[Bug gold/21751] New: Gold: GC of C++ functions behaves differently than LD.BFD with KEEP(*(.eh_frame))

[jan.smets at nokia dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=21751

            Bug ID: 21751
           Summary: Gold: GC of C++ functions behaves differently than
                    LD.BFD with KEEP(*(.eh_frame))
           Product: binutils
           Version: 2.29
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: gold
          Assignee: ccoutant at gmail dot com
          Reporter: jan.smets at nokia dot com
                CC: ian at airs dot com
  Target Milestone: ---

$ cat call.c main.c  unused.cpp 
void call(void)
{
    // nothing
}
extern void call(void);

int main(void)
{
    call();
    return 0;
}
void unused(void)
{
    // nothing
}


# build some "default" link script
# /opt/rh/devtoolset-6/root/usr/bin/ld --verbose > /tmp/link_script
# remove the header and footer, remove the SORT (otherwise gold won't accept
it)
# the only thing that is required in the link script is
#  .eh_frame       : ONLY_IF_RO { KEEP (*(.eh_frame)) *(.eh_frame.*) }


/opt/rh/devtoolset-6/root/usr/bin/gcc -ffunction-sections -c call.c -o call.o 
/opt/rh/devtoolset-6/root/usr/bin/gcc -ffunction-sections -c main.c -o main.o 
/opt/rh/devtoolset-6/root/usr/bin/g++ -ffunction-sections -c unused.cpp -o
unused.o -fno-exceptions

LD=ld.gold

/opt/rh/devtoolset-6/root/usr/bin/$LD -r main.o call.o unused.o -o bla.a

echo "unused function is GC'ed"
/opt/rh/devtoolset-6/root/usr/bin/$LD --gc-sections --print-gc-sections bla.a
-o test.exe -e main
echo
/opt/rh/devtoolset-6/root/usr/bin/$LD --gc-sections --print-gc-sections main.o
call.o unused.o -o test.exe -e main

echo "unused function is *NOT* GC'ed"
/opt/rh/devtoolset-6/root/usr/bin/$LD --gc-sections --print-gc-sections bla.a
-o test.exe  -T /tmp/link_script -e main
echo
/opt/rh/devtoolset-6/root/usr/bin/$LD --gc-sections --print-gc-sections main.o
call.o unused.o -o test.exe  -T /tmp/link_script -e main

# ld.gold + link script (-T) where the script has 
#   .eh_frame       : ONLY_IF_RO { KEEP (*(.eh_frame)) *(.eh_frame.*) }
# ld.bfd behaves 'as expected'

unused.o has

Relocation section '.rela.eh_frame' at offset 0x1b8 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000000020  000500000002 R_X86_64_PC32     0000000000000000 .text._Z6unusedv
+ 0

Let's call it a feature as the script explicitly said to KEEP eh_frame and as a
result everything that eh_framer references. But it may not be what end users
expect. Plus, LD.BFD happily removed the 'unused' function.

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