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

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Date:	Wed, 20 May 2015 13:59:35 -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?

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

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