[clang] [CIR] Upstream ShuffleDynamicOp for VectorType (PR #141411)

[Andy Kaylor via cfe-commits]

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@@ -1871,6 +1872,54 @@ mlir::LogicalResult CIRToLLVMVecCmpOpLowering::matchAndRewrite(
   return mlir::success();
 }
 
+mlir::LogicalResult CIRToLLVMVecShuffleDynamicOpLowering::matchAndRewrite(
+    cir::VecShuffleDynamicOp op, OpAdaptor adaptor,
+    mlir::ConversionPatternRewriter &rewriter) const {
+  // LLVM IR does not have an operation that corresponds to this form of
+  // the built-in.
+  //     __builtin_shufflevector(V, I)
+  // is implemented as this pseudocode, where the for loop is unrolled
+  // and N is the number of elements:
+  //     masked = I & (N-1)
----------------
andykaylor wrote:

This is not quite what the classic codegen does, and I think it will be wrong for non-power-of-two vector sizes. The classic codegen uses a mask value that is equal to X - 1 where X is the next power of two above 'N - 1'. So if the number of elements is a power of two, it will be equivalent to the code here, but if not, it will mask differently.

See https://godbolt.org/z/c764a1T5v

https://github.com/llvm/llvm-project/pull/141411