[llvm-dev] [ELF] [RFC] Padding between executable sections

[James Henderson via llvm-dev]

Hi,

I was doing some experiments with LLD and noticed that it pads out
OutputSections with null bytes in all cases (as far as I could tell).
However, for executable sections on some targets, 0x00 forms part of an
executable instruction that is not nop. In particular, for x86_64 targets
at least, the sequence 0x00 0x00 is an add instruction. This can result in
confusing disassembly.

For example, on x86_64, given a simple InputSection that is a single "0xc3
retq" instruction, and given an alignment of 16 bytes, 15 null bytes are
inserted between the end of that InputSection and the next. In the
disassembly I then see the retq instruction followed by a series of adds,
the last of which actually consumes 1 or more bytes of the next section to
form a valid instruction, which can in turn throw off the disassembly of
the following instructions at the start of the next section.

What do people think about using a target-specific nop instruction sequence
to pad executable sections? Another possible option, on x86_64 at least (I
haven't investigated other targets), is to use the 0xcc interrupt
instruction.

Regards,

James
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