[Bug c/12284] New: Add feature of preserving processor state in functions written in C

[mkgnu at gmx dot net]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

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

           Summary: Add feature of preserving processor state in functions
                    written in C
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mkgnu at gmx dot net
                CC: gcc-bugs at gcc dot gnu dot org

I am requesting the feature of telling gcc that it should create code for a
function that will leave the state of the registers exactly
as it was originally when the function was entered. So if the caller was already
using certain registers this feature could offload the responsibility of
preserving those to the callee. Ideally I would like to generate "a chunk of
code" that does not mess with the state of the processor, just like I would if
the same code was written intuitively  in asm.

The small example I can provide that further expains this is:

int a = 8;
int c = 5;

void test3()
{
  c = a + c;
}

Dump of assembler code for function test3:
0x80483cc <test3>:      push   %ebp
0x80483cd <test3+1>:    mov    %esp,%ebp
0x80483cf <test3+3>:    mov    0x80494f4,%eax
0x80483d4 <test3+8>:    add    %eax,0x80494f8
0x80483da <test3+14>:   leave  
0x80483db <test3+15>:   ret    
End of assembler dump.


I would like a way to flag function test3 so the value of the %eax register used
(or any other register that gcc may decide to use) will be preserved.


The need for this feature and possible approaches to implementing it are being
discussed in:

http://gcc.gnu.org/ml/gcc-bugs/2003-09/msg00946.html

Please check this thread or contact me for more details.