[llvm] [RISCV]: Implemented softening of `FCANONICALIZE` (PR #169234)

[Matt Arsenault via llvm-commits]

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@@ -311,6 +313,31 @@ SDValue DAGTypeLegalizer::SoftenFloatRes_FABS(SDNode *N) {
   return DAG.getNode(ISD::AND, SDLoc(N), NVT, Op, Mask);
 }
 
+SDValue DAGTypeLegalizer::SoftenFloatRes_FCANONICALIZE(SDNode *N) {
+  SDLoc dl(N);
+
+  // This implements llvm.canonicalize.f* by multiplication with 1.0, as
+  // suggested in
+  // https://llvm.org/docs/LangRef.html#llvm-canonicalize-intrinsic.
+  // It uses strict_fp operations even outside a strict_fp context in order
+  // to guarantee that the canonicalization is not optimized away by later
+  // passes. The result chain introduced by that is intentionally ignored
+  // since no ordering requirement is intended here.
+
+  // Create strict multiplication by 1.0.
+  SDValue Operand = N->getOperand(0);
+  EVT VT = Operand.getValueType();
+  SDValue One = DAG.getConstantFP(1.0, dl, VT);
+  SDValue Chain = DAG.getEntryNode();
+  // Propagate existing flags on canonicalize, and additionally set
+  // NoFPExcept.
+  SDNodeFlags CanonicalizeFlags = N->getFlags();
+  CanonicalizeFlags.setNoFPExcept(true);
+  SDValue Mul = DAG.getNode(ISD::STRICT_FMUL, dl, {VT, MVT::Other},
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arsenm wrote:

It made more sense to promote this to canonicalize. There's already a generic expansion from canonicalize to the strict_fmul

https://github.com/llvm/llvm-project/pull/169234