4.9 ld and PowerPC 32-bit ELF Support
Branches on PowerPC processors are limited to a signed 26-bit
displacement, which may result in ld giving
‘relocation truncated to fit’ errors with very large programs.
‘--relax’ enables the generation of trampolines that can access
the entire 32-bit address space. These trampolines are inserted at
section boundaries, so may not themselves be reachable if an input
section exceeds 33M in size. You may combine ‘-r’ and
‘--relax’ to add trampolines in a partial link. In that case
both branches to undefined symbols and inter-section branches are also
considered potentially out of range, and trampolines inserted.
- Current PowerPC GCC accepts a ‘-msecure-plt’ option that
generates code capable of using a newer PLT and GOT layout that has
the security advantage of no executable section ever needing to be
writable and no writable section ever being executable. PowerPC
ld will generate this layout, including stubs to access the
PLT, if all input files (including startup and static libraries) were
compiled with ‘-msecure-plt’. ‘--bss-plt’ forces the old
BSS PLT (and GOT layout) which can give slightly better performance.
- ld will use the new PLT and GOT layout if it is linking new
‘-fpic’ or ‘-fPIC’ code, but does not do so automatically
when linking non-PIC code. This option requests the new PLT and GOT
layout. A warning will be given if some object file requires the old
style BSS PLT.
- The new secure PLT and GOT are placed differently relative to other
sections compared to older BSS PLT and GOT placement. The location of
.plt must change because the new secure PLT is an initialized
section while the old PLT is uninitialized. The reason for the
.got change is more subtle: The new placement allows
.got to be read-only in applications linked with
‘-z relro -z now’. However, this placement means that
.sdata cannot always be used in shared libraries, because the
PowerPC ABI accesses
.sdata in shared libraries from the GOT
pointer. ‘--sdata-got’ forces the old GOT placement. PowerPC
GCC doesn't use
.sdata in shared libraries, so this option is
really only useful for other compilers that may do so.
- This option causes ld to label linker stubs with a local
symbol that encodes the stub type and destination.
- PowerPC ld normally performs some optimization of code
sequences used to access Thread-Local Storage. Use this option to
disable the optimization.