llvm dev - Aug 2017 - DWARF: Ranges base address specifier entries & Gold's gdb-index 32 bit bug

Adrian: any thoughts? Has LLDB been fixed to support this yet?

On Tue, Aug 8, 2017 at 6:33 AM Robinson, Paul <paul.robinson at sony.com>
wrote:
> My inclination would be to use "disable if 32-bit and
–ggnu-pubnames" as
> the default,
>
Unfortunately Nico points out that Chrome doesn't currently use
-ggnu-pubnames :/ So to continue to work "out of the box" we'd
have to
broaden the surface to "disable if 32 bit and targeting gdb".

-ggnu-pubnames makes gdb-index creation much more efficient by allowing
gdb-index generation not to have to parse most of the DWARF DIEs, and just
create the index straight from a table. (this is necessary for correctness
in -gsplit-dwarf mode (where the DIEs are not available at link time as
they're in the .dwo not the .o), and an efficiency gain otherwise)

If "disable if 32 bit and targeting gdb" is overly broad (I feel it
kind of
is, but I'm not wedded to it) we could require those working around the
gold bug to add -ggnu-pubnames to add a bit more of a specific hint that
they're intending to build gdb-index from these objects.

> and have a flag.
>
> This would be determined in DwarfDebug, right?
>
Yes

> If the check is in CodeGen then I'd think it would be insensitive to
LTO.
>
There's some desire that LTO works as-if it was non-LTO. So if you pass
flags (eg: debugger tuning flags, in this case) to each separate compile,
you get the same behavior if you LTO as when you didn't - so we preserve
things in the CU, and then do the specified thing per-CU. (eg: emission
kind (gmlt/limited/full), split-debug-inlining, debug-info-for-profiling,
etc)

In this case if, say, you're on an LLDB platform but prefer to use GDB
(&
happen to use Gold and gdb-index on 32 bit and LTO) - so you pass the
debugger tuning=gdb flag to your compiles, it wouldn't disable the feature,
because the flag would be consulted at CodeGen time and not at compile time
when you passed the tuning flag. (oh, actually the tuning is passed down
via TM Options anyway, not on the CU - so I guess you wouldn't be getitng
the tuning at all today because that flag already doesn't follow the per-CU
model... well I guess that makes it easier)

> Setting it via IR/metadata is adding a bunch of complexity just to work
> around a gold bug.  Not worth it IMO.
>
Fair enough. Any thoughts on whether it should be a frontend-visible option
or just a cc1 or even backend option? I'll probably just leave it as a
backend option at this point, could even implement the defaulting down in
LLVM without any changes to Clang, I suppose...

> --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Monday, August 07, 2017 6:58 PM
> *To:* llvm-dev; Robinson, Paul; Adrian Prantl; Eric Christopher; Nico
> Weber
> *Subject:* DWARF: Ranges base address specifier entries & Gold's
> gdb-index 32 bit bug
>
>
>
> Context:
>
> In r309526 (with a followup fix in r309529) I implemented the use of
> DWARF's debug_ranges base address specifier entries to reduce the
number of
> object file relocations needed for debug_ranges*.
>
> * in a particular binary internally, an optimized build had a 70%
> reduction in debug_ranges.reloc, a 16% decrease in total object size (with
> compressed debug info and fission)
>
> Nico noted that this broke the Chromium build (
> https://bugs.llvm.org/show_bug.cgi?id=34007 ). Turns out GNU Binutils
> Gold has a bug (I've reported it as:
> https://sourceware.org/bugzilla/show_bug.cgi?id=21894 - wouldn't expect
> much action on it, though) where it fails to parse a base address specifier
> entry when trying to build gdb-index (-Wl,-gdb-index) for a 32 bit build.
>
> To address this in the short term, I added a flag that disables this
> feature by default for now.
>
> So, two questions:
>
> 1) What does everyone reckon the best solution to this is?
>   * Keep the feature disabled by default - with a flag to enable it for
> those who want it/can use it
>   * Enable the feature everywhere - with a flag to disable it
>   * Conditionally disable (or conditionally enable) the feature, with a
> flag to enable/disable it
>     * best condition we probably have is "disabled if 32 bit and
> -ggnu-pubnames" (gnu-pubnames are necessary for efficient building of
> gdb-index, and isn't the default)
> 2) How to implement the flag:
>   * backend option as it is currently (-mllvm
> -use-dwarf-ranges-base-address-specifier (maybe drop "-specifier"
to make
> it a bit more terse))
>   * clang option that maps to
>     * backend option (as currently)
>     * MCOption (programmatic/C++ API rather than command line of a backend
> option)
>     * LLVM IR attribute on the CU (this feature could be supported on a
> per-CU basis easiyl enough, thus survive LTO, etc, exactly as the user
> requested (but really if the user requested this on at least one CU, they
> probably might as well have it on all their CUs))
>     * LLVM IR module metadata
>
>
> Any ideas/thoughts/other aspects?
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170808/71766a66/attachment.html>

