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[ELF][PPC] Allow PT_LOAD to have overlapping p_offset ranges

Description

[ELF][PPC] Allow PT_LOAD to have overlapping p_offset ranges

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

Details

Committed
MaskRayAug 20 2019, 1:34 AM
Reviewer
ruiu
Differential Revision
D64906: [ELF][PPC] Allow PT_LOAD to have overlapping p_offset ranges
Parents
rL369342: [test/Object] - Move/rewrite 2 more test cases.
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