Re: [PATCH][ARM]Use different startfile and endfile for elf target when generating shared object.

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi Renlin,


On 12 January 2017 at 16:50, Renlin Li <renlin.li@foss.arm.com> wrote:
> Hi Kugan,
>
> some of the targets do include pie, and use the same crtbegin file as shared
> object.
> For example, alpha/elf.h
>
> And there are targets which don't do that,
> For example, sh/elf.h
>
> Most of the elf target seem only consider the simple case.
>
> The purpose of this patch is to make it possible to correctly check whether
> current toolchain supports shared object.
>
> Current dejegnu target selector "shared" tries to compile a simple source
> code to with "-shared -fpic" options to check whether "-shared" is
> supported.
>
> For arm baremetal targets with, this is not sufficient.
>
> arm-none-eabi is built with multilib. When running this testcase, if it's
> compiled with "-march=armv7-a".
> The crtbegin.o for this architecture version contains relocations which
> cannot be used in shared object.
> This test will fail.
>
> if no cpu or architecture is specified, the default cpu will be arm7tdmi.
> The test will pass as crtbegin.o for this version doesn't contains any
> relocations
> only allowed in shared object.
>
> To make this "shared" target selector work for arm baremetal toolchain. The
> correct way is to use different startup file for shared and non-shared
> toolchain.
>
> If the toolchain doesn't support "-shared" option, and someone attempts to
> use it
> to create shared object, it will complaint that "crtbeginS.o" cannot not be
> found.
>

I have run validations with your patch, and noticed regressions on arm-none-eabi
when using default cpu or --with-cpu=cortex-m3:

  - PASS now FAIL             [PASS => FAIL]:

  gcc.dg/lto/pr54709 c_lto_pr54709_0.o-c_lto_pr54709_1.o link,  -fPIC
-fvisibility=hidden -flto
  gcc.dg/lto/pr61526 c_lto_pr61526_0.o-c_lto_pr61526_1.o link,  -fPIC
-flto -flto-partition=1to1
  gcc.dg/lto/pr64415 c_lto_pr64415_0.o-c_lto_pr64415_1.o link,  -O -flto -fpic

on the same configurations, I've noticed these improvements:

  g++.dg/ipa/devirt-28a.C  -std=gnu++11 (test for excess errors)
  g++.dg/ipa/devirt-28a.C  -std=gnu++14 (test for excess errors)
  g++.dg/ipa/devirt-28a.C  -std=gnu++98 (test for excess errors)
are now unsupported rather than fail.

Why is it different when the toolchain is configured --with-cpu=cortex-a9
for instance? Are the tests involving -shared already skipped in this case?

> A full history of discussion is here:
> https://gcc.gnu.org/ml/gcc-patches/2016-03/msg00322.html
>
>
> Regards,
> Renlin
>
>
>
> On 12/01/17 11:47, kugan wrote:
>>
>> Hi,
>>
>> On 16/06/16 21:04, Renlin Li wrote:
>>>
>>>  /* Now we define the strings used to build the spec file.  */
>>> -#define UNKNOWN_ELF_STARTFILE_SPEC    " crti%O%s crtbegin%O%s crt0%O%s"
>>> +#define UNKNOWN_ELF_STARTFILE_SPEC    \
>>> +  "crti%O%s \
>>> +  %{!shared:crtbegin%O%s} %{shared:crtbeginS%O%s} \
>>> +  crt0%O%s"
>>
>>
>> Some targets seems to use shared|pie. When you change it, shouldn't it
>> also include for pie?
>>
>> Thanks,
>> Kugan