[llvm-commits] [PATCH] ELF file alignments

[Krister Wombell]

The ELF spec says:

All data structures that the object file format defines follow the
> "natural'' size and alignment guidelines for the relevant class. If
> necessary, data structures contain explicit padding to ensure 4-byte
> alignment for 4-byte objects, to force structure sizes to a multiple of 4,
> and so on. Data also have suitable alignment from the beginning of the file.
> Thus, for example, a structure containing an Elf32_Addr member will be
> aligned on a 4-byte boundary within the file.
>

1. ELFObjectWriter doesn't write the following objects with correct file
alignment: symtab, relocation sections, section headers. It probably goes
unnoticed because most CPUs support unaligned memory accesses and get to the
data without a problem plus linkers tend to fix things up too.  CPUs without
unaligned accesses don't fare as well.

2. gas goes a step further than the rules above and ensures that even in
relocatable output that a section's file offset is also a multiple of the
section alignment. This behaviour falls out of the patch as well.

Krister
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