[GNU Tools Cauldron 2014] A proposal for type-safe RTL

[Prathamesh Kulkarni]

Hi,
    Please find attached my notes on "A proposal on type-safe RTL".
I would be grateful if you would review it for me.

Thanks,
Prathamesh
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A proposal for type-safe RTL

Author: David Malcolm

   RTL is a low-level intermediate langauge used in GCC to perform machine-dependent optimziations and
code generation. The current RTL data strucutures (rtx), is not type-safe, and type checking
cannot be performed during build-time.
The motivation of this project is to make RTL type-safe, so that type errors can be caught at build-time,
and to increase readability of code that operates on RTL.

Representation using C++ class heirarchy:
    Currently rtx is used to represent all RTL-types - instructions, expressions, etc.
The idea is to convert rtx into C++ class heirarchy, with rtx_def being base type, rtx_insn representing
instructions, etc. Using C++ class heirarchy makes it type-safe and enables type-errors to be caught at
build-time

Planned Class herirarchy: 

class rtx_def;
  class rtx_insn; /* (INSN_P (X) || NOTE_P (X)
                     || JUMP_TABLE_DATA_P (X) || BARRIER_P (X)
                     || LABEL_P (X)) */
    class rtx_real_insn;         /* INSN_P (X) */
      class rtx_debug_insn;      /* DEBUG_INSN_P (X) */
      class rtx_nonjump_insn;    /* NONJUMP_INSN_P (X) */
      class rtx_jump_insn;       /* JUMP_P (X) */
      class rtx_call_insn;       /* CALL_P (X) */
    class rtx_jump_table_data;   /* JUMP_TABLE_DATA_P (X) */
    class rtx_barrier;           /* BARRIER_P (X) */
    class rtx_code_label;        /* LABEL_P (X) */
    class rtx_note;              /* NOTE_P (X) */

Patch-series can be seen at:
http://dmalcolm.fedorapeople.org/gcc/patch-backups/rtx-classes/

Merge proposal for next GCC release:
- rtx_insn to represent RTL "instructions", subclass of rtx_def.
rtx_insn would be a big gain in type-safety.