forked from OSchip/llvm-project
01c7f4b606
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 |
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| COFF | ||
| Driver/Inputs | ||
| ELF | ||
| MinGW | ||
| Unit | ||
| darwin | ||
| mach-o | ||
| wasm | ||
| CMakeLists.txt | ||
| lit.cfg.py | ||
| lit.site.cfg.py.in | ||