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

[Davide Italiano via llvm-commits]

On Mon, Sep 7, 2015 at 9:57 PM, David Majnemer <david.majnemer at gmail.com> wrote:
>
>
> 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?
>

Yes. I'm in general opposed to write test in YAML when they can
expressed more concisely in ASM (see e.g. relocations) but in this
case it seems we have no choice (at least I don't see one).


-- 
Davide

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