[llvm-commits] patch: partial DSE

[Eli Friedman]

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