llvm dev - Nov 2020 - [LLD] Support DWARF64, debug_info "sorting"

>  Thinking about it, I wouldn't expect an LTO generated object itself to
have a mixture of DWARF32/64, although I guess the 32/64 bit state could be
encoded in the IR (I am not familiar enough with it to know if it actually is or
not). It might be necessary to find ways to configure LTO to generate DWARF64,
possibly via a link-time option.
I don’t think we need to encode dwarf32/64 in IR as attribute for each module.
We’re not going to emit mixed dwarf32/64 for merged LTO module anyways, so
allowing each module to express its dwarf setting would only introduce burden
for LTO to deal with inconsistency (warning?) among input modules. Having a
linker switch to pass the setting from driver to LTO sounds better to me.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org>
Date: Thursday, November 12, 2020 at 2:21 AM
To: Fangrui Song <maskray at google.com>
Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info "sorting"


On Thu, 12 Nov 2020 at 02:10, Fangrui Song <maskray at
google.com<mailto:maskray at google.com>> wrote:
On 2020-11-12, Alexander Yermolovich wrote:
>Thanks for feedback.
>
>I agree with patch and numbers this will be a more concrete discussion, but
I wanted to judge overall receptiveness to this approach and see maybe there was
a better way.
>
>"Whilst the majority of objects will only have a single CU in them,
there will be exceptions (LTO-generated objects, -r merged objects etc), so we
do need to consider this approach."
>David can you elaborate under which conditions LTO-generated objects will
have a mix of DWARF32/64 in same .debug_info? Looking at how dwarf64 was
implemented same flag will be used for the entirety of the dwarf output, even if
multiple CUs are included.
Thinking about it, I wouldn't expect an LTO generated object itself to have
a mixture of DWARF32/64, although I guess the 32/64 bit state could be encoded
in the IR (I am not familiar enough with it to know if it actually is or not).
It might be necessary to find ways to configure LTO to generate DWARF64,
possibly via a link-time option.
>
>On one hand since this is only applicable for when DWARF64 is used, special
option would be the way to go. Although the user will need to be aware of yet
another LLD option. Maybe an error when relocations overflow occur can be
modified to display this option along with -fdebug-types-section
I am quite happy with the relocation approach under a linker option. I'd
still
want to know generic-abi folks's thoughts, though. James may have prepared
something
he wants to share with generic-abi:) Let's wait...

I hadn't prepared anything if I'm honest (though if there's
widespread agreement that this would be useful, I certainly can - it would have
other positive improvements too, reducing the need for tools to rely on section
names to identify debug data for example). It was more a case of bouncing ideas
off of people to see what they thought. Any discussion we have will probably
also need circulating on the DWARF mailing list too, since it is more a DWARF
issue than a gABI issue (unless the solution is a new section type). Further
refinements to this idea that might make it more appealing to the generic group:
`SHT_DEBUG` for the section type name, with the first N bytes of the sh_info
used to specify the variant of debug data it represents (e.g. 0x1 for DWARF, 0x2
for SOME_OTHER_STANDARD etc), and the remainder for use as flags as defined by
the standard (I'm thinking for DWARF you could encode the 64-bit/32-bit
state in there, possibly the section variant (info/rnglists/line etc) and the
DWARF version too), on the understanding that consumers like the linker
wouldn't combine sections in a potentially broken way. This has the
advantage that it could be retrofitted to the existing standard versions, but as
has been pointed out, this won't help those with linker scripts - that could
only be solved with a new DWARF standard and separate names for 64/32 bit
sections, at least if we wanted to avoid the linker needing to do anything
beyond reading the section header.

