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llvm-project/lld/test/ELF/ppc64-tls-vaddr-align.s

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[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 llvm-svn: 369343
2019-08-20 16:34:25 +08:00
# REQUIRES: ppc
# RUN: llvm-mc -filetype=obj -triple=powerpc64le %s -o %t.o
# RUN: ld.lld %t.o -o %t
# RUN: llvm-readelf -S -l %t | FileCheck --check-prefix=SEC %s
# RUN: llvm-objdump -d %t | FileCheck --check-prefix=DIS %s
# SEC: Name Type Address Off Size ES Flg Lk Inf Al
# SEC: .tdata PROGBITS 0000000010020300 000300 000001 00 WAT 0 0 1
# SEC: .tbss NOBITS 0000000010020400 000301 000008 00 WAT 0 0 256
# SEC: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
# SEC: TLS 0x000300 0x0000000010020300 0x0000000010020300 0x000001 0x000108 R 0x100
## We currently have a hack in Writer.cpp:fixSectionAlignments() to force
## p_vaddr(PT_TLS)%p_align(PT_TLS)=0, to work around bugs in some dynamic loaders.
## p_vaddr rounded down to p_align has TP offset -0x7000.
## The first address of PT_TLS (p_vaddr) has TP offset (p_vaddr%p_align - 0x7000).
## Once we delete the hack, it is likely p_vaddr%p_align != 0.
## a@tprel = st_value(a) + p_vaddr%p_align - 0x7000 = .tbss-.tdata + p_vaddr%p_align - 0x7000
## = 0x10020400-0x10020300 + 0 - 0x7000 = -28416
# DIS: ld 3, -28416(13)
ld 3, a@tprel(13)
.section .tdata,"awT"
.byte 0
.section .tbss,"awT"
.p2align 8
a:
.quad 0