linux-kernel - Re: Avoiding unnecessary jump relocations in gas?

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Date:	Wed, 20 May 2015 14:58:49 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"H.J. Lu" <hjl.tools@...il.com>, Borislav Petkov <bp@...en8.de>,
	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Avoiding unnecessary jump relocations in gas?

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@...capital.net> wrote:
>On Wed, May 20, 2015 at 1:59 PM, H. Peter Anvin <hpa@...or.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

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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