llvm dev - Oct 2017 - [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> 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> 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?
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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On Thu, Oct 26, 2017 at 11:19 AM, Rui Ueyama <ruiu at google.com> wrote:
> 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.
>
I agree, we should fix it. :)
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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?

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


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On Thu, Oct 26, 2017 at 11:58 AM, Robinson, Paul <paul.robinson at
sony.com>
wrote:
> 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.
>
Yes, .eh_frame section consists of one or more CIE records followed by one
or more FDE records. Common information in FDEs is factored out to an CIE
to save space. So, if you split an .eh_frame into multiple smaller
.eh_frame, you end up having more CIEs.

But the good news is that even existing linkers deduplicate CIEs by
contents (that's why you saw only one CIE record in your executable, even
though each input object file has at least one CIE record.) So the linked
executables/DSOs would be the same size.

>
> 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>
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> 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?
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
>
>
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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.
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.

Thanks,
Evgeny Astigeevich
The Arm Compiler Optimization team


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

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


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