Re: [Xen-devel] [PATCH v3 5/5] x86: remove usage of .skip with non-absolute expressions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.01.18 at 13:43, <wei.liu2@xxxxxxxxxx> wrote:
> On Mon, Jan 29, 2018 at 12:26:43PM +0000, Roger Pau Monne wrote:
>> Clang assembler doesn't support using .skip with non-absolute
>> expressions:
>> 
> 
> But so is GNU as. From its manual for .skip:
> 
> "This directive emits size bytes, each of value fill. Both size and fill
> are absolute expressions."
> 
>> entry.S:109:15: error: expected absolute expression
>>         .skip .Lcr4_alt_end - .Lcr4_alt, 0x90
>>               ^
>> 
> 
> OOI what makes .Lcr4_alt_end - .Lcr4_alt non-absolute?

I guess they expect to be able to calculate the value right at the
point they evaluate .skip's arguments. Whereas gas either records
a fragment with variable size, or (less likely, as that could end up
wrong) evaluates the expression right away (iirc the .skip sits after
the definition of both symbols, and iirc further gas [at least some
versions] has problems if that wasn't the case).

Jan


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