4.10 ld and PowerPC64 64-bit ELF Support
- Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
by ld in stub sections located between groups of input sections.
--stub-group-size specifies the maximum size of a group of input
sections handled by one stub section. Since branch offsets are signed,
a stub section may serve two groups of input sections, one group before
the stub section, and one group after it. However, when using
conditional branches that require stubs, it may be better (for branch
prediction) that stub sections only serve one group of input sections.
A negative value for N chooses this scheme, ensuring that
branches to stubs always use a negative offset. Two special values of
N are recognized, 1 and -1. These both instruct
ld to automatically size input section groups for the branch types
detected, with the same behaviour regarding stub placement as other
positive or negative values of N respectively.
Note that --stub-group-size does not split input sections. A single input section larger than the group size specified will of course create a larger group (of one section). If input sections are too large, it may not be possible for a branch to reach its stub.
- This option causes ld to label linker stubs with a local symbol that encodes the stub type and destination.
- --dotsyms, --no-dotsyms
- These two options control how ld interprets version patterns in a version script. Older PowerPC64 compilers emitted both a function descriptor symbol with the same name as the function, and a code entry symbol with the name prefixed by a dot (.). To properly version a function foo, the version script thus needs to control both foo and .foo. The option --dotsyms, on by default, automatically adds the required dot-prefixed patterns. Use --no-dotsyms to disable this feature.
- PowerPC64 ld normally performs some optimization of code sequences used to access Thread-Local Storage. Use this option to disable the optimization.
- PowerPC64 ld normally removes
.opdsection entries corresponding to deleted link-once functions, or functions removed by the action of --gc-sections or linker script
/DISCARD/. Use this option to disable
- Some PowerPC64 compilers have an option to generate compressed
.opdentries spaced 16 bytes apart, overlapping the third word, the static chain pointer (unused in C) with the first word of the next entry. This option expands such entries to the full 24 bytes.
- PowerPC64 ld normally removes unused
.tocsection entries. Such entries are detected by examining relocations that reference the TOC in code sections. A reloc in a deleted code section marks a TOC word as unneeded, while a reloc in a kept code section marks a TOC word as needed. Since the TOC may reference itself, TOC relocs are also examined. TOC words marked as both needed and unneeded will of course be kept. TOC words without any referencing reloc are assumed to be part of a multi-word entry, and are kept or discarded as per the nearest marked preceding word. This works reliably for compiler generated code, but may be incorrect if assembly code is used to insert TOC entries. Use this option to disable the optimization.
- By default, PowerPC64 GCC generates code for a TOC model where TOC
entries are accessed with a 16-bit offset from r2. This limits the
total TOC size to 64K. PowerPC64 ld extends this limit by
grouping code sections such that each group uses less than 64K for its
TOC entries, then inserts r2 adjusting stubs between inter-group
calls. ld does not split apart input sections, so cannot
help if a single input file has a
.tocsection that exceeds 64K, most likely from linking multiple files with ld -r. Use this option to turn off this feature.