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Re: [Xen-devel] [PATCH] libx86: Work around GCC bug with ebx output constrants
- To: Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 19 Nov 2018 15:00:59 +0000
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 19 Nov 2018 15:02:07 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 19/11/2018 14:52, Mihai Donțu wrote:
> On Mon, 2018-11-19 at 13:29 +0000, Andrew Cooper wrote:
>> On 19/11/2018 13:11, Andrew Cooper wrote:
>>> Some versions of GCC can't compile cpuid.c, and fail with the rather
>>> cryptic:
>>>
>>> In file included from lib/x86/cpuid.c:3:0:
>>> lib/x86/cpuid.c: In function ‘x86_cpuid_policy_fill_native’:
>>> include/xen/lib/x86/cpuid.h:25:5: error: inconsistent operand constraints
>>> in an ‘asm’
>>> asm ( "cpuid"
>>> ^
>>>
>>> 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.
>> Actually, having just got GCC 5 working, that is also fine. I'll adjust
>> the wording/check, but won't bother posting a v2 if that is the only change.
> I wonder if it has anything to do with this:
>
> "Reuse of the PIC hard register, instead of using a fixed register, was
> implemented on x86/x86-64 targets. This improves generated PIC code
> performance as more hard registers can be used. Shared libraries can
> significantly benefit from this optimization. Currently it is switched
> on only for x86/x86-64 targets. As RA infrastructure is already
> implemented for PIC register reuse, other targets might follow this in
> the future."
That is almost certainly it. Looks like it is down to the pre 5.0 code
considering %rBX non-spillable
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54232
Thanks for digging this out. I missed it while scanning the 5
changelog. I think I'll tweak the text to reflect this.
~Andrew
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