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So after researching engines a lot I've been building a 2d framework for the iphone. As you know the world of engine architecture is vast so I've been trying to apply best practices as much as possible.

I've been using:

uint_fast8_t mId;

If I look up the definition of uint_fast8_t I find:

/* 7.18.1.3 Fastest-width integer types */
...
typedef uint8_t          uint_fast8_t;

And I've been using these types throughout my code - My question is, is there a performance benefit to using these types? And what exactly is going on behind the scenes? Besides the obvious fact that this is correct data type (unsigned 8 bit integer) for the data, is it worthwhile to have this peppered throughout my code?

Is this a needless optimization that the compiler would probably take care of anyways?

Thanks.

Edit: No responses/answers, so I'm putting a bounty on this!

mr-sk
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3 Answers3

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the "fast" integer types are defined to be the fastest integer type available with at least the amount of bits required (in this case 8).

If your platform defines uint_fast8_t as uint8_t then there will be absolutely no difference in speed.

The reason is that there may be architectures that are slower when not using their native word length. E.g. I could find one reference where for Alpha processors uint_fast_8_t was defined to be "unsigned int".

Axel Gneiting
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  • If your platform defines uint_fast8_t as uint8_t then there will be absolutely no difference in speed. - Ah, so why do they even bother? – mr-sk Jan 08 '10 at 18:19
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    @Mr-sk: Because you want your code to run on more than one platform. Say your algorithm requires at least 8 bits for the type, but does not require that it be *exactly* 8 bits. You would prefer to use a larger type on platforms where `uint8_t` will be a performance hazard. Solution? use `uint_fast8_t`. – Stephen Canon Jan 08 '10 at 18:27
  • only 3 y later :) I wonder what is it with overflow in alpha case... how much is 250+250... afaik uints should overflow nicely(modulo arithmetic) – NoSenseEtAl Nov 14 '12 at 12:11
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    And a year later ;) uint_fast8_t is NOT semantically same as uint8_t. Since uint_fast8_t is defined as 'the fastest integer type with at least 8 bits' you can't count on it overflowing like a 8-bit integer. If you need that, you need to use uint8_t. For the mentioned Alpha, uint_fast8_t 250+250 would equal 500. – Mikko Rantanen Nov 14 '13 at 23:30
  • it really depends on implementation, mingw doesn't guarantee that's the fastest int type – phuclv Jan 25 '14 at 06:22
3

An uint_fast8_t is the fastest integer guaranteed to be at least 8 bits wide. Depending on your platform it could be 8 or 16 or 32 bits wide.

It isnt taken care of by the compiler itself, it does indeed make your program execute faster

Here are some resource I found, You might already have seen them http://embeddedgurus.com/stack-overflow/2008/06/efficient-c-tips-1-choosing-the-correct-integer-size/

http://www.mail-archive.com/avr-gcc-list@nongnu.org/msg03149.html

Phyllostachys
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anijhaw
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0

The header in mingw64 said the fast types are "Not actually guaranteed to be fastest for all purposes"

/*  7.18.1.3  Fastest minimum-width integer types
 *  Not actually guaranteed to be fastest for all purposes <---------------------
 *  Here we use the exact-width types for 8 and 16-bit ints.
 */
typedef signed char int_fast8_t;
typedef unsigned char uint_fast8_t;
typedef short  int_fast16_t;
typedef unsigned short  uint_fast16_t;
typedef int  int_fast32_t;
typedef unsigned  int  uint_fast32_t;
__MINGW_EXTENSION typedef long long  int_fast64_t;
__MINGW_EXTENSION typedef unsigned long long   uint_fast64_t;

and that still applies to ARM or other architectures, because using a narrow type requires zero extension or sign extension in many situations which is less optimal than a native int.

However that'll benefit in large arrays or in case or slow operations (like division). I'm not sure how slow ARM divisions are but on x86 64-bit division is much slower than 32-bit or 8-bit division

phuclv
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