[PATCH]: BZ #14648: Define HAS_FMA with bit_FMA_Usable

[Andreas Jaeger]

On 10/01/2012 08:55 PM, H.J. Lu wrote:
> On Mon, Oct 1, 2012 at 11:01 AM, Andreas Jaeger <aj@suse.com> wrote:
>> On 10/01/2012 05:13 PM, H.J. Lu wrote:
>>>
>>> Hi,
>>>
>>> This patch properly detects FMA support and defines HAS_FMA with
>>> bit_FMA_Usable.  Tested on FMA machine.  OK for trunk and all applicable
>>> branches?
>>
>>
>> Please explain the reason for the change better:
>
> According to AVX spec:
>
> http://software.intel.com/sites/default/files/319433-014.pdf
>
> to detect FMA support, we should do
>
> INT supports_fma()
> {
> ; result in eax
> mov eax, 1
> cpuid
> and ecx, 018001000H
> cmp ecx, 018001000H; check OSXSAVE, AVX, FMA feature flags
> jne not_supported
> ; processor supports AVX,FMA instructions and XGETBV is enabled by OS
> mov ecx, 0; specify 0 for XFEATURE_ENABLED_MASK register
> XGETBV; result in EDX:EAX
> and eax, 06H
> cmp eax, 06H; check OS has enabled both XMM and YMM state support
> jne not_supported
> mov eax, 1
> jmp done
> NOT_SUPPORTED:
> mov eax, 0
> done:
> }
>
>> Do you only need this to implement 14649? Why do you need to backport this?
>
> It is a real bug.
>
>> Do we use HAS_FMA anywhere - and what is the bug if this does not go in?
>>
>
> The bug is similar to
>
> http://sourceware.org/bugzilla/show_bug.cgi?id=13583
> http://sourceware.org/bugzilla/show_bug.cgi?id=14059
>
> If not fixed, program may crash on FMA machines.

HJ, you need to explain better the reasons for your changes.
I would have expected something like:
  We currently enable FMA if cpuid says it's available but we should 
only enable it if the OS support XMM/YMM state support etc - like we do 
with AVX already.

Is that correct? If yes, then we should fix the issue - and your fix is 
ok for head. Carlos needs to approve it for the other branches,

Andreas
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