[llvm-dev] [RFC] Making .eh_frame more linker-friendly

Thu Oct 26 11:58:13 PDT 2017

The .eh_frame section (which is basically a DWARF .debug_frame section) was not designed with deduplication/gc in mind. I haven't studied it closely, but it looks like the bulk of it is frame descriptions which are divided up basically per-function, with some common overhead factored out.  If you want to put each per-function part into its own ELF section, there's overhead for that which you are more aware of than I am, and then either you need to replicate the common part into each per-function section or accept a relocation from each per-function section into the separate common section.

Looking at my latest clang build in Ubuntu, the executable has 96320 frame descriptions of which all but one use the same common part; in this case, that common part is 24 bytes.  The size is not fixed, but is guaranteed to be a multiple of the target address size, and it probably can't be any smaller than 24 on a normal machine.  This might help give you some estimates about the size effect of different choices.

HTH,
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Rui Ueyama via llvm-dev
Sent: Thursday, October 26, 2017 11:19 AM
To: Reid Kleckner
Cc: llvm-dev
Subject: Re: [llvm-dev] [RFC] Making .eh_frame more linker-friendly

No I haven't. Thank you for the pointer.

Looks like the problem of the inverted edges was discussed there. But I guess my bigger question is this: why do we still create one big .eh_frame even if -ffunction-sections is given?

When the option is given, Clang creates .text, .rela.text and .gcc_exception_table sections for each function, but it still creates a monolithic .eh_frame that covers all function sections, which seems odd to me.

On Thu, Oct 26, 2017 at 9:47 AM, Reid Kleckner <rnk at google.com<mailto:rnk at google.com>> wrote:
Have you seen the discussion of SHF_LINK_ORDER on the generic-abi@ mailing list? I think it implements exactly what you describe. My understanding is that ARM EHABI leverages this for the same purpose.

https://groups.google.com/forum/#!topic/generic-abi/_CbBM6T6WeM

On Wed, Oct 25, 2017 at 6:42 PM, Rui Ueyama via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi,

Many linkers including lld have a feature to eliminate unused sections from output to make output smaller (which is essentially a mark-sweep gc where sections are vertices and relocations are edges). lld and GNU gold have yet another feature, ICF, to merge functions by contents to save more space.

When we remove or merge a function, we want to eliminate its exception handling information as well. But that isn't very easy to do due to the format of .eh_frame. Here are reasons:

1. Linkers have to parse, split, eliminate exception handling information for dead functions, and then reconstruct an .eh_frame section. It is tedious, and it doesn't feel very much like a task that linkers have to do (linkers usually handle sections as opaque blobs and are agnostic of section contents.) That is contrary to other data where section is the atomic unit of inclusion/elimination.

2. From the viewpoint of gc, .eh_frame has reverse edges to sections. Usually, if section A depends on section B, there's a relocation in A pointing to B. But that isn't the case for .eh_frame, but opposite. If section A has exception handling information in .eh_frame section B, B has a relocation against A. This makes implementing a gc tricky, and when it is combined to (1), it is more tricky.

3. Comparing .eh_frame contents for equivalence is hard. In order to merge functions by contents, we need to verify that their exception handling information is also the same, but doing it isn't easy given the current .eh_frame format.

So, I don't feel .eh_frame needed to be designed that way. Maybe we can improve. Here is my rough idea:

1. We can emit an .eh_frame section for each .text section. So, if you pass -ffunction-sections, the resulting object file would have multiple .eh_frame sections. This makes .eh_frame a unit of garbage collection and eliminates the need to parse .eh_frame contents. It also makes it very easy to compare .eh_frame sections for function merging.

2. Make each .eh_frame section have a link to its .text section. We could set a section index of a .text section to its corresponding .eh_frame's sh_link field. This would make gc much easier. (If text section A is pointed by an .eh_frame section B via sh_link, that A is alive means B is alive. It is still reverse, but this is much more manageable.)

I think doing the above things doesn't break the compatibility with existing linkers, and new linkers can take advantage of the format that is more friendly to the linker. I don't think of any obvious disadvantage of doing them, except that we would have more sections, but I may be wrong as I'm no expert of .eh_frame.

What do you guys think?

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