Re: RFA: Simplifying truncation and integer lowpart subregs

[Richard Sandiford <rdsandiford at googlemail dot com>]

Thanks for the review.

Eric Botcazou <ebotcazou@adacore.com> writes:
>> Tested on x86_64-linux-gnu, mipsisa32-elf and mipsisa64-elf.  Also tested
>> by making sure that there were no code differences for a set of gcc .ii
>> files on gcc20 (-O2 -march=native).  OK to install?
>
> Are you sure that generating TRUNCATEs out of nowhere in simplify_subreg is 
> always correct?

We only use TRUNCATEs when truncating the operands of an existing operation.
We don't convert the SUBREG itself to a TRUNCATE.  I don't think it matters
whether the narrowing of the operation was brought about by a TRUNCATE
or by a SUBREG.

I think modelling it as a TRUNCATE operation is correct for
!TRULY_NOOP_TRUNCATION (it's the bug that Andrew pointed out).
And we shouldn't generate an actual TRUNCATE rtx for
TRULY_NOOP_TRUNCATION (the thing about making
simplify_gen_unary (TRUNCATE, ...) no worse than simplify_gen_subreg
for those targets).  I suppose:

      /* We can't handle truncation to a partial integer mode here
         because we don't know the real bitsize of the partial
         integer mode.  */
      if (GET_MODE_CLASS (mode) == MODE_PARTIAL_INT)
        break;

might be a problem though; we should still allow a subreg to be
generated.  Is that what you were thinking of, or something else?

I'm certainly open to other ideas though.  Or do you think that I was
wrong about doing this narrowing in simplify_subreg to begin with?

>> gcc/
>> 	* machmode.h (GET_MODE_UNIT_PRECISION): New macro.
>> 	* simplify-rtx.c (simplify_truncation): New function.
>
> You should say where it comes from.

OK.

>> 	(simplify_unary_operation_1): Use it.  Remove sign bit test
>> 	for !TRULY_NOOP_TRUNCATION_MODES_P.
>
> 	(simplify_unary_operation_1) <TRUNCATE>: ...
>
>> 	(simplify_subreg): Use simplify_int_lowpart for TRUNCATE.
>
> This function doesn't exist.  And this is misleading, it's not just for 
> TRUNCATE, it's for a truncation to the lowpart.

Curses, forgot to update that part.  Thanks.

>>  /* Try to simplify a unary operation CODE whose output mode is to be
>>     MODE with input operand OP whose mode was originally OP_MODE.
>>     Return zero if no simplification can be made.  */
>> @@ -689,12 +850,6 @@ simplify_unary_operation_1 (enum rtx_cod
>>  	    op_mode = mode;
>>  	  in2 = simplify_gen_unary (NOT, op_mode, in2, op_mode);
>> 
>> -	  if (GET_CODE (in2) == NOT && GET_CODE (in1) != NOT)
>> -	    {
>> -	      rtx tem = in2;
>> -	      in2 = in1; in1 = tem;
>> -	    }
>> -
>>  	  return gen_rtx_fmt_ee (GET_CODE (op) == IOR ? AND : IOR,
>>  				 mode, in1, in2);
>>  	}
>
> Why is this hunk here?

Sorry, I've no idea :-(

>> @@ -5595,14 +5730,6 @@ simplify_subreg (enum machine_mode outer
>>        return NULL_RTX;
>>      }
>> 
>> -  /* Merge implicit and explicit truncations.  */
>> -
>> -  if (GET_CODE (op) == TRUNCATE
>> -      && GET_MODE_SIZE (outermode) < GET_MODE_SIZE (innermode)
>> -      && subreg_lowpart_offset (outermode, innermode) == byte)
>> -    return simplify_gen_unary (TRUNCATE, outermode, XEXP (op, 0),
>> -			       GET_MODE (XEXP (op, 0)));
>> -
>>    /* SUBREG of a hard register => just change the register number
>>       and/or mode.  If the hard register is not valid in that mode,
>>       suppress this simplification.  If the hard register is the stack,
>
> Likewise.

This moved to simplify_truncation:

  /* (truncate:A (truncate:B X)) is (truncate:A X).  */
  if (GET_CODE (op) == TRUNCATE)
    return simplify_gen_unary (TRUNCATE, mode, XEXP (op, 0),
			       GET_MODE (XEXP (op, 0)));

I haven't included an updated patch because of your general concern above.

Richard