[cfe-dev] (not) initializing assembly outputs with -ftrivial-auto-var-init

[James Y Knight via cfe-dev]

I'm fairly certain it's simply an accident of how it's implemented in IR.
At IR level, we mostly treat it as a call to a function that takes a
pointer to memory. And we don't have any general function-parameter
attribute which says "this call definitely overwrites the memory pointed to
by this argument", nor special handling of the same sort for an inlineasm
call. So, as far as the optimizer is concerned, there's a call with a
pointer argument. And, as with any other call, the called "function" may
read, write, or do neither to the memory pointed to by the argument.

On the other hand, for register outputs, we do handle it as an output value
of the call in IR, which is why that works properly.

GCC does seem to optimize this properly, so I think the risk that fixing
this breaks a large body of code is fairly small.

On Thu, Mar 21, 2019 at 2:23 PM JF Bastien <jfbastien at apple.com> wrote:

>
>
> On Mar 21, 2019, at 11:14 AM, James Y Knight <jyknight at google.com> wrote:
>
> If an asm's constraints claim that the variable is an output, but then
> don't actually write to it, that's a bug (at least if the value is actually
> used afterwards). An output-only constraint on inline asm definitely does
> _not_ mean "pass through the previous value unchanged, if the asm failed to
> actually write to it". If you need that behavior, it's spelled "+m", not
> "=m".
>
> We do seem to fail to take advantage of this for memory outputs (again,
> this is not just for ftrivial-auto-var-init -- we ought to eliminate manual
> initialization just the same), which I'd definitely consider an
> missing-optimization bug.
>
>
> Agreed on both counts. Maybe we don’t do the optimization because code
> tends to be wrong? If that’s the case, the optimization would make people
> sad (even though their code is wrong). We might need to validate asm
> statement constraints…
>
> I don’t want us to just blindly do the optimization and break code, even
> if said code was incorrect.
>
>
> On Thu, Mar 21, 2019 at 10:16 AM Alexander Potapenko <glider at google.com>
> wrote:
>
>> On Thu, Mar 21, 2019 at 2:58 PM James Y Knight <jyknight at google.com>
>> wrote:
>> >
>> > Please be more specific about the problem, because your simplified
>> example doesn't actually show an issue. If I write this function:
>> > int foo() {
>> >   int retval;
>> >   asm("# ..." : "=r"(retval));
>> >   return retval;
>> > }
>> > it already does get treated as definitely writing retval, and optimizes
>> away the initialization (whether you explicitly initialize retval, or use
>> -ftrivial-auto-var-init).
>> > Example: https://godbolt.org/z/YYBCXL
>> This is probably because you're passing retval as a register output.
>> If you change "=r" to "=m" (https://godbolt.org/z/ulxSgx), it won't be
>> optimized away.
>> (I admit I didn't know about the difference)
>> > On Thu, Mar 21, 2019 at 8:35 AM Alexander Potapenko via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >>
>> >> Hi JF et al.,
>> >>
>> >> In the Linux kernel we often encounter the following pattern:
>> >>
>> >> type op(...) {
>> >>   type retval;
>> >>   inline asm(... retval ...);
>> >>   return retval;
>> >> }
>> >>
>> >> , which is used to implement low-level platform-dependent memory
>> operations.
>> >>
>> >> Some of these operations turn out to be very hot, so we probably don't
>> >> want to initialize |retval| given that it's always initialized in the
>> >> assembly.
>> >>
>> >> However it's practically impossible to tell that a variable is being
>> >> written to by the inline assembly, or figure out the size of that
>> >> write.
>> >> Perhaps we could speculatively treat every scalar output of an inline
>> >> assembly routine as an initialized value (which is true for the Linux
>> >> kernel, but I'm not sure about other users of inline assembly, e.g.
>> >> video codecs).
>> >>
>> >> WDYT?
>> >>
>> >>
>> >> --
>> >> Alexander Potapenko
>> >> Software Engineer
>> >>
>> >> Google Germany GmbH
>> >> Erika-Mann-Straße, 33
>> >> 80636 München
>> >>
>> >> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
>> >> Registergericht und -nummer: Hamburg, HRB 86891
>> >> Sitz der Gesellschaft: Hamburg
>> >> _______________________________________________
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>> >> cfe-dev at lists.llvm.org
>> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>>
>>
>> --
>> Alexander Potapenko
>> Software Engineer
>>
>> Google Germany GmbH
>> Erika-Mann-Straße, 33
>> 80636 München
>>
>> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
>> Registergericht und -nummer: Hamburg, HRB 86891
>> Sitz der Gesellschaft: Hamburg
>>
>
>
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