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

[Alexander Potapenko via cfe-dev]

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
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--
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