[llvm] [ms] [llvm-ml] Allow PTR casting of registers to their own size (PR #132751)

[Saleem Abdulrasool via llvm-commits]

================
@@ -2601,9 +2631,20 @@ bool X86AsmParser::parseIntelOperand(OperandVector &Operands, StringRef Name) {
       return Error(Start, "rip can only be used as a base register");
     // A Register followed by ':' is considered a segment override
     if (Tok.isNot(AsmToken::Colon)) {
-      if (PtrInOperand)
-        return Error(Start, "expected memory operand after 'ptr', "
-                            "found register operand instead");
+      if (PtrInOperand) {
+        if (!Parser.isParsingMasm())
+          return Error(Start, "expected memory operand after 'ptr', "
+                              "found register operand instead");
+
+        // If we are parsing MASM, we are allowed to cast registers to their own
+        // sizes, but not to other types.
+        if (RegSizeInBits(*getContext().getRegisterInfo(), RegNo) != Size)
+          return Error(
+              Start,
+              "cannot cast register '" +
+                  StringRef(getContext().getRegisterInfo()->getName(RegNo)) +
+                  "' to '" + SizeStr + "'; size does not match");
----------------
compnerd wrote:

I wonder if the size class in the diagnostic message is helpful. I don't think that
```
cannot cast register 'AL' to 'word'; size does not match
```
is particularly helpful. Perhaps something more along the lines of:
```
8-bit GPR 'AX' cannot be used as a 32-bit GPR
```

https://github.com/llvm/llvm-project/pull/132751