[PATCH] D20148: Idea for avoiding memory allocation calls.

[Xinliang David Li via llvm-commits]

On Wed, May 11, 2016 at 6:56 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Wed, May 11, 2016 at 12:25 AM, Rong Xu <xur at google.com> wrote:
>
>> xur added a comment.
>>
>> This avoids calloc in lprofCreateBufferIO() which is not used at all if
>> there is not value profiles.
>>
>> But how about the malloc calls from  setFilenamePossiblyWithPid() and
>> truncateCurrentFile(). They are not from value profiling, but they also
>> call dynamic memory allocation.
>>
>
> I suspect some combination of luck and that we officially expose only a
> limited API to users means we haven't hit those. This issue has only
> recently reached our priority list and we'll be looking more deeply at the
> issues and our requirements.
>
>
>>
>> Maybe we should convert all these into alloca or using pre-allocated
>> memory.
>>
>
> I would prefer pre-allocated, but I suppose alloca is fine and avoids a
> fixed limit. The downside of alloca is that in the worst case
> circumstances, we can overflow the stack and trigger an issue that is
> difficult to debug; at least for a fixed buffer we can return an error code.
>

I have same concern on using alloca, thus prefer fixed allocation. This
presents some challenge when serializing VP data, but it is doable.


>
>> David and I are working on to get the profile runtime malloc free (or at
>> least avoiding re-entrance to malloc) in the past few days. Hopefully we
>> can have some discussion on this soon.
>>
>
> Great! Glad we are working in the same direction.
>

yes -- I am in the process of getting rid of dynamic allocations one by
one. The first couple of ones are in profile dumper. The last/major one is
in the instrumenter.


>
> What do you mean "avoiding re-entrace to malloc"? Are you making calls
> into the runtime lib from malloc? (for value profiling presumably? I would
> not have guessed that malloc libs were using indirect calls)
>

It is the tcmalloc implementation (with source) -- it is feature rich that
also invokes various user define hooks .

David


>
> -- Sean Silva
>
>
>>
>>
>> Repository:
>>   rL LLVM
>>
>> http://reviews.llvm.org/D20148
>>
>>
>>
>>
>
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