Re: [PATCH 2/3] Revert "[AArch64][BZ #17711] Fix extern protected data handling"

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Florian,

Sure, something basic like this shows the issues:

int x;
int f(void) { return ++x; }
int main(void) { return f(); }

compile with -O2 -fPIC -flto -shared:

00000000000004f0 <f@plt>:
 4f0:   b0000090        adrp    x16, 11000 <__cxa_finalize@GLIBC_2.17>
 4f4:   f9400611        ldr     x17, [x16, #8]
 4f8:   91002210        add     x16, x16, #0x8
 4fc:   d61f0220        br      x17

0000000000000510 <main>:
 510:   17fffff8        b       4f0 <f@plt>

0000000000000600 <f>:
 600:   90000081        adrp    x1, 10000 <__FRAME_END__+0xf8f8>
 604:   f947e821        ldr     x1, [x1, #4048]
 608:   b9400020        ldr     w0, [x1]
 60c:   11000400        add     w0, w0, #0x1
 610:   b9000020        str     w0, [x1]
 614:   d65f03c0        ret

So f() does not get inlined into main, it is redirected via PLT, and it uses a GOT
indirection to access the global. The underlying problem is that ELF assumes by
default that you want interposition/export for all symbols. In reality that is
almost never needed. LLVM makes -fno-semantic-interposition the default,
which solves the PLT and inlining issue, but not the unnecessary GOT indirections.

Cheers,
Wilco