Re: [Xen-devel] [PATCH] x86/Rules: Use -mskip-rax-setup if the compiler supports it

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/01/18 12:04, Jan Beulich wrote:
>>>> On 08.01.18 at 11:46, <andrew.cooper3@xxxxxxxxxx> wrote:
>> This option is available from GCC 5 onwards, and was specifically introduced
>> as an optimisation for Linux.  When using varadic functions, the caller 
>> needs
>> to know how many floating point arguments were passed.  Xen, like Linux,
>> doesn't uses floating point arguments, so doesn't need to emit code to 
>> inform
>> varadic functions such as printk() that there are zero arguments.
>>
>> The net delta for a release build is:
>>
>>   add/remove: 0/0 grow/shrink: 35/625 up/down: 603/-5489 (-4886)
>>
>> with the single biggest change being:
>>
>>   x86_emulate                               101933  101751    -182
> What variadic function(s) are being used there? I can spot a couple
> of calls to printk() and one call to cpu_has_amd_erratum(), but
> that can hardly account to 182 saved bytes.

I wondered the same, until I remembered mkec(), which is behind every
generate_exception().

FWIW, I wasn't terribly happy with the way that one came out.

>
>> --- a/xen/arch/x86/Rules.mk
>> +++ b/xen/arch/x86/Rules.mk
>> @@ -30,3 +30,7 @@ CFLAGS += -fno-asynchronous-unwind-tables
>>  ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
>>  CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE
>>  endif
>> +
>> +# Xen doesn't ever pass SSE arguments.  Discard the SSE setup
>> +# (i.e. `xor %eax, %eax`) before varadic function calls if possible.
>> +$(call cc-option-add,CFLAGS,CC,-mskip-rax-setup)
> Considering the gcc documentation for the option, I think this wants
> to sit right next to -mno-sse. Also I think it is "variadic" (and then in
> the description as well). With that
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I'll see what I can do.

~Andrew

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