Thread: Speed of pointers vs. speed of arrays/structs

Originally Posted by Kempelen

I don't know why, but mingw get an error "syntax error before asm", any idea?

With the code I just posed, or with your post?

My code compiles with gcc-mignw 3.4.2.

What options are you using for gcc? If you give -ansi, then all asm-statements are syntax errors, since asm is not ansi compliant. You can still use inline assembler if you use __asm__ instead of asm, in which case my code also compiles with -ansi.

Code:

register int x __asm__("ebx");

--
Mats

Can't you use the global register declaration in a header-file (that is then included everywhere)[1]? I'm pretty sure that will work. Although I still think the compiler on it's own will do a better job, so you may well be right that you won't benefit much (if anything). I'd be interested to see what happens, however.

[1] This really shouldn't cause any problems, as there won't be a global symbol for something that the compiler makes into a register.

--
Mats

Originally Posted by matsp

Can't you use the global register declaration in a header-file (that is then included everywhere)[1]? I'm pretty sure that will work. Although I still think the compiler on it's own will do a better job, so you may well be right that you won't benefit much (if anything). I'd be interested to see what happens, however.

[1] This really shouldn't cause any problems, as there won't be a global symbol for something that the compiler makes into a register.

--
Mats

I can use it. I was refering that the version of the program that is declared as extern is running OK and I get the expected results from the program. If I declare as register in the way writed above, I can compile, but when run the program don't show the same result as the non-register version, even a crash. dont know why.
Also, when compile I get an error : "warning: unknow register name: reg" that I dont understand (dont know where 'reg' came from)

(EDIT: forget to say that the crash is in a line where the global reg var is not used, so it is more extrange.....isn't it?)

Originally Posted by Kempelen

I can use it. I was refering that the version of the program that is declared as extern is running OK and I get the expected results from the program. If I declare as register in the way writed above, I can compile, but when run the program don't show the same result as the non-register version, even a crash. dont know why.
Also, when compile I get an error : "warning: unknow register name: reg" that I dont understand (dont know where 'reg' came from)

I'll have a little play...

--
Mats

This works:

Code:

// asmvars.h
register int x __asm__("ebx");

//asmvars.c
#include <stdio.h>

#include "asmvars.h"

extern int getx(void);

int main()
{
  x = 7;
  printf("x = %d\n", getx());
  return 0;
}

// asm2.c
#include "asmvars.h"

int getx()
{
  return x;
}

compiled with

Code:

gcc -Wall -ansi asmvars.c asm2.c

And I looked at the code generated, it is using ebx when I expect it to.

--
Mats

I just had a couple of thoughts:
First of all, the register you choose must be one that is "preserved" by the compiler - otherwise, it won't work. EBX, EDI, ESI are your only choice (but 64-bit machine also has R8-R15 for you to use), as EAX, ECX, EDX, EBP and ESP are used by the compiler either as scratch-pad registers (presumed lost when calling functions) or as very specific use (ESP -> stack pointer, EBP -> frame pointer).

Second, I doubt this is a commonly used feature in gcc - there may well have bugs in the compiler that make things "go wrong" if you use it in a sufficiently complex piece of code - it may for example "forget" that the register is a global variable, and use it for temporary use.

--
Mats