Can gdb handle these?  i.e. is it just gold that has the problem?  Conditioning
on debugger tuning when it's not the debugger that has the problem… icky. 
If that's the case then asking people to add –ggnu-pubnames seems okay. 
Assuming that gets handed down through IR/metadata then the LTO problem goes
away too.
--paulr

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Tuesday, August 08, 2017 10:59 AM
To: Robinson, Paul; llvm-dev; Adrian Prantl; Eric Christopher; Nico Weber
Subject: Re: DWARF: Ranges base address specifier entries & Gold's
gdb-index 32 bit bug

Adrian: any thoughts? Has LLDB been fixed to support this yet?
On Tue, Aug 8, 2017 at 6:33 AM Robinson, Paul <paul.robinson at
sony.com<mailto:paul.robinson at sony.com>> wrote:
My inclination would be to use "disable if 32-bit and –ggnu-pubnames"
as the default,

Unfortunately Nico points out that Chrome doesn't currently use
-ggnu-pubnames :/ So to continue to work "out of the box" we'd
have to broaden the surface to "disable if 32 bit and targeting gdb".

-ggnu-pubnames makes gdb-index creation much more efficient by allowing
gdb-index generation not to have to parse most of the DWARF DIEs, and just
create the index straight from a table. (this is necessary for correctness in
-gsplit-dwarf mode (where the DIEs are not available at link time as they're
in the .dwo not the .o), and an efficiency gain otherwise)

If "disable if 32 bit and targeting gdb" is overly broad (I feel it
kind of is, but I'm not wedded to it) we could require those working around
the gold bug to add -ggnu-pubnames to add a bit more of a specific hint that
they're intending to build gdb-index from these objects.

and have a flag.
This would be determined in DwarfDebug, right?

Yes

If the check is in CodeGen then I'd think it would be insensitive to LTO.

There's some desire that LTO works as-if it was non-LTO. So if you pass
flags (eg: debugger tuning flags, in this case) to each separate compile, you
get the same behavior if you LTO as when you didn't - so we preserve things
in the CU, and then do the specified thing per-CU. (eg: emission kind
(gmlt/limited/full), split-debug-inlining, debug-info-for-profiling, etc)

In this case if, say, you're on an LLDB platform but prefer to use GDB
(& happen to use Gold and gdb-index on 32 bit and LTO) - so you pass the
debugger tuning=gdb flag to your compiles, it wouldn't disable the feature,
because the flag would be consulted at CodeGen time and not at compile time when
you passed the tuning flag. (oh, actually the tuning is passed down via TM
Options anyway, not on the CU - so I guess you wouldn't be getitng the
tuning at all today because that flag already doesn't follow the per-CU
model... well I guess that makes it easier)

Setting it via IR/metadata is adding a bunch of complexity just to work around a
gold bug.  Not worth it IMO.

Fair enough. Any thoughts on whether it should be a frontend-visible option or
just a cc1 or even backend option? I'll probably just leave it as a backend
option at this point, could even implement the defaulting down in LLVM without
any changes to Clang, I suppose...

--paulr

From: David Blaikie [mailto:dblaikie at gmail.com<mailto:dblaikie at
gmail.com>]
Sent: Monday, August 07, 2017 6:58 PM
To: llvm-dev; Robinson, Paul; Adrian Prantl; Eric Christopher; Nico Weber
Subject: DWARF: Ranges base address specifier entries & Gold's gdb-index
32 bit bug

Context:

In r309526 (with a followup fix in r309529) I implemented the use of DWARF's
debug_ranges base address specifier entries to reduce the number of object file
relocations needed for debug_ranges*.