The relocation approach sounds like a reasonable solution for the current
situation - even if we do decide to go the route of changing producers to start
emitting a new section type/update the standard etc, it doesn't resolve the
problem people may currently face.
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On Fri, Nov 13, 2020 at 8:35 AM Wenlei He via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> >  Thinking about it, I wouldn't expect an LTO generated object
itself to have a mixture of DWARF32/64, although I guess the 32/64 bit state
could be encoded in the IR (I am not familiar enough with it to know if it
actually is or not). It might be necessary to find ways to configure LTO to
generate DWARF64, possibly via a link-time option.
>
>
>
> I don’t think we need to encode dwarf32/64 in IR as attribute for each
module. We’re not going to emit mixed dwarf32/64 for merged LTO module anyways,
so allowing each module to express its dwarf setting would only introduce burden
for LTO to deal with inconsistency (warning?) among input modules. Having a
linker switch to pass the setting from driver to LTO sounds better to me.
Usually the issue there is that existing build systems may be setup
only to pass such flags to the compilations, and not to the link
invocations - like DWARF version, we pass that down through IR, emit
warnings/errors when two IR modules with different DWARF versions are
linked together, and then emit only one (the higher, I believe) DWARF
version out the other end.

We aren't 100% consistent on this "anything you could do without LTO,
you shuold be able to do with LTO/passing the same flags to the same
actions" kind of strategy (eg: type units and DWARF compression aren't
passed down through IR - if you want those you have to pass them to
the link invocation (via the clang driver) yourself). So it's more "is
there a systemic use of these flags already for the compilation and
would not supporting it there be a pain"? It's probably not for
DWARF64, since we haven't had any flag to support it at the moment
anyway.

- Dave
>
>
>
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org>
> Date: Thursday, November 12, 2020 at 2:21 AM
> To: Fangrui Song <maskray at google.com>
> Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info
"sorting"
>
>
>
>
>
> On Thu, 12 Nov 2020 at 02:10, Fangrui Song <maskray at google.com>
wrote:
>
> On 2020-11-12, Alexander Yermolovich wrote:
> >Thanks for feedback.
> >
> >I agree with patch and numbers this will be a more concrete discussion,
but I wanted to judge overall receptiveness to this approach and see maybe there
was a better way.
> >
> >"Whilst the majority of objects will only have a single CU in
them, there will be exceptions (LTO-generated objects, -r merged objects etc),
so we do need to consider this approach."
> >David can you elaborate under which conditions LTO-generated objects
will have a mix of DWARF32/64 in same .debug_info? Looking at how dwarf64 was
implemented same flag will be used for the entirety of the dwarf output, even if
multiple CUs are included.
>
>
>
> Thinking about it, I wouldn't expect an LTO generated object itself to
have a mixture of DWARF32/64, although I guess the 32/64 bit state could be
encoded in the IR (I am not familiar enough with it to know if it actually is or
not). It might be necessary to find ways to configure LTO to generate DWARF64,
possibly via a link-time option.
>
>
>
> >
> >On one hand since this is only applicable for when DWARF64 is used,
special option would be the way to go. Although the user will need to be aware
of yet another LLD option. Maybe an error when relocations overflow occur can be
modified to display this option along with -fdebug-types-section
>
> I am quite happy with the relocation approach under a linker option.
I'd still
> want to know generic-abi folks's thoughts, though. James may have
prepared something
> he wants to share with generic-abi:) Let's wait...
>
>
>
> I hadn't prepared anything if I'm honest (though if there's
widespread agreement that this would be useful, I certainly can - it would have
other positive improvements too, reducing the need for tools to rely on section
names to identify debug data for example). It was more a case of bouncing ideas
off of people to see what they thought. Any discussion we have will probably
also need circulating on the DWARF mailing list too, since it is more a DWARF
issue than a gABI issue (unless the solution is a new section type). Further
refinements to this idea that might make it more appealing to the generic group:
`SHT_DEBUG` for the section type name, with the first N bytes of the sh_info
used to specify the variant of debug data it represents (e.g. 0x1 for DWARF, 0x2
for SOME_OTHER_STANDARD etc), and the remainder for use as flags as defined by
the standard (I'm thinking for DWARF you could encode the 64-bit/32-bit
state in there, possibly the section variant (info/rnglists/line etc) and the
DWARF version too), on the understanding that consumers like the linker
wouldn't combine sections in a potentially broken way. This has the
advantage that it could be retrofitted to the existing standard versions, but as
has been pointed out, this won't help those with linker scripts - that could
only be solved with a new DWARF standard and separate names for 64/32 bit
sections, at least if we wanted to avoid the linker needing to do anything
beyond reading the section header.
>
>
>
> The relocation approach sounds like a reasonable solution for the current
situation - even if we do decide to go the route of changing producers to start
emitting a new section type/update the standard etc, it doesn't resolve the
problem people may currently face.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

