Speedup Function::getIntrinsicID() with caching of result

[Michael Ilseman]

On Feb 28, 2013, at 9:34 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Michael,
> 
> On 28/02/13 18:25, Michael Ilseman wrote:
>> 
>> On Feb 28, 2013, at 2:21 AM, Duncan Sands <baldrick at free.fr> wrote:
>> 
>>> Hi Jean-Luc,
>>> 
>>> On 28/02/13 02:16, Jean-Luc Duprat wrote:
>>>> The attached patch caches the result of Function::getIntrinsicID() in a DenseMap attached to the LLVMContext.  This reduces the time actually spent doing string to ID conversion and shows a 10% improvement in compile time for a particularly bad case that involves ARM Neon intrinsics (these have many overloads).
>>>> 
>>>> This changes passes the regression tests and the nightly tests suite.
>>> 
>>> if you delete an intrinsic function declaration from the module, don't you get a
>>> stale pointer to freed memory in the cache?  To avoid all such issues, maybe it
>>> is simpler to have the cache map the name (a StringRef) to the intrinsic ID,
>>> rather than mapping the function pointer to the intrinsic ID.  You probably lose
>>> some of the speedup then though.
>> 
>> Can he instead have the destructor remove itself from the cache? This would preserve the speedups of key-ing off of the Function pointer directly.
> 
> this would also mean that the code that clears the cache when a module is
> deleted wouldn't be needed.

Yes, I think the replacement is needed.

> 
>> 
>> Also, since it's the Function* that's the key, I believe the situation you described wouldn't provide an error so much as it would under-utilize the map by having extra meaningless entries. However, maybe there would be an error if anyone ever iterated over the cache. Replacing the Module's destructor clear() call with a Function's destructor erase() call might be a good way to go.
> 
> The problem is if that freed memory gets reallocated for another function.  Then
> the cache lookup will wrongly think that the new function has the intrinsic id
> of the freed function.
> 

Ah, yes, of course.

Jean-Luc, do you think you could replace Module's destructor clear() call with a call to erase() in Function's destructor?

> Ciao, Duncan.
> 
>> 
>>> 
>>> Ciao, Duncan.
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>> 
>