[llvm] [RISCV][GISEL] Legalize G_BITCAST for scalable vectors (PR #85970)

[Craig Topper via llvm-commits]

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@@ -155,6 +155,9 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST)
 
   getActionDefinitionsBuilder(G_BITREVERSE).maxScalar(0, sXLen).lower();
 
+  getActionDefinitionsBuilder(G_BITCAST).legalIf(
+      all(isScalableVector(0), isScalableVector(1)));
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topperc wrote:

> For scalable vectors there is no total vector size.

There is a symbolic representation of the total vector size that TypeSize can represent. As long as the the TypeSizes are both scalable and have the same minimum size, the bitcast is valid.

https://github.com/llvm/llvm-project/pull/85970