Don't combine fp_round (fp_round x) if f80 to f16 is generated

Summary:
This patch skips DAG combine of fp_round (fp_round x) if it results in
an fp_round from f80 to f16.

fp_round from f80 to f16 always generates an expensive (and as yet,
unimplemented) libcall to __truncxfhf2.  This prevents selection of
native f16 conversion instructions from f32 or f64.  Moreover, the first
(value-preserving) fp_round from f80 to either f32 or f64 may become a
NOP in platforms like x86.

Reviewers: ab

Subscribers: srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D17221

llvm-svn: 260769

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 3fca3b45757..8b1b6bbce30 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9019,6 +9019,17 @@ SDValue DAGCombiner::visitFP_ROUND(SDNode *N) {
   if (N0.getOpcode() == ISD::FP_ROUND) {
     const bool NIsTrunc = N->getConstantOperandVal(1) == 1;
     const bool N0IsTrunc = N0.getNode()->getConstantOperandVal(1) == 1;
+
+    // Skip this folding if it results in an fp_round from f80 to f16.
+    //
+    // f80 to f16 always generates an expensive (and as yet, unimplemented)
+    // libcall to __truncxfhf2 instead of selecting native f16 conversion
+    // instructions from f32 or f64.  Moreover, the first (value-preserving)
+    // fp_round from f80 to either f32 or f64 may become a NOP in platforms like
+    // x86.
+    if (N0.getOperand(0).getValueType() == MVT::f80 && VT == MVT::f16)
+      return SDValue();
+
     // If the first fp_round isn't a value preserving truncation, it might
     // introduce a tie in the second fp_round, that wouldn't occur in the
     // single-step fp_round we want to fold to.