[lld] r246901 - [elf2] Correctly handle sections with an alignment of 0. Spec says to treat it as an alignment of 1.

[David Majnemer via llvm-commits]

On Mon, Sep 7, 2015 at 4:09 PM, Davide Italiano via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> On Fri, Sep 4, 2015 at 10:01 PM, Michael Spencer <bigcheesegs at gmail.com>
> wrote:
> > On Fri, Sep 4, 2015 at 5:48 PM, Davide Italiano <davide at freebsd.org>
> wrote:
> >> On Fri, Sep 4, 2015 at 5:25 PM, Michael J. Spencer via llvm-commits
> >> <llvm-commits at lists.llvm.org> wrote:
> >>> Author: mspencer
> >>> Date: Fri Sep  4 19:25:33 2015
> >>> New Revision: 246901
> >>>
> >>> URL: http://llvm.org/viewvc/llvm-project?rev=246901&view=rev
> >>> Log:
> >>> [elf2] Correctly handle sections with an alignment of 0. Spec says to
> treat it as an alignment of 1.
> >>>
> >>> Added: lld/trunk/test/elf2/section-align-0.test
> >>> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/elf2/section-align-0.test?rev=246901&view=auto
> >>>
> ==============================================================================
> >>> --- lld/trunk/test/elf2/section-align-0.test (added)
> >>> +++ lld/trunk/test/elf2/section-align-0.test Fri Sep  4 19:25:33 2015
> >>> @@ -0,0 +1,19 @@
> >>> +# RUN: yaml2obj -format elf %s -o %t
> >>> +# RUN: lld -flavor gnu2 %t -o %tout
> >>> +
> >>
> >> Is there any reason why you can't use assembler for testing?
> >
> > Feel free to change it if you know the magic incantation necessary to
> > get llvm-mc to emit an alignment of 0.
> >
> > - Michael Spencer
> >
> >>
>
> I spent some time analyzing this -- and I noticed your commit
> indirectly exposes a couple of problems in llvm-mc:
>
> 1) .align directive refuses alignment 0 -- a comment in the code hints
> this is done for GNU as compatibility, but it seems GNU as accepts
> .align 0 (and silently rounds up alignment to 1). I have a patch to
> fix that that I'll submit shortly .
>
> $ ./llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu blah.s -o blah
> blah.s:4:8: error: alignment must be a power of 2
> .align 0
>        ^
> $ as blah.s -o blah
> $
>
> $ cat blah.s
> .global _start
> .global bar
>
> .align 0
> .text
> _start:
> bar:
>   movl $bar, %edx
>
> 2) If we change align to 1 -- it seems llvm-mc doesn't honor it, but
> silently rounds up that to 4. Not sure if this is a bug or intended
> behavior. CC:ing Rafael as he did most of the recent work on ELF MC.
>
> [llvm-mc]
>
> $ readelf -S blah
>   [ 2] .text             PROGBITS         0000000000000000  00000040
>        0000000000000005  0000000000000000  AX       0     0     4
>
> [GNU as]
>
> $ readelf -S blah
>   [ 1] .text             PROGBITS         0000000000000000  00000040
>        0000000000000005  0000000000000000  AX       0     0     1
>
> About your testcase -- I'm not sure if there's a way to emit a section
> with alignment zero as also GNU as seems to round up that to 1
> silently. I want to think a little bit harder about it, if somebody
> has ideas on how to handle, they're welcome.
>

Couldn't a readable testcase be crafted with yaml2obj?


>
> Thanks,
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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