clobber list in inline assembly for calls

clobber list in inline assembly for calls

[Daniel Mierswa]

Good day,

I was wondering which registers I have to clobber or which special
strings I have to put into the clobber list if all I'm going to do is:

asm("call %0;" ::"m"(addr_of_myfunc));

As I can't tell what the function is doing I may have to clobber all
registers or am I wrong?
I had in mind to clobber "cc" plus "eax", "ecx", "edx" which are by the
i386 ABI not preserved for the caller.

I couldn't find any relevant documentation about this in several
How-Tos. Thanks in advance for any reponses.

-- 
Mierswa, Daniel

If you still don't like it, that's ok: that's why I'm boss. I simply
know better than you do.
               --- Linus Torvalds, comp.os.linux.advocacy, 1996/07/22

Re: clobber list in inline assembly for calls

[Ian Lance Taylor]

Daniel Mierswa <impulze@impulze.org> writes:

> I was wondering which registers I have to clobber or which special
> strings I have to put into the clobber list if all I'm going to do is:
>
> asm("call %0;" ::"m"(addr_of_myfunc));
>
> As I can't tell what the function is doing I may have to clobber all
> registers or am I wrong?
> I had in mind to clobber "cc" plus "eax", "ecx", "edx" which are by the
> i386 ABI not preserved for the caller.

You are correct: you have to list all the caller-saved registers in the
clobber list.  On 32-bit x86 this is all the registers other than %ebp,
%ebx, %edi and %esi.

Ian

Re: clobber list in inline assembly for calls

[Daniel Mierswa]

On 08.01.2011 05:50, Ian Lance Taylor wrote:
> You are correct: you have to list all the caller-saved registers in the
> clobber list.  On 32-bit x86 this is all the registers other than %ebp,
> %ebx, %edi and %esi.
> 
> Ian

If I'm not mistaken that should leave eax, ecx and edx. What about "cc"
and "memory" though?

-- 
Mierswa, Daniel

If you still don't like it, that's ok: that's why I'm boss. I simply
know better than you do.
               --- Linus Torvalds, comp.os.linux.advocacy, 1996/07/22

Re: clobber list in inline assembly for calls

[Ian Lance Taylor]

Daniel Mierswa <impulze@impulze.org> writes:

> On 08.01.2011 05:50, Ian Lance Taylor wrote:
>> You are correct: you have to list all the caller-saved registers in the
>> clobber list.  On 32-bit x86 this is all the registers other than %ebp,
>> %ebx, %edi and %esi.
>> 
>> Ian
>
> If I'm not mistaken that should leave eax, ecx and edx. What about "cc"
> and "memory" though?

Good point, yes, you should add an explicit "memory" clobber.  You don't
need to explicitly clobber "cc".  That is a fixed register (actually gcc
calls it "flags") and as such gcc won't make any particular assumptions
about it in any case.

Ian

Re: clobber list in inline assembly for calls

[Daniel Mierswa]

On 08.01.2011 06:23, Ian Lance Taylor wrote:
> Good point, yes, you should add an explicit "memory" clobber.  You don't
> need to explicitly clobber "cc".  That is a fixed register (actually gcc
> calls it "flags") and as such gcc won't make any particular assumptions
> about it in any case.
> 
> Ian

Ok I'm curious as to why.

If the information at
http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#ss5.3 is
of any use then I'd have to:

[...]
If our instruction can alter the condition code register, we have to add
"cc" to the list of clobbered registers.

If our instruction modifies memory in an unpredictable fashion, add
"memory" to the list of clobbered registers.
[...]

I don't know if any of both situations may happen in my called function,
hence I'd have to supply both, or am I wrong?

-- 
Mierswa, Daniel

If you still don't like it, that's ok: that's why I'm boss. I simply
know better than you do.
               --- Linus Torvalds, comp.os.linux.advocacy, 1996/07/22

Re: clobber list in inline assembly for calls

[Ian Lance Taylor]

Daniel Mierswa <impulze@impulze.org> writes:

> On 08.01.2011 06:23, Ian Lance Taylor wrote:
>> Good point, yes, you should add an explicit "memory" clobber.  You don't
>> need to explicitly clobber "cc".  That is a fixed register (actually gcc
>> calls it "flags") and as such gcc won't make any particular assumptions
>> about it in any case.
>> 
>> Ian
>
> Ok I'm curious as to why.
>
> If the information at
> http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#ss5.3 is
> of any use then I'd have to:
>
> [...]
> If our instruction can alter the condition code register, we have to add
> "cc" to the list of clobbered registers.
>
> If our instruction modifies memory in an unpredictable fashion, add
> "memory" to the list of clobbered registers.
> [...]
>
> I don't know if any of both situations may happen in my called function,
> hence I'd have to supply both, or am I wrong?

You can add a clobber of "cc" if you like, it just won't make any
difference one way or another.  On x86 gcc already assumes that the
flags are clobbered by an asm.

Ian