[05/16] system_data_types.7: Add int_fastN_t family of types

  Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
---
 man7/system_data_types.7 | 76 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)
  

On Thu, 1 Oct 2020 at 11:25, Alejandro Colomar via Gcc <gcc@gcc.gnu.org> wrote:
>
> Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
> ---
>  man7/system_data_types.7 | 76 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 76 insertions(+)
>
> diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
> index a301c2309..07de6417f 100644
> --- a/man7/system_data_types.7
> +++ b/man7/system_data_types.7
> @@ -329,6 +329,82 @@ C99 and later; POSIX.1-2001 and later.
>  See also:
>  .BR imaxdiv (3)
>  .RE
> +.\"------------------------------------- int_fastN_t ------------------/
> +.TP
> +.IR int_fast N _t
> +.RS
> +Include:
> +.IR <stdint.h> .
> +Alternatively,
> +.IR <inttypes.h> .
> +.PP
> +.IR int_fast8_t ,
> +.IR int_fast16_t ,
> +.IR int_fast32_t ,
> +.I int_fast64_t
> +.PP
> +The fastest signed integer type
> +of a width of at least N bits,
> +N being the value specified in its type name.
> +According to the C language standard, they shall be
> +capable of storing values in the range
> +.RB [ INT_FAST N _MIN ,
> +.BR INT_FAST N _MAX ],
> +substituting N by the appropriate number.
> +.PP
> +The length modifiers for the
> +.IR int_fast N _t
> +types for the
> +.BR printf (3)
> +family of functions
> +are expanded by macros of the forms
> +.BR PRIdFAST N
> +and
> +.BR PRIiFAST N
> +(defined in
> +.IR <inttypes.h> );
> +resulting for example in
> +.B %"PRIdFAST64"
> +or
> +.B %"PRIiFAST64"
> +for printing
> +.I int_fast64_t
> +values.
> +The length modifiers for the
> +.IR int_fast N _t
> +types for the
> +.BR scanf (3)
> +family of functions
> +are expanded by macros of the forms
> +.BR SCNdFAST N
> +and
> +.BR SCNiFAST N,
> +(defined in
> +.IR <inttypes.h> );
> +resulting for example in
> +.B %"SCNdFAST8"
> +or
> +.B %"SCNiFAST8"
> +for scanning
> +.I int_fast8_t
> +values.
> +.PP
> +Conforming to:
> +C99 and later; POSIX.1-2001 and later.
> +.PP
> +Notes:
> +Some of these types may be optimized for size
> +instead of raw performance.

I'm not sure what this tells me as a programmer. What does "raw
performance" means exactly? The text above says it's "the fastest",
but then it says "may be optimized for size". I don't know how to
interpret this. Is it fast or is it small, or something else? Is it
optimized for small size? Natural word size? Cacheline size?

I prefer the phrasing of the caveats in the C and POSIX standards
which just say it might not be fastest for all purposes.

How about "Where there is no single type that is fastest for all
purposes, the implementation may choose any type with the required
signedness and at least the minimum width."

I don't see anything in this man page saying that the <stdint.h> types
are all typedefs, rather than new types that are distinct from the
standard integer types. That seems like useful information.
  

On 2020-10-01 13:07, Jonathan Wakely wrote:
[...]
>> +Notes:
>> +Some of these types may be optimized for size
>> +instead of raw performance.
> 
> I'm not sure what this tells me as a programmer. What does "raw
> performance" means exactly? The text above says it's "the fastest",
> but then it says "may be optimized for size". I don't know how to
> interpret this. Is it fast or is it small, or something else? Is it
> optimized for small size? Natural word size? Cacheline size?
> 
> I prefer the phrasing of the caveats in the C and POSIX standards
> which just say it might not be fastest for all purposes.
> 
> How about "Where there is no single type that is fastest for all
> purposes, the implementation may choose any type with the required
> signedness and at least the minimum width."
> 
> I don't see anything in this man page saying that the <stdint.h> types
> are all typedefs, rather than new types that are distinct from the
> standard integer types. That seems like useful information.
> 

Hi Jonathan,

I wasn't sure about how to word it.

In theory, they should be the fastest types; just that.
But then, for some reason, GCC decided that
int_fast8_t should be int8_t instead of int64_t,
because when using arrays of int_fast8_t,
it will create smaller arrays, which will be faster (less cache, etc.).

(I remember having read that a long time ago, but I don't remember the 
source, or if it's the actual reason).

How would you word that?

Thanks,

Alex
  

On Thu, 1 Oct 2020 at 12:15, Alejandro Colomar <colomar.6.4.3@gmail.com> wrote:
>
>
>
> On 2020-10-01 13:07, Jonathan Wakely wrote:
> [...]
> >> +Notes:
> >> +Some of these types may be optimized for size
> >> +instead of raw performance.
> >
> > I'm not sure what this tells me as a programmer. What does "raw
> > performance" means exactly? The text above says it's "the fastest",
> > but then it says "may be optimized for size". I don't know how to
> > interpret this. Is it fast or is it small, or something else? Is it
> > optimized for small size? Natural word size? Cacheline size?
> >
> > I prefer the phrasing of the caveats in the C and POSIX standards
> > which just say it might not be fastest for all purposes.
> >
> > How about "Where there is no single type that is fastest for all
> > purposes, the implementation may choose any type with the required
> > signedness and at least the minimum width."
> >
> > I don't see anything in this man page saying that the <stdint.h> types
> > are all typedefs, rather than new types that are distinct from the
> > standard integer types. That seems like useful information.
> >
>
> Hi Jonathan,
>
> I wasn't sure about how to word it.
>
> In theory, they should be the fastest types; just that.
> But then, for some reason, GCC decided that
> int_fast8_t should be int8_t instead of int64_t,
> because when using arrays of int_fast8_t,
> it will create smaller arrays, which will be faster (less cache, etc.).
>
> (I remember having read that a long time ago, but I don't remember the
> source, or if it's the actual reason).

So then that's still optimized for "raw performance", isn't it? The
"raw performance" of copying an array of bytes is better than the "raw
performance" of copying an array of 64-bit types. The meaning of "raw
performance" depends on what you're doing, so I don't think it's a
useful term without context.

> How would you word that?

I gave a suggestion above.

Don't use terms like "raw performance" that are meaningless without
context. Using "no single type that is fastest for all purposes" makes
it clearer that "fastest" isn't something universally true, it might
be fastest for some purposes and not others.
  

On 2020-10-01 13:27, Jonathan Wakely wrote:
>> How would you word that?
> 
> I gave a suggestion above.

Oops, I missed that.

Thanks,

Alex
  

On 2020-10-01 13:07, Jonathan Wakely wrote:

> 
> I don't see anything in this man page saying that the <stdint.h> types
> are all typedefs, rather than new types that are distinct from the
> standard integer types. That seems like useful information.
> 

Hello Jonathan,

I almost missed this.

We (Michael and I) chose not to give any information
about typedefs in this page.
So the reader should not assume that any type at all,
unless explicitly specified, is a typedef.
Any type could be defined however the implementation chooses to do,
and this page will only give the requirements for the implementation.

However, if you still think we should be clearer in this point,
we might create a NOTE about this in the end of the page.

Your thoughts?

Thanks,

Alex