X86: Do not return invalid cost for fp16 conversion (#114128)

Returning invalid instruction costs when converting from/to fp16 in
`X86TTIImpl::getCastInstrCost` when there is no hardware support
available was triggering asserts. This changes the code to return a
large (arbitrary) number to model the fact that libcalls are used to
implement the conversion.

This also simplifies the code by only reporting costs for the scalar
fp16 conversion; vectorized costs being left to the fallback assuming
scalarization.

This is a follow-up to assertion issues reported for the changes in
#113195

1 files changed, 7 insertions, 5 deletions

diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
index bae2232..520284d 100644
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -3068,6 +3068,13 @@ InstructionCost X86TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
         if (auto KindCost = Entry->Cost[CostKind])
           return *KindCost;
     }
+
+    if ((ISD == ISD::FP_ROUND && SimpleDstTy == MVT::f16) ||
+        (ISD == ISD::FP_EXTEND && SimpleSrcTy == MVT::f16)) {
+      // fp16 conversions not covered by any table entries require a libcall.
+      // Return a large (arbitrary) number to model this.
+      return InstructionCost(64);
+    }
   }
 
   // Fall back to legalized types.
@@ -3174,11 +3181,6 @@ InstructionCost X86TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
                             TTI::CastContextHint::None, CostKind);
   }
 
-  if (ISD == ISD::FP_ROUND && LTDest.second.getScalarType() == MVT::f16) {
-    // Conversion requires a libcall.
-    return InstructionCost::getInvalid();
-  }
-
   // TODO: Allow non-throughput costs that aren't binary.
   auto AdjustCost = [&CostKind](InstructionCost Cost,
                                 InstructionCost N = 1) -> InstructionCost {