[Bug c/18065] usual arithmetic conversion not applying correctly

[schlie at comcast dot net]

------- Additional Comments From schlie at comcast dot net  2004-10-20 21:19 -------
Subject: Re:  usual arithmetic conversion not applying
 correctly

Andrew,

Yes, and with respect to / and % operations:

- the modulo/reminder result can always be represented in the same type
  and size as its divisor/modulus operand, regardless of the dividend's
  type and size.

- the quotient result can always be represented in the same type and size
  as it's dividend, if it's divisor/modulus operand is unsigned, otherwise
  ideally to be promoted to it's next larger signed type to preserve
  accuracy if useful, but typically irrelevant, as operator result
  precision is in most practical circumstances determined by the precision
  of the result's need, therefore in typical cases like int = int / int
  no value is derived from pretending to preserve idealized accuracy,
  therefore CPU's typically implement instructions to yield the equivalent
  results that would have been attained if the precision were maintained,
  and then simply truncated when assigned to it's destination type for
  typical integer operations but mark an under or overflow flag, just as
  int = int + int; would overflow; therefore for all practical purposes,
  integer operands should never be promoted beyond their need, nor should
  operation precision be required beyond it's result's need.

(and just to double check, this needs to get fixed on 3.4.3 too, which
 I honestly suspect is related to the rtl interpretation, as other QI
 mode instruction specifications are properly matched as expected.)

(also it's likely that these type of problems predominantly express
 themselves on small target machines, as on larger 32/64 bit machines,
 operations physically occur on correspondingly wide cpu's, therefore
 don't enable inappropriate operand promotion errors to be easily
 observable.)

-paul-

> From: pinskia at gcc dot gnu dot org <gcc-bugzilla@gcc.gnu.org>
> Reply-To: <gcc-bugzilla@gcc.gnu.org>
> Date: 20 Oct 2004 19:29:13 -0000
> To: <schlie@comcast.net>
> Subject: [Bug c/18065] usual arithmetic conversion not applying correctly
> 
> 
> ------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-20
> 19:29 -------
> And the reason why we don't shoten back to signed char % signed char:
> 
>  /* Although it would be tempting to shorten always here, that loses
>     on some targets, since the modulo instruction is undefined if the
>     quotient can't be represented in the computation mode.  We shorten
>     only if unsigned or if dividing by something we know != -1.  */
> 
> So maybe this is correct.
> 
> -- 
> 
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18065
> 
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-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18065