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

Thu Oct 26 16:12:17 PDT 2017

Hi Rui,

It is my fault. I misread your RFC. Now I see it is about to do this in the compiler.
Yes, a linker does all needed magic. It combines all eh_frames, removes garbage and creates eh_frame_hdr.
And yes, your proposal will simplify garbage collection.  The main advantage is that you do not need to parse eh_frames.

Thanks,
Evgeny

From: Rui Ueyama <ruiu at google.com>
Date: Thursday, 26 October 2017 at 21:43
To: Evgeny Astigeevich <Evgeny.Astigeevich at arm.com>
Cc: "Robinson, Paul" <paul.robinson at sony.com>, Reid Kleckner <rnk at google.com>, "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>, nd <nd at arm.com>, Peter Smith <Peter.Smith at arm.com>
Subject: Re: [llvm-dev] [RFC] Making .eh_frame more linker-friendly

On Thu, Oct 26, 2017 at 1:13 PM, Evgeny Astigeevich <Evgeny.Astigeevich at arm.com<mailto:Evgeny.Astigeevich at arm.com>> wrote:
Hi,

There will be problems with eh_frame_hdr. Eh_frame_hdr is needed to use the binary search instead of the linear search. Having eh_frame per a function will cause no eh_frame_hdr or multiple eh_frame_hdr and will degrade search from binary to linear.

Linkers would combine .eh_frame sections into one .eh_frame, so that's not an issue, no?

As we create eh_frame_hdr in most cases there is no problem to filter out garbage eh_frame sections. If there is information about unused symbols, the implementation is very simple. BTW there is no need to do full decoding of eh_frame records to remove garbage.
Paul is right there will be code size overhead. Eh_frame is usually created per a compilation module with common information in CFI. Multiple eh_frames will cause a lot of redundant CFI. There might be a case when the total size of redundant CFIs will be greater than the total size of removed garbage.

As I wrote in the previous message, I don't think there's a size issue in link results because even existing linkers merge CIEs by contents.

Thanks,
Evgeny Astigeevich
The Arm Compiler Optimization team

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of "Robinson, Paul via llvm-dev" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Reply-To: "Robinson, Paul" <paul.robinson at sony.com<mailto:paul.robinson at sony.com>>
Date: Thursday, 26 October 2017 at 19:58
To: Rui Ueyama <ruiu at google.com<mailto:ruiu at google.com>>, Reid Kleckner <rnk at google.com<mailto:rnk at google.com>>
Cc: "llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>

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

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<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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