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Re: basic asm and memory clobbers


On Mon, Nov 23, 2015 at 05:39:17PM -0800, David Wohlferd wrote:
> On 11/23/2015 1:44 PM, Paul_Koning@Dell.com wrote:
> >>On Nov 23, 2015, at 4:36 PM, David Wohlferd <dw@LimeGreenSocks.com> wrote:
> >>
> >>...
> >>>The more I think about it, I'm just not keen on forcing all those 
> >>>old-style asms to change.
> >>If you mean you aren't keen to change them to "clobber all," I'm with 
> >>you.  If you are worried about changing them from basic to extended, what 
> >>kinds of problems do you foresee?  I've been reading a lot of basic asm 
> >>lately, and it seems to me that most of it would be fine with a simple 
> >>colon.  Certainly no worse than the current behavior.
> >I'm not sure.  I have some asm("sync") which I think assume that this 
> >means asm("sync"::"memory")
> 
> Another excellent reason to nudge people towards using extended asm.  If 
> you saw asm("sync":::"memory"), you would *know* what it did, without 
> having to read the docs (which don't say anyway).
> 
> I'm pretty confident that asm("") doesn't clobber memory on i386, but 
> maybe that behavior is platform-specific.  Since i386 doesn't have 
> "sync", I assume you are on something else?
> 
> If you have a chance to experiment, I'd love confirmation from other 
> platforms that asm("blah") is the same as asm("blah":).  Feel free to 
> email me off list to discuss.

The extended asm one is "asm_operands", the basic asm one is "asm_input".
There are places where those are handled differently.  If any of that
matters, who knows.

Here is a test that shows that on at least PowerPC the basic asm is
identical to the extended asm without clobber (compile with -O2 -S and
-fno-ipa-icf if you want to have it easier to read).  In this case,
the basic asm is treated as not clobbering memory at the tree level
already, before expanding to RTL.


Segher


===
int a;

void ext_m(void)
{
        int t = a;
        a = 42;
        asm("lolz" : : : "memory");
        a = t;
}

void ext(void)
{
        int t = a;
        a = 42;
        asm("lolz" :);
        a = t;
}

void bas(void)
{
        int t = a;
        a = 42;
        asm("lolz");
        a = t;
}
===

PowerPC output (-m32, redacted):

ext_m:
        lis 9,a@ha
        li 8,42
        lwz 10,a@l(9)
        stw 8,a@l(9)
        lolz
        stw 10,a@l(9)
        blr

ext:
        lolz
        blr

bas:
        lolz
        blr


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