llvm dev - Jul 2021 - Questions about support __llvm_profile_set_file_object on continuous mode

Hi,

When merging is disabled, a possible solution is to unmap the mapped file
region, copy the old profile file to the new file, and then mmap the
CounterBegin to the new file region. Does this sound correct?
When merging is enabled, multiple profile files will be created. I'm not
sure how to set the file object in this case as only one file descriptor is
passed to __llvm_profile_set_file_object.

Zequan
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Hi Zequan,
> On Jul 27, 2021, at 6:50 PM, Zequan Wu <zequanwu at google.com>
wrote:
> 
> Hi,
> 
> When merging is disabled, a possible solution is to unmap the mapped file
region, copy the old profile file to the new file, and then mmap the
CounterBegin to the new file region. Does this sound correct?
If the instrumented program is multithreaded, it might be possible for a thread
to racily update a counter in the un-mmap'd region.

Do you think it's viable to sidestep the issue by restricting how/when
__llvm_profile_set_file_object can be called, e.g. either by documenting or
requiring that no other threads are active at the time of the call?
> When merging is enabled, multiple profile files will be created. I'm
not sure how to set the file object in this case as only one file descriptor is
passed to __llvm_profile_set_file_object.
Can you share how __llvm_profile_set_file_object(..., EnableMerge = true) works
in non-continuous mode? It's not something I'm familiar with.

thanks,
vedant
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