[llvm] [DAG] Lower frem of power-2 using div/trunc/mul+sub (PR #91148)

[Matt Arsenault via llvm-commits]

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@@ -17264,17 +17264,32 @@ SDValue DAGCombiner::visitFREM(SDNode *N) {
   EVT VT = N->getValueType(0);
   SDNodeFlags Flags = N->getFlags();
   SelectionDAG::FlagInserter FlagsInserter(DAG, N);
+  SDLoc DL(N);
 
   if (SDValue R = DAG.simplifyFPBinop(N->getOpcode(), N0, N1, Flags))
     return R;
 
   // fold (frem c1, c2) -> fmod(c1,c2)
-  if (SDValue C = DAG.FoldConstantArithmetic(ISD::FREM, SDLoc(N), VT, {N0, N1}))
+  if (SDValue C = DAG.FoldConstantArithmetic(ISD::FREM, DL, VT, {N0, N1}))
     return C;
 
   if (SDValue NewSel = foldBinOpIntoSelect(N))
     return NewSel;
 
+  // Lower frem N0, N1 => x - trunc(N0 / N1) * N1, providing N1 is an integer
+  // power of 2.
+  if (DAG.isKnownToBeAPowerOfTwoFP(N1) &&
+      (Flags.hasNoSignedZeros() || DAG.isKnownNonNegativeFP(N0)) &&
+      !TLI.isOperationLegal(ISD::FREM, VT) &&
+      TLI.isOperationLegalOrCustom(ISD::FMUL, VT) &&
+      TLI.isOperationLegalOrCustom(ISD::FDIV, VT) &&
+      TLI.isOperationLegalOrCustom(ISD::FTRUNC, VT)) {
+    SDValue Div = DAG.getNode(ISD::FDIV, DL, VT, N0, N1);
+    SDValue Rnd = DAG.getNode(ISD::FTRUNC, DL, VT, Div);
+    SDValue Mul = DAG.getNode(ISD::FMUL, DL, VT, Rnd, N1);
+    return DAG.getNode(ISD::FSUB, DL, VT, N0, Mul);
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arsenm wrote:

Preserve the flags 

https://github.com/llvm/llvm-project/pull/91148