Revert of linux: Improve gdb startup time for debug builds from over 4 minutes to 35s.

Revert of linux: Improve gdb startup time for debug builds from over 4 minutes to 35s. (patchset #4 id:60001 of https://codereview.chromium.org/2770933009/ )

Reason for revert:
Suspicion that it cause a compile failure on https://build.chromium.org/p/chromium.linux/builders/Linux%20Builder%20%28dbg%29%2832%29/builds/65207

Original issue's description:
> linux: Improve gdb startup time for debug builds from over 4 minutes to 35s.
> 
> This change contains two parts, for full-symbol (symbol_level=2, default)
> and reduced-symbol (symbol_level=1) debug builds. It has no effect on release
> builds.
> 
> 1.) For symbol_level=2, pass -Wl,--gdb-index to the linker. This lets the linker
> write an index that lets it load binaries much faster. 
> gdb startup time for target chrome from goes from 60s to 10s and time from
> `run` in gdb to the program actually starting from 270s to 45s.
> In return, this slows down linking a bit, but for target chrome in a
> debug build, it increases link time from 37s to 42s, which is better than
> making people who want to use gdb wait several minutes every time they start
> gdb.
> 
> There's some history here: We used to pass -Wl,--gdb-index long ago, and
> then removed it in https://codereview.chromium.org/335903002/, with the
> recommendation that people who want to use gdb could run build/gdb-add-index.
> But running `gdb-add-index chrome` takes 73s nowadays, a lot more than the 5s
> that gold needs. (Back then, gdb-add-index was faster, and gold, due to us
> not yet defaulting to component builds in debug builds, was slower. Also,
> people were on an older default Ubuntu and used an older gdb version.)
> 
> People who don't use gdb should use symbol_level=1 for their builds anyhow
> (and bots do too), so this small regression in link time shouldn't affect
> them.
> 
> Remove the explicit gdb_index gn arg now that this has a good default.
> 
> 2) For symbol_level=1, make this mode actually work again after the gn swtich.
> In symbol_level=1 builds, gn would pass `-g1 -gsplit-dwarf` to clang (*).
> Surprisingly, -gsplit-dwarf implies -g2 with clang, so the -g1 gets
> overriden by -g2 immediately.
> 
> Before this, symbol_level=1 in debug builds would produce full debug info.
> Since all bots set symbol_level=1, this might help with build speed
> on debug bots.  For people who set this locally, it'll also speed up
> gdb startup time for target chrome from 39s to 13s and time from
> `run` in gdb to the program actually starting from 255s to 35s.
> 
> *: clang always writes stack debug info to both .o and .dwo files, and lets
> the linker link them into the executable, so -g1 -gsplit-dwarf would make
> no sense. This was used as justification to make -gsplit-dwarf imply -g2,
> as it otherwise wouldn't have an effect.
> 
> BUG=374952
> 
> Review-Url: https://codereview.chromium.org/2770933009
> Cr-Commit-Position: refs/heads/master@{#459790}
> Committed: https://chromium.googlesource.com/chromium/src/+/7b26c51270cebd5de672a155b34f30bc8282c35a

TBR=dpranke@chromium.org,piman@chromium.org,tansell@chromium.org,thakis@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=374952
Review-Url: https://codereview.chromium.org/2776193002
Cr-Commit-Position: refs/heads/master@{#459800}
Committed: https://chromium.googlesource.com/chromium/src/+/745ab003bddd9043058e1f19db831a9b4f0f6dbf

[jwd, 2017-03-27 16:29:33]

The CQ bit was checked by jwd@chromium.org

[jwd, 2017-03-27 16:29:33]

Created Revert of linux: Improve gdb startup time for debug builds from over 4
minutes to 35s.

[commit-bot: I haz the power, 2017-03-27 16:30:11]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-27 16:30:52]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1490632173362100, "parent_rev":
"0e44ebc7dcf7c343d45694b97b2edc455f1a6256", "commit_rev":
"745ab003bddd9043058e1f19db831a9b4f0f6dbf"}

[commit-bot: I haz the power, 2017-03-27 16:31:39]

Description was changed from

==========
Revert of linux: Improve gdb startup time for debug builds from over 4 minutes
to 35s. (patchset #4 id:60001 of https://codereview.chromium.org/2770933009/ )

Reason for revert:
Suspicion that it cause a compile failure on
https://build.chromium.org/p/chromium.linux/builders/Linux%20Builder%20%28dbg...

