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[lld][Alignment] Use Align in ELF Sections
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Authored by gchatelet on Dec 2 2022, 7:57 AM.

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MaskRay
Summary

Follow up on D139181.
Here we assume that alignExpr contains a valid value (i.e. non zero power of 2).
It was already somewhat assumed but not checked. This will now crash if it's not the case.

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gchatelet created this revision.Dec 2 2022, 7:57 AM
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gchatelet requested review of this revision.Dec 2 2022, 7:57 AM
Herald added a project: Restricted Project. · View Herald TranscriptDec 2 2022, 7:57 AM
arichardson added inline comments.Dec 2 2022, 8:01 AM
lld/ELF/InputSection.h
74

Moving it after partition should allow it to fit into padding bytes

gchatelet added inline comments.Dec 2 2022, 8:48 AM
lld/ELF/InputSection.h
74

I moved the field around but it didn't improve the situation
https://godbolt.org/z/vYchbnME5

If you look at the -Xclang -fdump-record-layouts output you can see that it does improve the requirerd size, but due to the 8 byte alignment requirement it is still rounded up to 48 (for 64-bit architectures). However, for 32-bit arch reordering does help: 36 vs 40 bytes.

*** Dumping AST Record Layout
         0 | struct OutputSegment
     0:0-2 |   uint8_t sectionKind
     0:3-3 |   uint8_t bss
     0:4-4 |   uint8_t keepUnique
         1 |   uint8_t partition
         2 |   struct llvm::Align addralign
         2 |     uint8_t ShiftValue
         4 |   uint32_t type
         8 |   class llvm::StringRef name
         8 |     const char * Data
        16 |     size_t Length
        24 |   uint64_t flags
        32 |   uint32_t entsize
        36 |   uint32_t link
        40 |   uint32_t info
           | [sizeof=48, dsize=44, align=8,
           |  nvsize=44, nvalign=8]

Whereas the current patch has sizeof==dsize:

*** Dumping AST Record Layout
         0 | struct OutputSegment
     0:0-2 |   uint8_t sectionKind
     0:3-3 |   uint8_t bss
     0:4-4 |   uint8_t keepUnique
         1 |   uint8_t partition
         4 |   uint32_t type
         8 |   class llvm::StringRef name
         8 |     const char * Data
        16 |     size_t Length
        24 |   uint64_t flags
        32 |   struct llvm::Align addralign
        32 |     uint8_t ShiftValue
        36 |   uint32_t entsize
        40 |   uint32_t link
        44 |   uint32_t info
           | [sizeof=48, dsize=48, align=8,
           |  nvsize=48, nvalign=8]
MaskRay added a comment.EditedDec 2 2022, 10:40 AM

With this patch, the final lld executable is larger and some call sites are more complex.
llvm::Align probably won't improve lld/ELF (and the extra dependency llvm/Support/Alignment.h and MathExtras.h is somewhat unnecessary).
I've checked using p2align for lld::elf::Symbol, but I think it won't decrease the struct size and will likely lead to no less code size in most call sites.

Note: I think struct size for 32-bit systems isn't really necessary. The 64-bit system behavior is much more interesting.

MaskRay requested changes to this revision.Dec 2 2022, 10:40 AM
This revision now requires changes to proceed.Dec 2 2022, 10:40 AM
gchatelet abandoned this revision.Dec 19 2022, 5:44 AM