[PATCH] D60133: [DAGCombiner] Improve detection of unmergable stores, based on type size (WIP)

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D60133#1452430 <https://reviews.llvm.org/D60133#1452430>, @niravd wrote:

> It's not sufficient to check if you can merge two stores into a valid node; there are backends where you need 4 or more to get a legal merged store.
>
> If you look at target-specific implementations of CanMergeStoresTo it essentially serves as a context-specific find maximum store which is what we need here.  If you massage that interface a bit you can fold most of this check in there.


Sorry for the long delay with getting back to this one! I had a look at the X86 and AArch64 implementations of CanMergeStoresTo and it looks like they are not too helpful with getting a reasonable upper bound. They return true for functions without noimplicitfloat and otherwise limit the size to 64 bits or 32 bits for X86 32 bit mode. I think one thing that's more important to check is whether the merged type is legal for the target. This will weed out stores with types too large for the target.

So I think we need to do more work than just checking canMergeStoresTo unfortunately. What we would need is the maximum size for integer, float and vector stores I think, but I am not sure if any existing interface provides that info.


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https://reviews.llvm.org/D60133