[PATCH] Compile time reduction for small programs.

[Puyan Lotfi]

On Feb 4, 2014, at 10:16 PM, Eric Christopher <echristo at gmail.com> wrote:

> On Tue, Feb 4, 2014 at 9:47 PM, Puyan Lotfi <plotfi at apple.com> wrote:
>> 
>> On Feb 4, 2014, at 6:52 PM, Eric Christopher <echristo at gmail.com> wrote:
>> 
>>> Hey cool,
>>> 
>>> +/// reinitPhysRegEntries - Initializes PhysRegEntries to new
>>> +/// zeroed out memory of size NumPhysRegs. This is faster than using
>>> +/// llvm::SmallVector::assign(N, 0). It's used in the below code to
>>> +/// also supports when a pass manager could be reused for different
>>> +/// but similar targets that could have varying numbers of PhysRegs.
>>> +void InterferenceCache::reinitPhysRegEntries() {
>>> +  if (PhysRegEntriesCount == TRI->getNumRegs()) return;
>>> +  free(PhysRegEntries);
>>> +  PhysRegEntriesCount = TRI->getNumRegs();
>>> +  PhysRegEntries = (unsigned char*) calloc(PhysRegEntriesCount,
>>> sizeof(unsigned char));
>>> +}
>>> 
>>> Looks to be faster since we're not actually zeroing out the array if
>>> it's the same size as the previous allocation?
>>> 
>> 
>> Not exactly; the compile time improvement gained here over the current implementation comes from the fact that we're using a SmallVector:
>> 
>> SmallVector<unsigned char, 2> PhysRegEntries;
>> 
>> that we zero out using assign:
>> 
>> PhysRegEntries.assign(TRI->getNumRegs(), 0);
>> 
>> when the SmallVector might not actually be that small at all.
>> 
>> Compile time was being spent in zeroing out the SmallVector when calling llvm::SmallVector::assign, which uses an iterator to assign each element in the SmallVector to zero.
>> I found that just by changing the SmallVector to a std::vector I got a compile time reduction since std::vector ends up using __platform_bzero to do the job.
>> 
>> Jakob suggested I just calloc the memory in the first place, rather than using a std::vector or calling bzero.
>> And actually, it would be nice if SmallVector had some platform specific speed improvements eventually, the way that std::vector does.
>> 
> 
> Yeah, it would be. That's a bit crazy.
> 
> At any rate, I was wondering why reinit didn't 0 the mem if it
> happened to be the same size?
> 

Hmm, I am actually not sure what is correct in this case. I’ll have to ask Jakob. 
But the reinit here is more or less a precaution.
I believe the existing code would have used the same already populated SmallVector memory if the pass manager we to be reused too. 

>> 
>>> +    for (auto RegI = PhysRegs.begin(), E = PhysRegs.end(); RegI != E; ++RegI)
>>> +      if (!MRI->reg_nodbg_empty(*RegI))
>>> +        MRI->setPhysRegUsed(*RegI);
>>> 
>>> Can't use auto quite yet :)
>>> 
>> 
>> Doe! I was under the impression that C++11 support had been enabled several weeks ago.
> 
> Almost there :)
> 
> -eric
> 
>> 
>>> 
>>> -eric
>>> 
>> 
>> -Thanks
>> 
>> Puyan
>> 
>>> On Tue, Feb 4, 2014 at 3:44 PM, Puyan Lotfi <plotfi at apple.com> wrote:
>>>> 
>>>> On Feb 4, 2014, at 2:52 PM, Eric Christopher <echristo at gmail.com> wrote:
>>>> 
>>>>> On Tue, Feb 4, 2014 at 2:47 PM, Andrew Trick <atrick at apple.com> wrote:
>>>>>> On Feb 4, 2014, at 2:41 PM, Puyan Lotfi <plotfi at apple.com> wrote:
>>>>>> 
>>>>>>> Here is a patch for reducing compile time on small programs, especially if the target has a large register file.
>>>>>>> I've already done sanity checks to determine that it doesn't regress compile time on large programs.
>>>>>> 
>>>>>> LGTM. (mainly I'm trusting Jakob's informal review earlier).
>>>>> 
>>>>> Knowing this helps :)
>>>>> 
>>>>> It'd be nice to get some comments in the code as to what's going on
>>>>> where. There aren't a lot in there as it is, but getting some more
>>>>> would be nice.
>>>>> 
>>>>> -eric
>>>>> 
>>>> 
>>>> Here's a better commented version, Eric:
>>>> 
>>>> 
>>>> 
>>>> 
>>>> -Puyan
>>>> 
>>>> 
>>>>>> -Andy
>>>>>> 
>>>>>>> 
>>>>>>> -Puyan
>>>>>>> 
>>>>>>> <CompileTimeReductionSmall.patch>_______________________________________________
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>>>>>>> llvm-commits at cs.uiuc.edu
>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>> 
>>>>>> 
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>>>> 
>>>> 
>>