Re: [Xen-devel] [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

[Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>]

On Thu, 8 Mar 2012, Richard Earnshaw wrote:

> On 02/03/12 21:15, Nicolas Pitre wrote:
> > So, to me, the gcc documentation is perfectly clear on this topic.
> > there really _is_ a guarantee that those asm marked variables will be in
> > the expected registers on entry to the inline asm, given that the
> > variable is _also_ listed as an operand to the asm statement.  But only
> > in that case.
> >
> > It is true that gcc may reorder other function calls or other code
> > around the inline asm and then intervening code can clobber any
> > registers.  Then it is up to gcc to preserve the variable's content
> > elsewhere when its register is used for other purposes, and restore it
> > when some inline asm statement is referring to it.
> >
> > And if gcc does not do this then it is buggy.  Version 3.4.0 of gcc was
> > buggy.  No other gcc versions in the last 7 years had such a problem or
> > the __asmeq macro in the kernel would have told us.
> >
> >> Or, to summarise another way, there is no way to control which register
> >> is used to pass something to an inline asm in general (often we get away
> >> with this, and there are a lot of inline asms in the kernel that assume
> >> it works, but the more you inline the more likely you are to get nasty
> >> surprises).
> >
> > This statement is therefore unfounded and wrong.  Please direct the
> > tools guy who mislead you to the above gcc documentation.
> >
> 
> The problem is not really about re-ordering functions but about implicit
> functions that come from the source code; for example
> 
> int foo (int a, int b)
> {
>   register int x __asm__("r0") = 33;
> 
>   register int c __asm__("r1") = a / b; /* Ooops, clobbers r0 with
> division function call.  */
> 
>   asm ("svc 0" : : "r" (x));
> }
> 
> There's nothing in the specification to say what happens if there's a
> statement in the code that causes an implicit clobber of your assembly
> register.

I'm sure gcc is full of implicit behaviors that are not mentioned in 
the specification.  But as long as the specification is respected, then 
there is no need to mention any unobservable side effects from a program 
flow point of view, right?

Why wouldn't gcc be able to respect the documented feature by 
preventing live variable from being clobbered and reloading them in 
the specified register at the inline asm entry point, just like it does 
for function calls?

Here's an example code that shows that, unfortunately, gcc is still 
broken with regards to the documented behavior:

extern int bar(int);
int foo(int y)
{
        register int x __asm__("r1") = 33;
        y += bar(x);
        asm ("@ x should be live in %0 here" : "+r" (x) : "r" (y));
        y += bar(x);
        asm ("@ x should be live in %0 here" : "+r" (x) : "r" (y));
        return x;
}

Result is:

foo:
        stmfd   sp!, {r4, lr}
        mov     r4, r0
        mov     r0, #33
        bl      bar
        add     r4, r0, r4
        @ x should be live in r1 here
        mov     r0, r1
        bl      bar
        add     r0, r0, r4
        @ x should be live in r1 here
        mov     r0, r1
        ldmfd   sp!, {r4, lr}
        bx      lr

To me this is clearly a bug if gcc is not able to meet the documented 
expectation.  And the documented expectation is not at all unreasonable.


Nicolas

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