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 the position 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.
- 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.
- 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.
Does this comment need updating. What does page aligned mean now? Moreover does the comment about PT_GNU_RELRO make sense?