I got replies from Nick Clifton and Michael Matz:
https://sourceware.org/pipermail/binutils/2020-November/114116.html
(and its reply).
I have mentioned (a) the difficulty of the
detecting-DWARF64-by-first-relocation approach and (b) the section
type approach  in my reply there
https://sourceware.org/pipermail/binutils/2020-November/114125.html

(a) My prototype has made me feel uneasy with this approach.

<quote>
In DWARF v4 or if .debug_str_offset is not used, it is a problem. A
heuristic is: if an input section in a file is marked DWARF64, we mark
all other .debug_* DWARF64. This makes me feel a bit uneasy because
for an output section description

   .debug_str 0 : { *(.debug_str) }

Now the behavior of `*` (or, if we invent a `SORT_*` keyword) is also
dependent on other output sections.
</quote>

(b)
* It needs a section type (either a gABI one or a SHT_GNU_* in GNU
ABI). Seeking for a gABI one is not that I think this is particularly
related to gABI but that I don't want Solaris (which LLVM also
supports) uses a different section type to unnecessarily cause
friction on our implementation
* It needs a clarification on multiple output section descriptions
with the same name.
* It needs a linker script feature to match input sections by type.

On Fri, Nov 13, 2020 at 9:52 AM David Blaikie <dblaikie at gmail.com>
wrote:
>
> On Fri, Nov 13, 2020 at 8:35 AM Wenlei He via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > >  Thinking about it, I wouldn't expect an LTO generated object
itself to have a mixture of DWARF32/64, although I guess the 32/64 bit state
could be encoded in the IR (I am not familiar enough with it to know if it
actually is or not). It might be necessary to find ways to configure LTO to
generate DWARF64, possibly via a link-time option.
> >
> >
> >
> > I don’t think we need to encode dwarf32/64 in IR as attribute for each
module. We’re not going to emit mixed dwarf32/64 for merged LTO module anyways,
so allowing each module to express its dwarf setting would only introduce burden
for LTO to deal with inconsistency (warning?) among input modules. Having a
linker switch to pass the setting from driver to LTO sounds better to me.
>
> Usually the issue there is that existing build systems may be setup
> only to pass such flags to the compilations, and not to the link
> invocations - like DWARF version, we pass that down through IR, emit
> warnings/errors when two IR modules with different DWARF versions are
> linked together, and then emit only one (the higher, I believe) DWARF
> version out the other end.
>
> We aren't 100% consistent on this "anything you could do without
LTO,
> you shuold be able to do with LTO/passing the same flags to the same
> actions" kind of strategy (eg: type units and DWARF compression
aren't
> passed down through IR - if you want those you have to pass them to
> the link invocation (via the clang driver) yourself). So it's more
"is
> there a systemic use of these flags already for the compilation and
> would not supporting it there be a pain"? It's probably not for
> DWARF64, since we haven't had any flag to support it at the moment
> anyway.
>
> - Dave
>
> >
> >
> >
> > From: llvm-dev <llvm-dev-bounces at lists.llvm.org>
> > Date: Thursday, November 12, 2020 at 2:21 AM
> > To: Fangrui Song <maskray at google.com>
> > Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> > Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info
"sorting"
> >
> >
> >
> >
> >
> > On Thu, 12 Nov 2020 at 02:10, Fangrui Song <maskray at
google.com> wrote:
> >
> > On 2020-11-12, Alexander Yermolovich wrote:
> > >Thanks for feedback.
> > >
> > >I agree with patch and numbers this will be a more concrete
discussion, but I wanted to judge overall receptiveness to this approach and see
maybe there was a better way.
> > >
> > >"Whilst the majority of objects will only have a single CU in
them, there will be exceptions (LTO-generated objects, -r merged objects etc),
so we do need to consider this approach."
> > >David can you elaborate under which conditions LTO-generated
objects will have a mix of DWARF32/64 in same .debug_info? Looking at how
dwarf64 was implemented same flag will be used for the entirety of the dwarf
output, even if multiple CUs are included.
> >
> >
> >
> > Thinking about it, I wouldn't expect an LTO generated object
itself to have a mixture of DWARF32/64, although I guess the 32/64 bit state
could be encoded in the IR (I am not familiar enough with it to know if it
actually is or not). It might be necessary to find ways to configure LTO to
generate DWARF64, possibly via a link-time option.
> >
> >
> >
> > >
> > >On one hand since this is only applicable for when DWARF64 is
