1 files changed, 28 insertions, 1 deletions
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 20e749ba569..4ef16cb7186 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11571,6 +11571,34 @@ SDValue DAGCombiner::visitFPOW(SDNode *N) {
   if (!ExponentC)
     return SDValue();
 
+  // Try to convert x ** (1/3) into cube root.
+  // TODO: Handle the various flavors of long double.
+  // TODO: Since we're approximating, we don't need an exact 1/3 exponent.
+  //       Some range near 1/3 should be fine.
+  EVT VT = N->getValueType(0);
+  if ((VT == MVT::f32 && ExponentC->getValueAPF().isExactlyValue(1.0f/3.0f)) ||
+      (VT == MVT::f64 && ExponentC->getValueAPF().isExactlyValue(1.0/3.0))) {
+    // pow(-0.0, 1/3) = +0.0; cbrt(-0.0) = -0.0.
+    // pow(-inf, 1/3) = +inf; cbrt(-inf) = -inf.
+    // pow(-val, 1/3) =  nan; cbrt(-val) = -num.
+    // For regular numbers, rounding may cause the results to differ.
+    // Therefore, we require { nsz ninf nnan afn } for this transform.
+    // TODO: We could select out the special cases if we don't have nsz/ninf.
+    SDNodeFlags Flags = N->getFlags();
+    if (!Flags.hasNoSignedZeros() || !Flags.hasNoInfs() || !Flags.hasNoNaNs() ||
+        !Flags.hasApproximateFuncs())
+      return SDValue();
+
+    // Do not create a cbrt() libcall if the target does not have it, and do not
+    // turn a pow that has lowering support into a cbrt() libcall.
+    if (!DAG.getLibInfo().has(LibFunc_cbrt) ||
+        (!DAG.getTargetLoweringInfo().isOperationExpand(ISD::FPOW, VT) &&
+         DAG.getTargetLoweringInfo().isOperationExpand(ISD::FCBRT, VT)))
+      return SDValue();
+
+    return DAG.getNode(ISD::FCBRT, SDLoc(N), VT, N->getOperand(0), Flags);
+  }
+
   // Try to convert x ** (1/4) into square roots.
   // x ** (1/2) is canonicalized to sqrt, so we do not bother with that case.
   // TODO: This could be extended (using a target hook) to handle smaller
@@ -11587,7 +11615,6 @@ SDValue DAGCombiner::visitFPOW(SDNode *N) {
       return SDValue();
 
     // Don't double the number of libcalls. We are trying to inline fast code.
-    EVT VT = N->getValueType(0);
     if (!DAG.getTargetLoweringInfo().isOperationLegalOrCustom(ISD::FSQRT, VT))
       return SDValue();