[RISCV][WIP] Treat bf16->f32 as separate ExtKind in combineOp_VLToVWOp_VL.

This allows us to better track the narrow type we need and to fix
miscompiles if f16->f32 and bf16->f32 extends are mixed.

Fixes #144651.

Still need to add tests, but it's late and I need sleep.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -16309,7 +16309,12 @@ namespace {
 // apply a combine.
 struct CombineResult;
 
-enum ExtKind : uint8_t { ZExt = 1 << 0, SExt = 1 << 1, FPExt = 1 << 2 };
+enum ExtKind : uint8_t {
+  ZExt = 1 << 0,
+  SExt = 1 << 1,
+  FPExt = 1 << 2,
+  BF16Ext = 1 << 3
+};
 /// Helper class for folding sign/zero extensions.
 /// In particular, this class is used for the following combines:
 /// add | add_vl | or disjoint -> vwadd(u) | vwadd(u)_w
@@ -16344,8 +16349,10 @@ struct NodeExtensionHelper {
   /// instance, a splat constant (e.g., 3), would support being both sign and
   /// zero extended.
   bool SupportsSExt;
-  /// Records if this operand is like being floating-Point extended.
+  /// Records if this operand is like being floating point extended.
   bool SupportsFPExt;
+  /// Records if this operand is extended from bf16.
+  bool SupportsBF16Ext;
   /// This boolean captures whether we care if this operand would still be
   /// around after the folding happens.
   bool EnforceOneUse;
@@ -16381,6 +16388,7 @@ struct NodeExtensionHelper {
     case ExtKind::ZExt:
       return RISCVISD::VZEXT_VL;
     case ExtKind::FPExt:
+    case ExtKind::BF16Ext:
       return RISCVISD::FP_EXTEND_VL;
     }
     llvm_unreachable("Unknown ExtKind enum");
@@ -16402,13 +16410,6 @@ struct NodeExtensionHelper {
     if (Source.getValueType() == NarrowVT)
       return Source;
 
-    // vfmadd_vl -> vfwmadd_vl can take bf16 operands
-    if (Source.getValueType().getVectorElementType() == MVT::bf16) {
-      assert(Root->getSimpleValueType(0).getVectorElementType() == MVT::f32 &&
-             Root->getOpcode() == RISCVISD::VFMADD_VL);
-      return Source;
-    }
-
     unsigned ExtOpc = getExtOpc(*SupportsExt);
 
     // If we need an extension, we should be changing the type.
@@ -16451,7 +16452,8 @@ struct NodeExtensionHelper {
     // Determine the narrow size.
     unsigned NarrowSize = VT.getScalarSizeInBits() / 2;
 
-    MVT EltVT = SupportsExt == ExtKind::FPExt
+    MVT EltVT = SupportsExt == ExtKind::BF16Ext ? MVT::bf16
+                : SupportsExt == ExtKind::FPExt
                     ? MVT::getFloatingPointVT(NarrowSize)
                     : MVT::getIntegerVT(NarrowSize);
 
@@ -16628,17 +16630,17 @@ struct NodeExtensionHelper {
     EnforceOneUse = false;
   }
 
-  bool isSupportedFPExtend(SDNode *Root, MVT NarrowEltVT,
-                           const RISCVSubtarget &Subtarget) {
+  bool isSupportedFPExtend(MVT NarrowEltVT, const RISCVSubtarget &Subtarget) {
+    if (NarrowEltVT == MVT::f32)
+      return true;
     // Any f16 extension will need zvfh
-    if (NarrowEltVT == MVT::f16 && !Subtarget.hasVInstructionsF16())
-      return false;
-    // The only bf16 extension we can do is vfmadd_vl -> vfwmadd_vl with
-    // zvfbfwma
-    if (NarrowEltVT == MVT::bf16 && (!Subtarget.hasStdExtZvfbfwma() ||
-                                     Root->getOpcode() != RISCVISD::VFMADD_VL))
-      return false;
-    return true;
+    if (NarrowEltVT == MVT::f16 && Subtarget.hasVInstructionsF16())
+      return true;
+    return false;
+  }
+
+  bool isSupportedBF16Extend(MVT NarrowEltVT, const RISCVSubtarget &Subtarget) {
+    return NarrowEltVT == MVT::bf16 && Subtarget.hasStdExtZvfbfwma();
   }
 
