[llvm-commits] patch: partial DSE

[Evan Cheng]

This might be something we have to defer until we add "what's native types" to TargetData. I'm worried when / if this does something bad, the performance impact can be very significant.

Evan

On Nov 1, 2011, at 3:12 PM, Eli Friedman wrote:

> On Tue, Nov 1, 2011 at 2:47 PM, Eric Christopher <echristo at apple.com> wrote:
>> 
>> On Oct 31, 2011, at 5:38 PM, Peter Cooper wrote:
>> 
>>> Hi
>>> 
>>> Please review this patch to allow DSE to trim stores as opposed to deleting them.
>>> 
>>> The logic here is that if the end of the earlier store is dead because of a later store then the length of the earlier store will be trimmed in size to avoid writing dead memory.  The only time i won't do this is if the original store was likely to use vector writes which if shortened would end up as multiple scalar writes and so is less efficient.
>>> 
>> 
>> Not a huge fan of this style:
>> 
>> +          (OR = isOverwrite(Loc, DepLoc, *AA,
>> +                            DepWriteOffset,
>> +                            InstWriteOffset)) != OverwriteUnknown &&
>> 
>> in large conditionals. Things that return booleans, or set something for a block, e.g.:
>> 
>> if (ConstantInt *CI = dyn_cast<ConstantInt>(Inst)) {
>> }
>> 
>>> Any help removing the magic vector size (16) constant would be good too :)
>> 
>> Something like this maybe?
>> 
>> bool isLegalVector = false;
>> if (VectorType *VecTy = dyn_cast<VectorType>(Store->getType()) {
>>  EVT VT = TLI.getValueType(VecTy);
>>  isLegalVector = TLI.isTypeLegal(VT);
>> }
>> 
>> does require target info though and I'm not sure how kosher that is in AA.
> 
> There aren't actually any vector types involved here; the issue is
> that, for example, a 32 byte memset is cheaper than a 31-byte memset
> under the default settings on x86-64.  I'm not sure what the right
> approach is here.
> 
> -Eli
> 
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