On 03/07/2013 10:57 AM, Tobias Grosser wrote:
On 03/07/2013 12:35 AM, Redmond, Paul wrote:
On 2013-03-06, at 3:42 PM, Pekka Jääskeläinen wrote:
Hi Paul,
On 03/06/2013 10:11 PM, Redmond, Paul wrote:
I have updated the patch to not add metadata on loads and stores
where the
pointer comes from an alloca. I wonder if the check should be more
conservative and only include pointers coming from Arguments and
GlobalValues
(perhaps Constants too?)
I think it's safer that way around for now.
Hmm.. I'm not sure that this approach (or my original) is general
enough. Here's a simple loop and the corresponding IR as generated by
clang:
Before taking decisions on what exactly we need to do during code
generation, I would appreciate if we could write down the exact
semantics of #pragma ivdep on the C level. We should get a list of
interesting test cases and investigate what other compilers gcc and icc
do in such cases. Pekka and me gave a couple of test cases, that may be
a good start.
We can obviously take a very conservative approach here. Meaning it
should be enough to define the semantics of #pragma ivdeps for the cases
where we actually introduce memory annotations and to only confirm for
those cases, that we are in line what other compilers do.
For the 'safe start' I mentioned further down, this seems easy to do.
Regarding your current approach. I wonder if, instead of analyzing the
IR that clang generates, we could take decisions according to how
a certain memory access is/was represented at the C level. This is the
level the #pragma ivdep semantics are defined and this should allow us
to take the right conclusions.
A safe start would e.g. be to only annotate loads/stores that come from
array style memory accesses ala 'A[i][j]', where A was declared outside
of the loop that was marked with #pragma ivdep.
Cheers,
Tobi
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