[CodeGen] Forbid passing a PointerType to MVT::getVT and EVT::getEVT

There is the expectation throughout CodeGen that, for types representing
"real" values, the MVT or EVT is self-contained. However, MVT::iPTR is
challenging, because it has no address space, and even if it did, there
often is no DataLayout immediately accessible to determine what actually
is the underlying type.

Historically it was documented as being TableGen-only, but that was lost
in 631bfdb's conversion to using the
generated defines. Let's preserve that intent by not allowing it to
originate through accidental calls to get(E)VT with a PointerType. If
you need to support that, be sure to use something like TargetLowering's
getValueType, which takes a DataLayout and can map pointers to their
concrete MVTs. Whilst here, reintroduce documentation about these value
types being TableGen-only.

Author: jrtc27

llvm/include/llvm/CodeGen/ValueTypes.h

@@ -488,8 +488,10 @@ namespace llvm {
     Type *getTypeForEVT(LLVMContext &Context) const;
 
     /// Return the value type corresponding to the specified type.
-    /// This returns all pointers as iPTR.  If HandleUnknown is true, unknown
-    /// types are returned as Other, otherwise they are invalid.
+    /// If HandleUnknown is true, unknown types are returned as Other,
+    /// otherwise they are invalid.
+    /// NB: This includes pointer types, which require a DataLayout to convert
+    /// to a concrete value type.
     static EVT getEVT(Type *Ty, bool HandleUnknown = false);
 
     intptr_t getRawBits() const {

llvm/include/llvm/CodeGen/ValueTypes.td

@@ -296,18 +296,23 @@ def MetadataVT : ValueType<0, 249> { // Metadata
 def iPTRAny    : VTAny<250>;
 
 // Pseudo valuetype to represent "vector of any size"
+// Should only be used in TableGen.
 def vAny       : VTAny<251>;
 
 // Pseudo valuetype to represent "float of any format"
+// Should only be used in TableGen.
 def fAny       : VTAny<252>;
 
 // Pseudo valuetype to represent "integer of any bit width"
+// Should only be used in TableGen.
 def iAny       : VTAny<253>;
 
 // Pseudo valuetype mapped to the current pointer size.
+// Should only be used in TableGen.
 def iPTR       : ValueType<0, 254>;
 
 // Pseudo valuetype to represent "any type of any size".
+// Should only be used in TableGen.
 def Any        : VTAny<255>;
 
 } // end defset ValueTypes

llvm/include/llvm/CodeGenTypes/MachineValueType.h

@@ -476,9 +476,11 @@ namespace llvm {
       return getVectorVT(VT, EC.getKnownMinValue());
     }
 
-    /// Return the value type corresponding to the specified type.  This returns
-    /// all pointers as iPTR.  If HandleUnknown is true, unknown types are
-    /// returned as Other, otherwise they are invalid.
+    /// Return the value type corresponding to the specified type.
+    /// If HandleUnknown is true, unknown types are returned as Other,
+    /// otherwise they are invalid.
+    /// NB: This includes pointer types, which require a DataLayout to convert
+    /// to a concrete value type.
     static MVT getVT(Type *Ty, bool HandleUnknown = false);
 
   public:

llvm/lib/CodeGen/ValueTypes.cpp

@@ -579,9 +579,11 @@ Type *EVT::getTypeForEVT(LLVMContext &Context) const {
   // clang-format on
 }
 
-/// Return the value type corresponding to the specified type.  This returns all
-/// pointers as MVT::iPTR.  If HandleUnknown is true, unknown types are returned
-/// as Other, otherwise they are invalid.
+/// Return the value type corresponding to the specified type.
+/// If HandleUnknown is true, unknown types are returned as Other, otherwise
+/// they are invalid.
+/// NB: This includes pointer types, which require a DataLayout to convert
+/// to a concrete value type.
 MVT MVT::getVT(Type *Ty, bool HandleUnknown){
   assert(Ty != nullptr && "Invalid type");
   switch (Ty->getTypeID()) {
@@ -611,7 +613,6 @@ MVT MVT::getVT(Type *Ty, bool HandleUnknown){
   case Type::X86_AMXTyID:   return MVT(MVT::x86amx);
   case Type::FP128TyID:     return MVT(MVT::f128);
   case Type::PPC_FP128TyID: return MVT(MVT::ppcf128);
-  case Type::PointerTyID:   return MVT(MVT::iPTR);
   case Type::FixedVectorTyID:
   case Type::ScalableVectorTyID: {
     VectorType *VTy = cast<VectorType>(Ty);
@@ -622,9 +623,11 @@ MVT MVT::getVT(Type *Ty, bool HandleUnknown){
   }
 }
 
-/// getEVT - Return the value type corresponding to the specified type.  This
-/// returns all pointers as MVT::iPTR.  If HandleUnknown is true, unknown types
-/// are returned as Other, otherwise they are invalid.
+/// getEVT - Return the value type corresponding to the specified type.
+/// If HandleUnknown is true, unknown types are returned as Other, otherwise
+/// they are invalid.
+/// NB: This includes pointer types, which require a DataLayout to convert
+/// to a concrete value type.
 EVT EVT::getEVT(Type *Ty, bool HandleUnknown){
   switch (Ty->getTypeID()) {
   default: