[PATCH] Compile time reduction for small programs.

[Puyan Lotfi]

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.

> +  PhysRegEntries = (unsigned char*) calloc(PhysRegEntriesCount,
> sizeof(unsigned char));
> 
> 80-col?
> 

Ah, yes. Will change it to:
+  PhysRegEntries = (unsigned char*)
+    calloc(PhysRegEntriesCount, sizeof(unsigned char));

> +  reinitPhysRegEntries();
> 
> Might be nice to get a comment of when we're reinitializing why we're
> doing so there.
> 

I mentioned it in the comment for reinitPhysRegEntries(), but I will add it here too:
that to support when the same pass manager could be reused for similar targets that have different register file sizes,
we need to check to see if the PhysRegEntries buffer needs to be reallocated to a different size.

> +    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.

> 
> -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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>> 
>>