Re: Forcing inline assembly code to be produced

[Kristis Makris <kristis dot makris at asu dot edu>]

>  > In a more complicated example, I'm trying to position uncommonly used
>  > code in a separate memory region for better cache locality. One of the
>  > inline assembly statements I'm using is not produced. This results in
>  > not changing back to the ".text" section, resulting in further assembler
>  > problems as described in:
> 
> Use an attribute to set the section.
> 
>          extern void foobar (void) __attribute__ ((section ("bar")));
> 
>      puts the function `foobar' in the `bar' section.

This definition method is not granular enough. I don't want the entire
function in the 'bar' section. I want the entire function in the 'text'
section but in some cases in the function where there is an if-statement
like this:

if (something) {
/* body /*
}

...I want to place the body, which is written in C, in the 'bar'
section. And everything after the if-statement should remain in the
'text' section.



I'm currently producing 

if (something) {
  relocate_pre("if_something");
  /* body */
  relocate_post();
}

And then defining:

#define relocate_pre(name) \
   asm volatile ( "jmp 1f\n\t" \
 	          ".subsection 2\n\t" \
                  ".section .bar\n" \
  		  "hcu_uncommon_section_" name ":\n\t" \
                  "1:\n\t" );

#define relocate_post() \
   asm volatile( "jmp 1f\n\t" \
		 ".text\n\t" \
                 "1:\n" );


The problems begin when I hit a case like:

if (something) {
  relocate_pre("if_something");
  /* parts of the body */
  goto somewhere_else;
  relocate_post();
}

or

if (something) {
  relocate_pre("if_something");
  /* parts of the body */
  return;
  relocate_post();
}

relocate_post() seems to be eliminated, hence I don't get the switch
back to the ".text" section.

>  
> If you really need naked assembly code, do it *outside* a function:
> 
> asm ("\nfoo:\t\n"
>      "\t# wibble");
> 
> int main(void)
> {
>   return 0;
> }

The problem with using naked assembly code outside a function is that
after such code I may need to execute additional code written in C, and
presently there's no way to write such code in C outside a function. I
requested a feature like this in the past, but it was not accepted:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12118
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12284

I'm applying an automated transformation to 3rd-party source code to
produce such uncommon code in separate memory regions. Certainly,
interpreting the body of if-statements as C code and automatically
producing the corresponding naked assembly outside a function image
seems like something the compiler or assembler should be doing. It
shouldn't be hand written.

I can't help but feel that the compiler should be helping here.