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

[David Malcolm <dmalcolm at redhat dot com>]

On Tue, 2014-08-05 at 03:20 +0530, Prathamesh Kulkarni wrote:

> 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
> 


> 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.

Not quite: there is an option ENABLE_RTL_CHECKING to do this, but it's
so expensive that AIUI it's rarely used.

> 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.

Note that to "make RTL type-safe" could be ambiguous: I'm looking at
typesafety of GCC's own types, rather than the user's.  My plans
involving making a distinction between RTL instructions vs expressions
i.e. the types of GCC's own implementation.  AIUI, by the time we reach
rtx most of the higher-level types in the *code being compiled* don't
really exist anymore; we're just working with bit patterns of varying
lengths - and it's not my intention to change the latter.

> 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. 

(nods)   I've been cleaning up and testing the patch series (currently
at 236 patches) and hope to post it for review later today.