used, special option would be the way to go. Although the user will need to be
aware of yet another LLD option. Maybe an error when relocations overflow occur
can be modified to display this option along with -fdebug-types-section
> >
> > I am quite happy with the relocation approach under a linker option.
I'd still
> > want to know generic-abi folks's thoughts, though. James may have
prepared something
> > he wants to share with generic-abi:) Let's wait...
> >
> >
> >
> > I hadn't prepared anything if I'm honest (though if
there's widespread agreement that this would be useful, I certainly can - it
would have other positive improvements too, reducing the need for tools to rely
on section names to identify debug data for example). It was more a case of
bouncing ideas off of people to see what they thought. Any discussion we have
will probably also need circulating on the DWARF mailing list too, since it is
more a DWARF issue than a gABI issue (unless the solution is a new section
type). Further refinements to this idea that might make it more appealing to the
generic group: `SHT_DEBUG` for the section type name, with the first N bytes of
the sh_info used to specify the variant of debug data it represents (e.g. 0x1
for DWARF, 0x2 for SOME_OTHER_STANDARD etc), and the remainder for use as flags
as defined by the standard (I'm thinking for DWARF you could encode the
64-bit/32-bit state in there, possibly the section variant (info/rnglists/line
etc) and the DWARF version too), on the understanding that consumers like the
linker wouldn't combine sections in a potentially broken way. This has the
advantage that it could be retrofitted to the existing standard versions, but as
has been pointed out, this won't help those with linker scripts - that could
only be solved with a new DWARF standard and separate names for 64/32 bit
sections, at least if we wanted to avoid the linker needing to do anything
beyond reading the section header.
> >
> >
> >
> > The relocation approach sounds like a reasonable solution for the
current situation - even if we do decide to go the route of changing producers
to start emitting a new section type/update the standard etc, it doesn't
resolve the problem people may currently face.
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


-- 
宋方睿

On Fri, Nov 13, 2020 at 9:52 AM David Blaikie via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Fri, Nov 13, 2020 at 8:35 AM Wenlei He via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > >  Thinking about it, I wouldn't expect an LTO generated object
itself
> to have a mixture of DWARF32/64, although I guess the 32/64 bit state could
> be encoded in the IR (I am not familiar enough with it to know if it
> actually is or not). It might be necessary to find ways to configure LTO to
> generate DWARF64, possibly via a link-time option.
> >
> >
> >
> > I don’t think we need to encode dwarf32/64 in IR as attribute for each
> module. We’re not going to emit mixed dwarf32/64 for merged LTO module
> anyways, so allowing each module to express its dwarf setting would only
> introduce burden for LTO to deal with inconsistency (warning?) among input
> modules. Having a linker switch to pass the setting from driver to LTO
> sounds better to me.
>
> Usually the issue there is that existing build systems may be setup
> only to pass such flags to the compilations, and not to the link
> invocations - like DWARF version, we pass that down through IR, emit
> warnings/errors when two IR modules with different DWARF versions are
> linked together, and then emit only one (the higher, I believe) DWARF
> version out the other end.
>
> We aren't 100% consistent on this "anything you could do without
LTO,
> you shuold be able to do with LTO/passing the same flags to the same
> actions" kind of strategy (eg: type units and DWARF compression
aren't
> passed down through IR - if you want those you have to pass them to
> the link invocation (via the clang driver) yourself). So it's more
"is
> there a systemic use of these flags already for the compilation and
> would not supporting it there be a pain"? It's probably not for
> DWARF64, since we haven't had any flag to support it at the moment
> anyway.
>
Haven't followed this whole discussion closely, but +1 on this. Sounds like
a good use of module flags that automatically pick the "max" value on
merge. For anything that in a non-LTO build would only need to be passed to
the compile step and not affect the link, we should strive to do the same
with LTO (there are some legacy things that are passed through the driver
at link time, but want to avoid new cases).

