From: Daniel Jacobowitz <drow@false.org>
To: Paul Brook <paul@codesourcery.com>
Cc: binutils@sourceware.org
Subject: Re: [patch] R_ARM_REL32 and Thumb functions.
Date: Fri, 21 Apr 2006 03:23:00 -0000 [thread overview]
Message-ID: <20060420184921.GA21343@nevyn.them.org> (raw)
In-Reply-To: <200604201929.50393.paul@codesourcery.com>
On Thu, Apr 20, 2006 at 07:29:49PM +0100, Paul Brook wrote:
> The attached patch makes the R_ARM_REL32 relocation set the low bit of the
> value when the relocated symbol is a Thumb function, as required by the ARM
> EABI.
>
> I don't know if the current behavior is considered a bug or a feature for
> pre-EABI objects. For now I'm assuming bug, and making the change
> unconditionally. I can preserver the old behavior if anything depends on it.
>
> A slight complication is that gas will try and fold relative expressions at
> assembly time. This folding is done in generic code, so gets the value wrong.
> e.g.
>
> .thumb_func
> foo:
> .word foo - .
>
> Will be folded to zero by gas/expr.c:expr. I've [ab]used md_optimize_expr to
> prevent this happening.
>
> Initially I disabled the folding if either symbol was a Thumb function.
> However this ends up breaking the explicit cfi directives in libgcc, so I've
> only disabled the folding when the first symbol is a Thumb function.
Wow, this is nasty!
I can understand why Richard, and presumably the ABI, wants this to
happen:
ldr ip, 1f
2: add ip, pc, ip
bx ip
1: .word foo - . - (2b - .)
But I'd be mighty confused if, say, some code tried to copy itself from
one location to another and expected "foo - ." to describe the number
of bytes to be copied. And now "foo - ." and ". - foo" are assymetric.
--
Daniel Jacobowitz
CodeSourcery
next prev parent reply other threads:[~2006-04-20 18:49 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-21 3:05 Paul Brook
2006-04-21 3:23 ` Daniel Jacobowitz [this message]
2006-04-21 13:19 ` Richard Earnshaw
2006-04-21 14:36 ` Richard Earnshaw
2006-04-28 19:03 ` Paul Brook
2006-05-02 12:23 ` Nick Clifton
2006-05-02 13:20 ` Paul Brook
2006-05-02 13:36 ` Daniel Jacobowitz
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