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

[Igor Kudrin via llvm-dev]

Hi Evgeny,

> Yes, a linker needs some details but not all of them. It needs to know sizes of records and initial locations (PC Begin) to find out which functions FDEs belong to.

So, it still needs some details. Not all of them, but anyway, handling of .eh_frame sections is still a special case, even if we split all the content at compile time.

> What do you mean "one place or two"?

If I understand it right, the RFC is about helping a linker to eliminate unneeded .eh_frame items when performing GC. But if we still need to deal with CIEs and generate .eh_frame_hdr in a special way, does it make sense to make this change to simplify only a small part of a linker?

?Best Regards,
Igor Kudrin
C++ Developer, Access Softek, Inc.
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Evgeny Astigeevich via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Friday, November 10, 2017 6:41 PM
To: Igor Kudrin
Cc: llvm-dev at lists.llvm.org; nd
Subject: Re: [llvm-dev] [RFC] Making .eh_frame more linker-friendly

Hi Igor,

> It sounds like the linker has to be aware of the .eh_frame section details to be able to generate .eh_frame_hdr and eliminate duplicate CIEs, right?

Yes, a linker needs some details but not all of them. It needs to know sizes of records and initial locations (PC Begin) to find out which functions FDEs belong to.

> So, is there any difference whether it knows that in one place or two?

What do you mean "one place or two"? If .eh_frame_hdr is not created a linker does not need to parse .eh_frame sections. It simply merges them into one section. The format of .eh_frame allows to do this without parsing .eh_frame sections.

Thanks,
Evgeny Astigeevich

From: Igor Kudrin <ikudrin.dev at gmail.com>
Date: Thursday, 9 November 2017 at 11:29
To: Rui Ueyama <ruiu at google.com>, Evgeny Astigeevich <Evgeny.Astigeevich at arm.com>
Cc: "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>, nd <nd at arm.com>
Subject: Re: [llvm-dev] [RFC] Making .eh_frame more linker-friendly


It sounds like the linker has to be aware of the .eh_frame section details to be able to generate .eh_frame_hdr and eliminate duplicate CIEs, right?

So, is there any difference whether it knows that in one place or two?
Best Regards,
Igor Kudrin
C++ Developer, Access Softek, Inc.
On 27-Oct-17 3:43, Rui Ueyama via llvm-dev wrote:
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.
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