basic asm and memory clobbers

[Segher Boessenkool]

On Sun, Nov 08, 2015 at 04:10:01PM -0800, David Wohlferd wrote:
> It seems like a doc update is what is needed to close PR24414 (Old-style 
> asms don't clobber memory).

What is needed to close the bug is to make the compiler work properly.

Whether that means clobbering memory or not, I don't much care -- with
the status quo, if you want your asm to clobber memory you have to use
extended asm; if basic asm is made to clobber memory, if you want your
asm to *not* clobber memory you have to use extended asm (which you
can with no operands by writing e.g.  asm("bork" : );  ).  So both
behaviours are available whether we make a change or not.

But changing things now will likely break user code.

>  Safely accessing C data and calling functions from basic @code{asm} is more 
> -complex than it may appear. To access C data, it is better to use extended 
> +complex than it may appear. To access C data (including both local and
> +global register variables), use extended
>  @code{asm}.

I don't think this makes things clearer.  Register vars are described
elsewhere already; if you really think it needs mentioning here, put
it at the end (in its own sentence), don't break up this sentence.

(dot space space).

> +Basic @code{asm} statements are not treated as though they used a "memory"
> +clobber, although they do implicitly perform a clobber of the flags
> +(@pxref{Clobbers}).

They do not clobber the flags.  Observe:

===
void f(int a)
{
        a = a >> 2;
        if (a <= 0)
                asm("OHAI");
        if (a >= 0)
                asm("OHAI2");
}
===

Compiling this for powerpc gives (-m32, edited):

f:
        srawi. 9,3,2    # this sets cr0
        ble 0,.L5       # this uses cr0
.L2:
        OHAI2
        blr
        .p2align 4,,15
.L5:
        OHAI
        bnelr 0         # this uses cr0
        b .L2

which shows that CR0 (which is "cc") is live over the asm.  So are all
other condition regs.

It is true for cc0 targets I guess, but there aren't many of those left.

> Also, there is no implicit clobbering of registers,
> +so any registers changed must be restored to their original value before
> +exiting the @code{asm}.

One of the important uses of asm is to set registers GCC does not know
about, so you might want to phrase this differently.


Segher