[Skiboot] [PATCH 03/13] cpu.h: implement this_cpu for clang
joel at jms.id.au
Thu May 3 19:33:12 AEST 2018
On 3 May 2018 at 17:11, Joel Stanley <joel at jms.id.au> wrote:
> On 3 May 2018 at 12:43, Joel Stanley <joel at jms.id.au> wrote:
>> On 3 May 2018 at 07:19, Balbir Singh <bsingharora at gmail.com> wrote:
>>> On Wed, 2018-05-02 at 18:07 +0930, Joel Stanley wrote:
>>>> Clang can't use the global register variable, so instead load struct
>>>> cpu_thread from r13 and return it.
>>> How do we know clang is not using r13 for anything else? Does -ffixed-reg
>>> work with r13?
>> That is a really good question. I was really surprised (excited!) that
>> it booted once I had fixed all of the issues I knew about, I forgot to
>> go back and ensure clang was reserving r13.
> Here's the answer:
> // On PPC64, r13 is the thread pointer. Never allocate this register.
> if (TM.isPPC64())
> markSuperRegs(Reserved, PPC::R13);
> So there we go. I will add this information to the commit message.
Even better, this patch and the other one to cpu.h can be dropped as
clang supports the global register variable.
I think clang was warning about this back before I was properly
telling it to build for ppc64, where r13 is allowed to be used by a
global register variable (as clang reserves, as we discover above).
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