[llvm] [RISCV] Use slideup to lower build_vector when its last operand is an extraction (PR #154450)

[Min-Yih Hsu via llvm-commits]

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@@ -4512,42 +4512,98 @@ static SDValue lowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG,
          "Illegal type which will result in reserved encoding");
 
   const unsigned Policy = RISCVVType::TAIL_AGNOSTIC | RISCVVType::MASK_AGNOSTIC;
+  auto getVSlide = [&](bool SlideUp, EVT ContainerVT, SDValue Passthru,
+                       SDValue Vec, SDValue Offset, SDValue Mask,
+                       SDValue VL) -> SDValue {
+    if (SlideUp)
+      return getVSlideup(DAG, Subtarget, DL, ContainerVT, Passthru, Vec, Offset,
+                         Mask, VL, Policy);
+    return getVSlidedown(DAG, Subtarget, DL, ContainerVT, Passthru, Vec, Offset,
+                         Mask, VL, Policy);
+  };
+
+  // General case: splat the first operand and slide other operands down one
+  // by one to form a vector. Alternatively, if the last operand is an
+  // extraction from element 0 of a vector, we can use the original vector
+  // reduction result as the start value and slide up instead of slide down.
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mshockwave wrote:

Fixed

https://github.com/llvm/llvm-project/pull/154450