Re: [Xen-devel] [PATCH] libx86: Work around GCC bug with ebx output constrants

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.11.18 at 15:08, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 19/11/2018 13:51, Jan Beulich wrote:
>>>>> On 19.11.18 at 14:11, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> In practice, this is a collision between the output constraint and the GOT
>>> which is held in %ebx when compiling with -fPIC for libraries.
>>>
>>> This affects at least GCC 4.9 as shipped in Debian Jessie, but 
>>> experimentally
>>> is fixed in GCC 6 and later.  Curiously, it only affects 32-bit builds.
>> I don't think there's anything curious here: The GOT (or actually
>> .rodata here) gets accessed by %rip-relative addressing in small
>> model 64-bit code, iirc.
> 
> Aah - good point, and also explains the other conditionals in GCC's
> intrinsic header.
> 
> I'll update the logic to cope with the larger 64bit models.

Do you need to? The ABI specifies that the GOT pointer lives
in %r15 in that case.

Jan



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