[llvm] [RISCV] Handle fixed length vectors with exact VLEN in lowerINSERT_SUBVECTOR (PR #84107)

[Philip Reames via llvm-commits]

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@@ -9596,6 +9596,21 @@ SDValue RISCVTargetLowering::lowerVPREDUCE(SDValue Op,
                            Vec, Mask, VL, DL, DAG, Subtarget);
 }
 
+/// Returns true if \p LHS is known to be equal to \p RHS, taking into account
+/// if VLEN is exactly known by \p Subtarget and thus vscale when handling
+/// scalable quantities.
+static bool isKnownEQ(ElementCount LHS, ElementCount RHS,
+                      const RISCVSubtarget &Subtarget) {
+  if (auto VLen = Subtarget.getRealVLen()) {
+    const unsigned Vscale = *VLen / RISCV::RVVBitsPerBlock;
+    if (LHS.isScalable())
+      LHS = ElementCount::getFixed(LHS.getKnownMinValue() * Vscale);
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preames wrote:

You've got this pattern repeating a few times, maybe extract a helper which returns the ElementCount of a vector which returns a fixed value if possible given exact vlen?

https://github.com/llvm/llvm-project/pull/84107