asm("") matter, mostly on i386

[Etienne LORRAIN <etienne_lorrain at yahoo dot fr>]

 Hello,

 Here is two problems, the first is only for i386:
---------------------
bash-2.02$ cat bug1.c
void fct (void)
  {
  asm ("" : : : "ebp");
  }
bash-2.02$ xgcc -S bug1.c
bash-2.02$ cat bug1.s
        .file   "bug1.c"
        .version        "01.01"
gcc2_compiled.:
___gnu_compiled_c:
.text
        .align 16
.globl _fct
        .def    _fct;   .scl    2;      .type   32;     .endef
_fct:
        pushl   %ebp
        movl    %esp, %ebp
/APP
/NO_APP
L2:
        movl    %ebp, %esp ###### %ebp is reused even if clobbered !
        popl    %ebp
        ret
bash-2.02$ xgcc -v
Using builtin specs.
gcc version 2.96 19991201 (experimental)
bash-2.02$
---------------------
  I do not know if the problem is that "ebp" is accepted
 on the clobber list or that there is a problem in its
 save/restore. It does not work neither when -fomit-frame-pointer
 is specified - but only for functions using the %ebp register
 to store a variable (else %ebp is correctly pushed/poped).

  It would be nice to be able to also use this "ebp" register in
 input/output of asm("int $0x10" : "+B"(...) ) - but I would
 be the only guy to use it - so it is OK to give an error
 message.


  The second thing is something I have already reported, but in
 another form - i.e. the const'ness of an inline function:

---------------------
bash-2.02$ cat bug3.c
static unsigned inline peekl (unsigned addr)
  {
  unsigned val;

  asm (" mov %%gs:%a1,%0 " : "=r" (val) : "r" (addr));
  return val;
  }

unsigned fct (unsigned addr)
  {
  unsigned val1, val2;

  val1 = peekl (addr);
  asm ( "" ::: "memory");
  val2 = peekl (addr);

  return val2 - val1;
  }
bash-2.02$ xgcc -S -O -fomit-frame-pointer bug3.c
bash-2.02$ cat bug3.s
        .file   "bug3.c"
        .version        "01.01"
gcc2_compiled.:
___gnu_compiled_c:
.text
        .align 16
.globl _fct
        .def    _fct;   .scl    2;      .type   32;     .endef
_fct:
/APP
/NO_APP
        movl    $0, %eax
        ret
bash-2.02$ xgcc -v
Using builtin specs.
gcc version 2.96 19991201 (experimental)
---------------------

  I know that 'asm (" mov %%gs:%a1,%0 " :::)' could be declared
 volatile - but look at what it does - it is not _volatile_ (for
 instance if the returned value is ignored, it does not worth
 inserting the assembly) it is just _NOT_ const - it depends
 on "memory" (and it is not clobbering it). Declaring a function
 "volatile" is not the opposite of declaring it "const" - and
 it seems that "peekl()" has been optimised like a const function,
 without having explicitely the const attribute - the asm() does
 not have the const attribute neither.

 Have a nice day,
  Etienne.
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