[llvm] [RISCV][TTI] Improve SiFive7 reduction cost (PR #90951)

[Craig Topper via llvm-commits]

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@@ -76,37 +113,63 @@ RISCVTTIImpl::getRISCVInstructionCost(ArrayRef<unsigned> OpCodes, MVT VT,
     case RISCV::VFREDUSUM_VS: {
       unsigned VL = VT.getVectorMinNumElements();
       if (!VT.isFixedLengthVector())
-        VL *= *getVScaleForTuning();
-      Cost += Log2_32_Ceil(VL);
+        VL *= VScale;
+      // For the cases with small VL, we use a lookup table for accurate
+      // cost estimation.
+      unsigned LookUpSiFive7ReduceLatency[] = {0,  20, 27, 32, 34,
+                                               38, 40, 41, 42};
+      if (VL <= 32) {
+        Cost += LookUpSiFive7ReduceLatency[(VL + 3) >> 2];
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topperc wrote:

Use divideCeil instead of `(VL + 3) >> 2`

https://github.com/llvm/llvm-project/pull/90951