[PATCH] D30886: [ELF] Pad x86 executable sections with 0xcc int3 instructions

[James Henderson via Phabricator via llvm-commits]

jhenderson added a comment.

In https://reviews.llvm.org/D30886#703297, @ruiu wrote:

> In this patch, one thread writes to a large memory region to initialize that with a fixed pattern, and then immediately after that, other threads read from and write to that memory region. I think that memory access pattern can be quite expensive on a mult-processor machine. But I don't know why that doesn't happen on your machine.


Me neither - perhaps we've got slightly different build settings, e.g. you've got -ffunction-sections enabled, which would increase the amount of work for the linker to do. However, I do see where performance should be negatively affected, and so potential for an alternative implementation. As far as I can understand from the code as is, the fill is written to the whole output section, and then the input sections written over the top. This is doing unnecessary extra work, I would think - typically the amount of padding in the .text section is far outweighed by the amount of executable instructions, so would it not make more sense to write the fill only to the places that need it, presumably after each InputSection is written? I would expect this to significantly improve performance, because much less work is being done.


https://reviews.llvm.org/D30886