[Bug target/94504] New: On powerpc, -ffunction-sections -fdata-sections is not as effective as expected for PIE executables.

[gcc-bugzilla at mkarcher dot dialup.fu-berlin.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94504

            Bug ID: 94504
           Summary: On powerpc, -ffunction-sections -fdata-sections is not
                    as effective as expected for PIE executables.
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-bugzilla at mkarcher dot dialup.fu-berlin.de
  Target Milestone: ---

I try to compile the following test program using

gcc -ffunction-sections -fdata-sections -pie -Wl,--gc-sections input.c

The linking step fails, because g is not defined. On most architectures except
powerpc (32 bit), the garbage collection is able to discard both fptr and f,
and so the reference to g vanishes. This is not the case on powerpc, where f is
referencing fptr through a GOT entry in the .got2 section.

This issue (I don't dare to call it "bug" yet) is the cause of of
https://bugs.debian.org/955845. The librsvg build process is quirky and
building the tests only works if garbage collection is able to collect a hughe
amount of dead code. Garbage collection is able to do that on all architectures
Debian tried it on except for powerpc (and possibly ppc64, see
https://bugs.debian.org/895723). The attached example program does compile fine
on ppc64, though.

/* dead code */
extern void g(int x, ...);
extern void (*fptr)();
void f()
{
        /* using fptr creates a GOT entry for fptr */
        g(0, fptr);
}
/* fptr is reference from the GOT. Let's reference f from fptr */
void (*fptr)() = f;

/* Non-dead code */
int x = 5;
int main(void)
{
        /* using x goes through the GOT. This prevents the GC to kill it */
        return x;
}