From: Fangrui Song <i@maskray.me>
To: Alan Modra <amodra@gmail.com>
Cc: binutils <binutils@sourceware.org>
Subject: Re: [ld] section address : ALIGN(align) and the maximum of input section alignments
Date: Wed, 04 Mar 2020 00:03:00 -0000 [thread overview]
Message-ID: <BY5PR07MB7316EFFEE876DEA501ED5614CBE50@BY5PR07MB7316.namprd07.prod.outlook.com> (raw)
In-Reply-To: <20200303223901.GO5384@bubble.grove.modra.org>
On Tue, Mar 3, 2020 at 2:39 PM Alan Modra <amodra@gmail.com> wrote:
>
> On Tue, Feb 25, 2020 at 10:30:03PM -0800, Fangrui Song wrote:
> > > Why doesn't `.bss . : ALIGN(16)` respect the maximum of input section alignments (32)?
> > >
> > > (I have verified that .bss being SHT_NOBITS is unrelated.)
> > >
> > > The rule of sh_addr computation appears to be:
> > >
> > > * ADDR is unset, ALIGN is unset => max_input_alignment
> > > * ADDR is unset, ALIGN is set => max(ALIGN, max_input_alignment)
> > > * ADDR is set, ALIGN is unset => max_input_alignment
> > > * ADDR is set, ALIGN is set => ALIGN
> >
> > When ADDR is set, ALIGN is used while max_input_alignment is ignored.
> >
> > ld/ldlang.c:5582
> >
> > if (os->addr_tree == NULL) {
> > ...
> > newdot = os->region->current;
> > // maximum of ALIGN and input section alignments
> > section_alignment = os->bfd_section->alignment_power;
> > }
> > else
> > // ALIGN
> > section_alignment = exp_get_power (os->section_alignment, "section alignment");
> >
> > Alan, is the `else` branch supposed to use `section_alignment = os->bfd_section->alignment_power;` as well?
>
> No. When you specify the address of an output section that is the
> address it should have. Except that specifying both an address and
> alignment aligns the address given. Specifying both is somewhat
> contradictory and it would probably have been better if we didn't
> support ALIGN here.
Thanks. For the example below, do you agree with the comments in a.x below?
% cat a.s
.globl _start; _start: ret
.section .data.rel.ro,"aw"; .balign 8; .byte 0
.data; .byte 0
.section .data2,"aw"; .balign 8; .byte 0
.section .data3,"aw"; .balign 32; .byte 0
.bss; .balign 32; .byte 0
% as a.s -o a.o
% cat a.x
SECTIONS {
.text 0x10000 : { *(.text) }
/* sh_addr is 0x10010. Specifying both address and ALIGN should be
disallowed. */
.data.rel.ro . : ALIGN(16) { *(.data.rel.ro) }
.data 0x20000 : { *(.data) }
/* sh_addr is 0x20001. Should there be a warning that sh_addralign
is 8? Even --warn-section-align does not warn. */
.data2 . : { *(.data2) }
/* sh_addr is 0x20020. The input section alignment wins. */
.data3 : ALIGN(16) { *(.data3) }
/* sh_addr is 0x20030. Specifying both address and ALIGN should be
disallowed. */
.bss . : ALIGN(16) { *(.bss) }
}
% ld.bfd -T a.x a.s -o a
If specifying both Output Section Address and ALIGN is disallowed,
there should be no "changing start of section" warning when
--warn-section-align is not specified. Is my understanding correct?
BTW, I filed a bug about duplicate "changing start of section"
warnings https://sourceware.org/bugzilla/show_bug.cgi?id=25570
next prev parent reply other threads:[~2020-03-04 0:03 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-26 5:23 Fangrui Song
2020-02-26 6:31 ` Fangrui Song
2020-03-03 5:46 ` Fangrui Song
2020-03-03 22:39 ` Alan Modra
2020-03-04 0:03 ` Fangrui Song [this message]
[not found] ` <BY5PR07MB7316674DD98011D2AD812A63CBE40@BY5PR07MB7316.namprd07.prod.outlook.com>
2020-03-04 5:56 ` Alan Modra
2020-03-04 6:40 ` Fangrui Song
2020-03-04 8:04 ` Alan Modra
2020-03-05 6:41 ` Fangrui Song
2020-03-05 11:21 ` Alan Modra
2020-03-10 19:00 ` Fangrui Song
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