Thanks,
Teresa

- Dave
>
> >
> >
> >
> > From: llvm-dev <llvm-dev-bounces at lists.llvm.org>
> > Date: Thursday, November 12, 2020 at 2:21 AM
> > To: Fangrui Song <maskray at google.com>
> > Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> > Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info
"sorting"
> >
> >
> >
> >
> >
> > On Thu, 12 Nov 2020 at 02:10, Fangrui Song <maskray at
google.com> wrote:
> >
> > On 2020-11-12, Alexander Yermolovich wrote:
> > >Thanks for feedback.
> > >
> > >I agree with patch and numbers this will be a more concrete
discussion,
> but I wanted to judge overall receptiveness to this approach and see maybe
> there was a better way.
> > >
> > >"Whilst the majority of objects will only have a single CU in
them,
> there will be exceptions (LTO-generated objects, -r merged objects etc), so
> we do need to consider this approach."
> > >David can you elaborate under which conditions LTO-generated
objects
> will have a mix of DWARF32/64 in same .debug_info? Looking at how dwarf64
> was implemented same flag will be used for the entirety of the dwarf
> output, even if multiple CUs are included.
> >
> >
> >
> > Thinking about it, I wouldn't expect an LTO generated object
itself to
> have a mixture of DWARF32/64, although I guess the 32/64 bit state could be
> encoded in the IR (I am not familiar enough with it to know if it actually
> is or not). It might be necessary to find ways to configure LTO to generate
> DWARF64, possibly via a link-time option.
> >
> >
> >
> > >
> > >On one hand since this is only applicable for when DWARF64 is
used,
> special option would be the way to go. Although the user will need to be
> aware of yet another LLD option. Maybe an error when relocations overflow
> occur can be modified to display this option along with
> -fdebug-types-section
> >
> > I am quite happy with the relocation approach under a linker option.
I'd
> still
> > want to know generic-abi folks's thoughts, though. James may have
> prepared something
> > he wants to share with generic-abi:) Let's wait...
> >
> >
> >
> > I hadn't prepared anything if I'm honest (though if
there's widespread
> agreement that this would be useful, I certainly can - it would have other
> positive improvements too, reducing the need for tools to rely on section
> names to identify debug data for example). It was more a case of bouncing
> ideas off of people to see what they thought. Any discussion we have will
> probably also need circulating on the DWARF mailing list too, since it is
> more a DWARF issue than a gABI issue (unless the solution is a new section
> type). Further refinements to this idea that might make it more appealing
> to the generic group: `SHT_DEBUG` for the section type name, with the first
> N bytes of the sh_info used to specify the variant of debug data it
> represents (e.g. 0x1 for DWARF, 0x2 for SOME_OTHER_STANDARD etc), and the
> remainder for use as flags as defined by the standard (I'm thinking for
> DWARF you could encode the 64-bit/32-bit state in there, possibly the
> section variant (info/rnglists/line etc) and the DWARF version too), on the
> understanding that consumers like the linker wouldn't combine sections
in a
> potentially broken way. This has the advantage that it could be retrofitted
> to the existing standard versions, but as has been pointed out, this
won't
> help those with linker scripts - that could only be solved with a new DWARF
> standard and separate names for 64/32 bit sections, at least if we wanted
> to avoid the linker needing to do anything beyond reading the section
> header.
> >
> >
> >
> > The relocation approach sounds like a reasonable solution for the
> current situation - even if we do decide to go the route of changing
> producers to start emitting a new section type/update the standard etc, it
> doesn't resolve the problem people may currently face.
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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