Next: , Previous: PowerPC ELF32, Up: Machine Dependent

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 .opd section 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 .opd optimization.

Some PowerPC64 compilers have an option to generate compressed .opd entries 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 .toc section 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 .toc section that exceeds 64K, most likely from linking multiple files with ld -r. Use this option to turn off this feature.