture the recent discussion (plus my own
> ideas/understanding) in this new section:
> <https://gcc.gnu.org/wiki/Offloading#Partial_Offloading>. Please
> change/extend, as required.
Thanks for summarizing this.
I'm not very happy how -foffload=disable works in GCC 6, here is a testcase:
int main ()
{
int x = 10;
#pragma omp target data map (from: x)
#pragma omp target map (alloc: x)
x = 20;
if (x != 10 && x != 20)
__builtin_abort ();
}
On the system with non-shared accelerator it will abort, because "#pragma omp
target data" behaves like offloading is enabled, but "#pragma omp target" runs
on the host. As the result, at the end of the *target data* region, it tries to
receive x from target and receives 0, or crashes.
We can forbid -foffload=disable option, but I think it's very useful, e.g. for
comparing performance of host vs. accelerator using the same compiler, etc.
Or if the system contains 2 different accelerators, someone might want to
compile only for the first, but libgomp will load 2 plugins, and the program
will crash (instead of doing fallback) if it will try to use the second device.
So, maybe we still need something like this patch?
https://gcc.gnu.org/ml/gcc-patches/2015-04/msg01033.html
-- Ilya
