Re: [Android] -fpic default option

[Alexander Ivchenko <aivchenk at gmail dot com>]

>> The canonical way of building native applications for Android is to add -fno-pic to the compiler flags.
That’s true for programs in userspace, but for Android system-level
programs (standalone executables) like system tools, console apps or
tests it seems -fpic doesn’t make much sense.

>> The intent was to make the most common behavior the default, and for Android that
>> happens to be building shared libraries that can then be called through JNI.
Okay, but for building shared libs we either provide -shared to the
driver, or -c, right? In these cases
we can go with -fPIC by default. In other cases we can safely assume
that the executable will be created and
in such case it would be a good idea to use -fPIE.
That would solve the problem with a lot of tests that failed on
Android and also with native applications (executables, not libs) that
are built
with -fPIC now. What do you think?


2012/11/15 H.J. Lu <hjl.tools@gmail.com>:
> On Wed, Nov 14, 2012 at 5:26 AM, Alexander Ivchenko <aivchenk@gmail.com> wrote:
>> By default in Android we always compile with -fpic or -fPIC, even when
>> compiling executable. Because of that we have some test fails on
>> Android:
>>
>> For example:
>> gcc/testsuite/gcc.target/i386/pr47312.c
>> /* { dg-do run } */
>> /* { dg-options "-O2" } */
>>
>> void exit (int);
>> void noreturn_autodetection_failed ();
>> __attribute__ ((noinline))
>> detect_noreturn ()
>> {
>>   exit (0);
>> }
>> int
>> main (void)
>> {
>>   detect_noreturn ();
>>   noreturn_autodetection_failed ();
>>   return 0;
>> }
>>
>> If gcc knew, that we are not going to make a shared library (which is
>> implied if we are using pic option), then it would delete the call of
>> noreturn_autodetection_failed as a dead code. But in case of pic we
>> cannot rely on the fact that detect_noreturn () will not be
>> overwritten on runtime, eventhough we are compiling executable and
>> that will never happen! So in addition to several testfails, it seems
>> that we are losing an optimization opportunity here also.
>> I'm wondering, maybe we can use -fpie instead of -fpic by default in
>> the compiler (of course if there are no -shared or -c options; those
>> cases will mean that we cannot rely on the fact that we will have an
>> executable after linking)? It seems that it would solve the problem..
>> Of course we always can add something like { target nonpic } to all
>
> We should add  { target nonpic } independent of Android.
>
>> tests that fail on Android, but probably the problem is deeper than
>> that and that would be only hiding our head in the sand.
>
> Using -fPIE to compile executables is a good idea.
>
> --
> H.J.