WIP: Fix char declarations

[j2kun]

Draft. Working notes:

The AST is

CompoundStmt 0x5621e73b38c0
|-DeclStmt 0x5621e73b37c0
| `-VarDecl 0x5621e73b3720  used one 'char' cinit
|   `-ImplicitCastExpr 0x5621e73b37a8 'char' <IntegralCast>
|     `-IntegerLiteral 0x5621e73b3788 'int' 1
`-ReturnStmt 0x5621e73b38b0
  `-ImplicitCastExpr 0x5621e73b3898 'char' <IntegralCast>
    `-BinaryOperator 0x5621e73b3878 'int' '+'
      |-ImplicitCastExpr 0x5621e73b3830 'int' <IntegralCast>
      | `-ImplicitCastExpr 0x5621e73b3818 'char' <LValueToRValue>
      |   `-DeclRefExpr 0x5621e73b37d8 'char' lvalue ParmVar 0x5621e73b3550 'x' 'char'
      `-ImplicitCastExpr 0x5621e73b3860 'int' <IntegralCast>
        `-ImplicitCastExpr 0x5621e73b3848 'char' <LValueToRValue>
          `-DeclRefExpr 0x5621e73b37f8 'char' lvalue Var 0x5621e73b3720 'one' 'char'

Looks like the right place to fix this is in MLIRASTConsumer::getMLIRType in tools/cgeist/Lib/clang-mlir.cc:5825 where it parses the integer type.

Either that or add a separate visitor for ImplicitCastExpr and trunc the arith.constant to the size of the VarDecl.

[j2kun]

Actually, it looks like the problem is in how the AST parses the binary op as a cast to int before a cast back to char.

`-ImplicitCastExpr 'char' <IntegralCast>
    `-BinaryOperator 'int' '+'
      |-ImplicitCastExpr 'int' <IntegralCast>
      | `-ImplicitCastExpr 'char' <LValueToRValue>
      |   `-DeclRefExpr 'char' lvalue ParmVar 'x' 'char'
      `-ImplicitCastExpr 'int' <IntegralCast>
        `-ImplicitCastExpr 'char' <LValueToRValue>
          `-DeclRefExpr 'char' lvalue Var 'one' 'char'

This aligns with the generated MLIR, but how is that semantically correct?

[j2kun]

OK now I'm learning that this is just part of the C standard: all arithmetic is done at int level or higher. Closing.