* in a particular binary internally, an optimized build had a 70% reduction in
debug_ranges.reloc, a 16% decrease in total object size (with compressed debug
info and fission)

Nico noted that this broke the Chromium build (
https://bugs.llvm.org/show_bug.cgi?id=34007 ). Turns out GNU Binutils Gold has a
bug (I've reported it as:
https://sourceware.org/bugzilla/show_bug.cgi?id=21894 - wouldn't expect much
action on it, though) where it fails to parse a base address specifier entry
when trying to build gdb-index (-Wl,-gdb-index) for a 32 bit build.

To address this in the short term, I added a flag that disables this feature by
default for now.

So, two questions:

1) What does everyone reckon the best solution to this is?
  * Keep the feature disabled by default - with a flag to enable it for those
who want it/can use it
  * Enable the feature everywhere - with a flag to disable it
  * Conditionally disable (or conditionally enable) the feature, with a flag to
enable/disable it
    * best condition we probably have is "disabled if 32 bit and
-ggnu-pubnames" (gnu-pubnames are necessary for efficient building of
gdb-index, and isn't the default)
2) How to implement the flag:
  * backend option as it is currently (-mllvm
-use-dwarf-ranges-base-address-specifier (maybe drop "-specifier" to
make it a bit more terse))
  * clang option that maps to
    * backend option (as currently)
    * MCOption (programmatic/C++ API rather than command line of a backend
option)
    * LLVM IR attribute on the CU (this feature could be supported on a per-CU
basis easiyl enough, thus survive LTO, etc, exactly as the user requested (but
really if the user requested this on at least one CU, they probably might as
well have it on all their CUs))
    * LLVM IR module metadata


Any ideas/thoughts/other aspects?
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170808/365944a5/attachment-0001.html>

On Tue, Aug 8, 2017 at 10:50 AM Robinson, Paul <paul.robinson at sony.com>
wrote:
> Can gdb handle these?  i.e. is it just gold that has the problem?
>
Yep, it's just gold when it's building the gdb-index (an accelerator
table
for GDB)

>   Conditioning on debugger tuning when it's not the debugger that has
the
> problem… icky.
>
It does. Though to a lesser degree even gnu-pubnames isn't a perfect
signal. One, I think, could use that even without a linker-generated
gdb-index - presumably GDB's still faster by reading gnu-pubnames than by
reading the raw DIEs to do lookup.

>   If that's the case then asking people to add –ggnu-pubnames seems
okay.
> Assuming that gets handed down through IR/metadata then the LTO problem
> goes away too.
>
gnu-pubnames has such problems (it's done as a backend option, passed down
from the clang driver via -mllvm -generate-gnu-dwarf-pub-sections, but
they're already there... - mostly the discussion around flags was "If
we're
adding a new one, what's the right tradeoff here for a workaround flag
compared to the platonic ideals of CU-based flags as much as possible") but
it's already that way...

