From the PowerPC ELFv2 ABI section 4.2.3. Global Offset Table.
```
The GOT consists of an 8-byte header that contains the TOC base (the first TOC
base when multiple TOCs are present), followed by an array of 8-byte addresses.
```
Due to the introduction of PC Relative code it is now possible to require a GOT
without having a .TOC. symbol in the object that is being linked. Since LLD uses
the .TOC. symbol to determine whether or not a GOT is required the GOT header is
not setup correctly and the 8-byte header is missing.
This patch allows the Power PC GOT setup to happen when an element is added to
the GOT instead of at the very begining. When this header is added a .TOC.
symbol is also added.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D91426
Thunk alignment is added in thie patch when using pc-rel instructions
to avoid crossing the 64 byte boundary.
Patched by: nemanjai, NeHuang
Reviewed By: sfertile, MaskRay
Differential Revision: https://reviews.llvm.org/D85973
```
// llvm-objdump -d output (before)
0: bl .-4
4: bl .+0
8: bl .+4
// llvm-objdump -d output (after) ; GNU objdump -d
0: bl 0xfffffffc / bl 0xfffffffffffffffc
4: bl 0x4
8: bl 0xc
```
Many Operand's are not annotated as OPERAND_PCREL.
They are not affected (e.g. `b .+67108860`). I plan to fix them in future patches.
Modified test/tools/llvm-objdump/ELF/PowerPC/branch-offset.s to test
address space wraparound for powerpc32 and powerpc64.
Reviewed By: sfertile, jhenderson
Differential Revision: https://reviews.llvm.org/D76591
ThunkSection contains 4-byte instructions on all targets that use
thunks. Thunks should not be used in any performance sensitive places,
and locality/cache line/instruction fetching arguments should not apply.
We use 16 bytes as preferred function alignments for modern PowerPC cores.
In any case, 8 is not optimal.
Differential Revision: https://reviews.llvm.org/D72819
This change affects the non-linker script case (precisely, when the
`SECTIONS` command is not used). It deletes 3 alignments at PT_LOAD
boundaries for the default case: the size of a powerpc64 binary can be
decreased by at most 192kb. The technique can be ported to other
targets.
Let me demonstrate the idea with a maxPageSize=65536 example:
When assigning the address to the first output section of a new PT_LOAD,
if the end p_vaddr of the previous PT_LOAD is 0x10020, we advance to
the next multiple of maxPageSize: 0x20000. The new PT_LOAD will thus
have p_vaddr=0x20000. Because p_offset and p_vaddr are congruent modulo
maxPageSize, p_offset will be 0x20000, leaving a p_offset gap [0x10020,
0x20000) in the output.
Alternatively, if we advance to 0x20020, the new PT_LOAD will have
p_vaddr=0x20020. We can pick either 0x10020 or 0x20020 for p_offset!
Obviously 0x10020 is the choice because it leaves no gap. At runtime,
p_vaddr will be rounded down by pagesize (65536 if
pagesize=maxPageSize). This PT_LOAD will load additional initial
contents from p_offset ranges [0x10000,0x10020), which will also be
loaded by the previous PT_LOAD. This is fine if -z noseparate-code is in
effect or if we are not transiting between executable and non-executable
segments.
ld.bfd -z noseparate-code leverages this technique to keep output small.
This patch implements the technique in lld, which is mostly effective on
targets with large defaultMaxPageSize (AArch64/MIPS/PPC: 65536). The 3
removed alignments can save almost 3*65536 bytes.
Two places that rely on p_vaddr%pagesize = 0 have to be updated.
1) We used to round p_memsz(PT_GNU_RELRO) up to commonPageSize (defaults
to 4096 on all targets). Now p_vaddr%commonPageSize may be non-zero.
The updated formula takes account of that factor.
2) Our TP offsets formulae are only correct if p_vaddr%p_align = 0.
Fix them. See the updated comments in InputSection.cpp for details.
On targets that we enable the technique (only PPC64 now),
we can potentially make `p_vaddr(PT_TLS)%p_align(PT_TLS) != 0`
if `sh_addralign(.tdata) < sh_addralign(.tbss)`
This exposes many problems in ld.so implementations, especially the
offsets of dynamic TLS blocks. Known issues:
FreeBSD 13.0-CURRENT rtld-elf (i386/amd64/powerpc/arm64)
glibc (HEAD) i386 and x86_64 https://sourceware.org/bugzilla/show_bug.cgi?id=24606
musl<=1.1.22 on TLS Variant I architectures (aarch64/powerpc64/...)
So, force p_vaddr%p_align = 0 by rounding dot up to p_align(PT_TLS).
The technique will be enabled (with updated tests) for other targets in
subsequent patches.
Reviewed By: ruiu
Differential Revision: https://reviews.llvm.org/D64906
llvm-svn: 369343
Stefan Pintilie