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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 13:59:35 -0700
- Subject: Re: Avoiding unnecessary jump relocations in gas?
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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.
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.
-hpa