- Dave

> --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Tuesday, August 08, 2017 10:59 AM
> *To:* Robinson, Paul; llvm-dev; Adrian Prantl; Eric Christopher; Nico
> Weber
> *Subject:* Re: DWARF: Ranges base address specifier entries &
Gold's
> gdb-index 32 bit bug
>
>
>
> Adrian: any thoughts? Has LLDB been fixed to support this yet?
>
> On Tue, Aug 8, 2017 at 6:33 AM Robinson, Paul <paul.robinson at
sony.com>
> wrote:
>
> My inclination would be to use "disable if 32-bit and
–ggnu-pubnames" as
> the default,
>
>
> Unfortunately Nico points out that Chrome doesn't currently use
> -ggnu-pubnames :/ So to continue to work "out of the box"
we'd have to
> broaden the surface to "disable if 32 bit and targeting gdb".
>
> -ggnu-pubnames makes gdb-index creation much more efficient by allowing
> gdb-index generation not to have to parse most of the DWARF DIEs, and just
> create the index straight from a table. (this is necessary for correctness
> in -gsplit-dwarf mode (where the DIEs are not available at link time as
> they're in the .dwo not the .o), and an efficiency gain otherwise)
>
> If "disable if 32 bit and targeting gdb" is overly broad (I feel
it kind
> of is, but I'm not wedded to it) we could require those working around
the
> gold bug to add -ggnu-pubnames to add a bit more of a specific hint that
> they're intending to build gdb-index from these objects.
>
>
> and have a flag.
>
> This would be determined in DwarfDebug, right?
>
>
> Yes
>
>
> If the check is in CodeGen then I'd think it would be insensitive to
LTO.
>
>
> There's some desire that LTO works as-if it was non-LTO. So if you pass
> flags (eg: debugger tuning flags, in this case) to each separate compile,
> you get the same behavior if you LTO as when you didn't - so we
preserve
> things in the CU, and then do the specified thing per-CU. (eg: emission
> kind (gmlt/limited/full), split-debug-inlining, debug-info-for-profiling,
> etc)
>
> In this case if, say, you're on an LLDB platform but prefer to use GDB
(&
> happen to use Gold and gdb-index on 32 bit and LTO) - so you pass the
> debugger tuning=gdb flag to your compiles, it wouldn't disable the
feature,
> because the flag would be consulted at CodeGen time and not at compile time
> when you passed the tuning flag. (oh, actually the tuning is passed down
> via TM Options anyway, not on the CU - so I guess you wouldn't be
getitng
> the tuning at all today because that flag already doesn't follow the
per-CU
> model... well I guess that makes it easier)
>
>
> Setting it via IR/metadata is adding a bunch of complexity just to work
> around a gold bug.  Not worth it IMO.
>
>
> Fair enough. Any thoughts on whether it should be a frontend-visible
> option or just a cc1 or even backend option? I'll probably just leave
it as
> a backend option at this point, could even implement the defaulting down in
> LLVM without any changes to Clang, I suppose...
>
>
> --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Monday, August 07, 2017 6:58 PM
> *To:* llvm-dev; Robinson, Paul; Adrian Prantl; Eric Christopher; Nico
> Weber
> *Subject:* DWARF: Ranges base address specifier entries & Gold's
> gdb-index 32 bit bug
>
>
>
> Context:
>
> In r309526 (with a followup fix in r309529) I implemented the use of
> DWARF's debug_ranges base address specifier entries to reduce the
number of
> object file relocations needed for debug_ranges*.
>
> * in a particular binary internally, an optimized build had a 70%
> reduction in debug_ranges.reloc, a 16% decrease in total object size (with
> compressed debug info and fission)
>
> Nico noted that this broke the Chromium build (
> https://bugs.llvm.org/show_bug.cgi?id=34007 ). Turns out GNU Binutils
> Gold has a bug (I've reported it as:
> https://sourceware.org/bugzilla/show_bug.cgi?id=21894 - wouldn't expect
> much action on it, though) where it fails to parse a base address specifier
> entry when trying to build gdb-index (-Wl,-gdb-index) for a 32 bit build.
>
> To address this in the short term, I added a flag that disables this
> feature by default for now.
>
> So, two questions:
>
> 1) What does everyone reckon the best solution to this is?
>   * Keep the feature disabled by default - with a flag to enable it for
> those who want it/can use it
>   * Enable the feature everywhere - with a flag to disable it
>   * Conditionally disable (or conditionally enable) the feature, with a
> flag to enable/disable it
>     * best condition we probably have is "disabled if 32 bit and
> -ggnu-pubnames" (gnu-pubnames are necessary for efficient building of
> gdb-index, and isn't the default)
> 2) How to implement the flag:
>   * backend option as it is currently (-mllvm
> -use-dwarf-ranges-base-address-specifier (maybe drop "-specifier"
to make
> it a bit more terse))
>   * clang option that maps to
>     * backend option (as currently)
>     * MCOption (programmatic/C++ API rather than command line of a backend
> option)
>     * LLVM IR attribute on the CU (this feature could be supported on a
> per-CU basis easiyl enough, thus survive LTO, etc, exactly as the user
> requested (but really if the user requested this on at least one CU, they
> probably might as well have it on all their CUs))
>     * LLVM IR module metadata
>
>
> Any ideas/thoughts/other aspects?
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170808/fdbcc06e/attachment.html>