Re: [Xen-devel] [PATCH] x86/alternatives: Force inline stac() and clac()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/08/2014 21:40, Jan Beulich wrote:
>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 08/18/14 6:16 PM >>>
>> In this case, we know better than the compiler.
>>
>> gcc 4.7 (Debian Wheezy) chooses to create translation-unit-local functions
>> (even for non-debug builds) named stac() and clac(), and calls them.
>>
>> $ objdump -d xen-syms | grep -c "<stac>:"
>> 6
>>
>> $ objdump -d xen-syms | grep -o "callq  [0-9a-f]\+ <stac>" | uniq -c
>       >5 callq  ffff82d0801166c9 <stac>
>      >20 callq  ffff82d08015ef99 <stac>
>       >4 callq  ffff82d080165169 <stac>
>       >8 callq  ffff82d080188cb9 <stac>
>       >3 callq  ffff82d080228779 <stac>
>       >4 callq  ffff82d08022c5c9 <stac>
>> Forcing always_inline removes these functions, and replaces each of the 
>> callqs
>> with the expected 3byte nops.
> I'm fine putting the patch in, but isn't this a compiler bug? Creating a 
> 5-byte
> call instruction instead of a 3-byte inline expansion should even preclude 
> outdoe
> of line placement under -Os (which otherwise is the most likely reason for
> functions not getting inlined).

I am not sure.  A static inline function in a header file is never a
guarantee for forcing the inlining the function, which is why
always_inline exists.

The ALTERNATIVE() macro does contain three push/pop sections, and the
alternative() macro contains a memory clobber.  It is entirely possible
that gcc has decided early on that abstracting this as a local function
is the easiest automated way to deal with the potential side effects.

It might even be rather more clever, and deciding to optimise fewer
entries being placed in the .altinstructions section, which is an
optimisation we specifically wish to avoid.

Either way, this is a grey area, and I wouldn't say for certain that
this is a compiler bug, but certainly an outcome which we would wish to
avoid.

~Andrew

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