   /// Helper method to set the various fields of this struct based on the
@@ -16648,6 +16650,7 @@ struct NodeExtensionHelper {
     SupportsZExt = false;
     SupportsSExt = false;
     SupportsFPExt = false;
+    SupportsBF16Ext = false;
     EnforceOneUse = true;
     unsigned Opc = OrigOperand.getOpcode();
     // For the nodes we handle below, we end up using their inputs directly: see
@@ -16679,9 +16682,11 @@ struct NodeExtensionHelper {
     case RISCVISD::FP_EXTEND_VL: {
       MVT NarrowEltVT =
           OrigOperand.getOperand(0).getSimpleValueType().getVectorElementType();
-      if (!isSupportedFPExtend(Root, NarrowEltVT, Subtarget))
-        break;
-      SupportsFPExt = true;
+      if (isSupportedFPExtend(NarrowEltVT, Subtarget))
+        SupportsFPExt = true;
+      if (isSupportedBF16Extend(NarrowEltVT, Subtarget))
+        SupportsBF16Ext = true;
+
       break;
     }
     case ISD::SPLAT_VECTOR:
@@ -16698,16 +16703,16 @@ struct NodeExtensionHelper {
       if (Op.getOpcode() != ISD::FP_EXTEND)
         break;
 
-      if (!isSupportedFPExtend(Root, Op.getOperand(0).getSimpleValueType(),
-                               Subtarget))
-        break;
-
       unsigned NarrowSize = VT.getScalarSizeInBits() / 2;
       unsigned ScalarBits = Op.getOperand(0).getValueSizeInBits();
       if (NarrowSize != ScalarBits)
         break;
 
-      SupportsFPExt = true;
+      if (isSupportedFPExtend(Op.getOperand(0).getSimpleValueType(), Subtarget))
+        SupportsFPExt = true;
+      if (isSupportedBF16Extend(Op.getOperand(0).getSimpleValueType(),
+                                Subtarget))
+        SupportsBF16Ext = true;
       break;
     }
     default:
@@ -16940,6 +16945,11 @@ canFoldToVWWithSameExtensionImpl(SDNode *Root, const NodeExtensionHelper &LHS,
     return CombineResult(NodeExtensionHelper::getFPExtOpcode(Root->getOpcode()),
                          Root, LHS, /*LHSExt=*/{ExtKind::FPExt}, RHS,
                          /*RHSExt=*/{ExtKind::FPExt});
+  if ((AllowExtMask & ExtKind::BF16Ext) && LHS.SupportsBF16Ext &&
+      RHS.SupportsBF16Ext && Root->getOpcode() == RISCVISD::VFMADD_VL)
+    return CombineResult(NodeExtensionHelper::getFPExtOpcode(Root->getOpcode()),
+                         Root, LHS, /*LHSExt=*/{ExtKind::BF16Ext}, RHS,
+                         /*RHSExt=*/{ExtKind::BF16Ext});
   return std::nullopt;
 }
 
@@ -16953,9 +16963,10 @@ static std::optional<CombineResult>
 canFoldToVWWithSameExtension(SDNode *Root, const NodeExtensionHelper &LHS,
                              const NodeExtensionHelper &RHS, SelectionDAG &DAG,
                              const RISCVSubtarget &Subtarget) {
-  return canFoldToVWWithSameExtensionImpl(
-      Root, LHS, RHS, ExtKind::ZExt | ExtKind::SExt | ExtKind::FPExt, DAG,
-      Subtarget);
+  return canFoldToVWWithSameExtensionImpl(Root, LHS, RHS,
+                                          ExtKind::ZExt | ExtKind::SExt |
+                                              ExtKind::FPExt | ExtKind::BF16Ext,
+                                          DAG, Subtarget);
 }
 
 /// Check if \p Root follows a pattern Root(LHS, ext(RHS))