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

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Message-ID: <CALCETrUQ_uMLEn3O-Vmu0vOm_4qRYxgj8YZOUJE13SDMqQmBPQ@mail.gmail.com>
Date:	Wed, 20 May 2015 13:53:45 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Mon, May 18, 2015 at 1:34 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/18/2015 01:28 PM, H. Peter Anvin wrote:
>>
>> OK, that is probably too recent.  The simplest answer I think is just to
>> .balign 16 each vector.  This is init space... some extra padding really
>> doesn't matter.
>>
>> Patch attached (still in compile test).
>>
>
> Corrected.

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.

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.

--Andy
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