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Re: Avoiding unnecessary jump relocations in gas?
- From: "H. Peter Anvin" <hpa at zytor dot com>
- To: Andy Lutomirski <luto at amacapital dot net>
- Cc: "H.J. Lu" <hjl dot tools at gmail dot com>, Borislav Petkov <bp at alien8 dot de>, Jan Beulich <JBeulich at suse dot com>, Binutils <binutils at sourceware dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>
- Date: Wed, 20 May 2015 14:58:49 -0700
- Subject: Re: Avoiding unnecessary jump relocations in gas?
- Authentication-results: sourceware.org; auth=none
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Okay, let's go with it. that also let us merge this constant with the length of the array in the C header file.
On May 20, 2015 2:47:10 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Wed, May 20, 2015 at 1:59 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 05/20/2015 01:53 PM, Andy Lutomirski wrote:
>>> Egads. Now I understand what that code is. I don't like the
>balign,
>>> since this has nothing to do with alignment -- we're creating an
>array
>>> of functions.
>>
>> Actually it does... we align to the beginning of each slot. If
>.balign
>> could be something other than a power of 2 that would work, too.
>
>When I see "align", I think that we want to align to a multiple of X
>but we don't particularly care which multiple of X. Here we want a
>specific address and any other address would be an error.
>
>>
>> I was mostly looking to minimize the amount of gas magic we rely on.
>>
>>> Can't we make it explicit?
>>>
>>> #define EARLY_IDT_HANDLER_STRIDE 9
>>>
>>> ...
>>>
>>> .rept NUM_EXCEPTION_VECTORS
>>> . = early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE
>>> .if (EXCEPTION_ERRCODE_MASK >> i) & 1
>>> ASM_NOP2
>>> .else
>>> pushl $0 # Dummy error code, to make stack frame uniform
>>> .endif
>>> pushl $i # 20(%esp) Vector number
>>> jmp early_idt_handler
>>> i = i + 1
>>> .endr
>>>
>>> gas will error out if we try to move . backwards, so this should be
>safe.
>>
>> If that works too with all the versions of gas we care about, that
>would
>> be fine (and I do appreciate the explicitness.) However, .[b]align
>is
>> something that will have been well exercised in every version of gas,
>so
>> I do feel slightly safer with it.
>
>I have no idea how to easily test my approach on really old binutils
>versions, but I'm reasonably confident that assigning to . is common.
>
>--Andy
--
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