[PATCH] D119469: [AArch64] Turn truncating buildvectors into truncates

[Dave Green via Phabricator via llvm-commits]

dmgreen added inline comments.


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Comment at: llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:9269
+// concats and truncate nodes.
+static SDValue ReconstructTruncateFromBuildVector(SDValue V, SelectionDAG &DAG) {
+  if (V.getValueType() != MVT::v16i8)
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david-arm wrote:
> Hi @dmgreen, this feels a bit *too* specific to a particular optimisation. For example, there is a test in CodeGen/AArch64/neon-extracttruncate.ll called `@extract_2_v4i32` that could also benefit from something like this. Also, some of the code in here looks very similar to what ReconstructShuffle attempts to do, which also looks at BUILD_VECTORS that are constructed from extractelement operations. Would it be better to simply extend ReconstructShuffle to cover this case?
ReconstructShuffle is for reconstructing buildvectors into Legal Shuffles, and if you look at the code only handles 2 sources. This method is for reconstructing a truncates, not shuffles, and needs to handle 4 sources. As this isn't a legal shuffle, it doesn't really fit there.
The tests in neon-extracttruncate.ll I just added as a quick test. They are not expected to come up in practice. https://godbolt.org/z/eEser71Gq. It is the lowering of large fptosi.sat that we are targeting.


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  https://reviews.llvm.org/D119469/new/

https://reviews.llvm.org/D119469