[PATCH] D49242: [Intrinsics] define funnel shift IR intrinsics + DAG builder support

[Fabian Giesen via Phabricator via llvm-commits]

fabiang added inline comments.


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Comment at: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:5678
+    SDValue ShX = DAG.getNode(ISD::SHL, sdl, VT, X, IsFSHL ? ShAmt : InvShAmt);
+    SDValue ShY = DAG.getNode(ISD::SRL, sdl, VT, Y, IsFSHL ? InvShAmt : ShAmt);
+    SDValue Res = DAG.getNode(ISD::OR, sdl, VT, ShX, ShY);
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It's definitely rare and, perhaps more importantly, not nearly as commonly supported as rotates, especially for the variable-amount case.

OTOH, even if the resulting DAG is a bit weird, any backends that want to match it can do so easily _if_ it has a canonical form. It's the almost-but-not-quite-canonical cases where the general shape is clear but there are lots of equivalent minor variations that turn out to be brittle and over-specific in my experience. Plus there's always the option of adding real DAG funnel shift ops in a later patch, should the demand arise.


https://reviews.llvm.org/D49242