Re: [RFC] gcc feature request: Moving blocks into sections

H. Peter Anvin
2013-08-12T17:47:43Z

On 08/12/2013 09:09 AM, Peter Zijlstra wrote:
>>
>> On the majority of architectures, including x86, you cannot simply copy
>> a piece of code elsewhere and have it still work.
>
> I thought we used -fPIC which would allow just that.
>

Doubly wrong. The kernel is not compiled with -fPIC, nor does -fPIC
allow this kind of movement for code that contains intramodule
references (that is *all* references in the kernel). Since we really
doesn't want to burden the kernel with a GOT and a PLT, that is life.

>> You end up doing a
>> bunch of the work that a JIT would do anyway, and would end up with
>> considerably higher complexity and worse results than a true JIT.
>
> Well, less complexity but worse result, yes. We'd only poke the specific
> static_branch sites with either NOPs or the (relative) jump target for
> each of these branches. Then copy the result.

Once again, you can't "copy the result". You end up with a full
disassembler.

-hpa