Original issue's description:
> linux: Improve gdb startup time for debug builds from over 4 minutes to 35s.
> 
> This change contains two parts, for full-symbol (symbol_level=2, default)
> and reduced-symbol (symbol_level=1) debug builds. It has no effect on release
> builds.
> 
> 1.) For symbol_level=2, pass -Wl,--gdb-index to the linker. This lets the
linker
> write an index that lets it load binaries much faster. 
> gdb startup time for target chrome from goes from 60s to 10s and time from
> `run` in gdb to the program actually starting from 270s to 45s.
> In return, this slows down linking a bit, but for target chrome in a
> debug build, it increases link time from 37s to 42s, which is better than
> making people who want to use gdb wait several minutes every time they start
> gdb.
> 
> There's some history here: We used to pass -Wl,--gdb-index long ago, and
> then removed it in https://codereview.chromium.org/335903002/, with the
> recommendation that people who want to use gdb could run build/gdb-add-index.
> But running `gdb-add-index chrome` takes 73s nowadays, a lot more than the 5s
> that gold needs. (Back then, gdb-add-index was faster, and gold, due to us
> not yet defaulting to component builds in debug builds, was slower. Also,
> people were on an older default Ubuntu and used an older gdb version.)
> 
> People who don't use gdb should use symbol_level=1 for their builds anyhow
> (and bots do too), so this small regression in link time shouldn't affect
> them.
> 
> Remove the explicit gdb_index gn arg now that this has a good default.
> 
> 2) For symbol_level=1, make this mode actually work again after the gn swtich.
> In symbol_level=1 builds, gn would pass `-g1 -gsplit-dwarf` to clang (*).
> Surprisingly, -gsplit-dwarf implies -g2 with clang, so the -g1 gets
> overriden by -g2 immediately.
> 
> Before this, symbol_level=1 in debug builds would produce full debug info.
> Since all bots set symbol_level=1, this might help with build speed
> on debug bots.  For people who set this locally, it'll also speed up
> gdb startup time for target chrome from 39s to 13s and time from
> `run` in gdb to the program actually starting from 255s to 35s.
> 
> *: clang always writes stack debug info to both .o and .dwo files, and lets
> the linker link them into the executable, so -g1 -gsplit-dwarf would make
> no sense. This was used as justification to make -gsplit-dwarf imply -g2,
> as it otherwise wouldn't have an effect.
> 
> BUG=374952
> 
> Review-Url: https://codereview.chromium.org/2770933009
> Cr-Commit-Position: refs/heads/master@{#459790}
> Committed:
https://chromium.googlesource.com/chromium/src/+/7b26c51270cebd5de672a155b34f...

TBR=dpranke@chromium.org,piman@chromium.org,tansell@chromium.org,thakis@chrom...
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=374952
==========

to

==========
Revert of linux: Improve gdb startup time for debug builds from over 4 minutes
to 35s. (patchset #4 id:60001 of https://codereview.chromium.org/2770933009/ )

Reason for revert:
Suspicion that it cause a compile failure on
https://build.chromium.org/p/chromium.linux/builders/Linux%20Builder%20%28dbg...

Original issue's description:
> linux: Improve gdb startup time for debug builds from over 4 minutes to 35s.
> 
> This change contains two parts, for full-symbol (symbol_level=2, default)
> and reduced-symbol (symbol_level=1) debug builds. It has no effect on release
> builds.
> 
> 1.) For symbol_level=2, pass -Wl,--gdb-index to the linker. This lets the
linker
> write an index that lets it load binaries much faster. 
> gdb startup time for target chrome from goes from 60s to 10s and time from
> `run` in gdb to the program actually starting from 270s to 45s.
> In return, this slows down linking a bit, but for target chrome in a
> debug build, it increases link time from 37s to 42s, which is better than
> making people who want to use gdb wait several minutes every time they start
> gdb.
> 
> There's some history here: We used to pass -Wl,--gdb-index long ago, and
> then removed it in https://codereview.chromium.org/335903002/, with the
> recommendation that people who want to use gdb could run build/gdb-add-index.
> But running `gdb-add-index chrome` takes 73s nowadays, a lot more than the 5s
> that gold needs. (Back then, gdb-add-index was faster, and gold, due to us
> not yet defaulting to component builds in debug builds, was slower. Also,
> people were on an older default Ubuntu and used an older gdb version.)
> 
> People who don't use gdb should use symbol_level=1 for their builds anyhow
> (and bots do too), so this small regression in link time shouldn't affect
> them.
> 
> Remove the explicit gdb_index gn arg now that this has a good default.
> 
> 2) For symbol_level=1, make this mode actually work again after the gn swtich.
> In symbol_level=1 builds, gn would pass `-g1 -gsplit-dwarf` to clang (*).
> Surprisingly, -gsplit-dwarf implies -g2 with clang, so the -g1 gets
> overriden by -g2 immediately.
> 
> Before this, symbol_level=1 in debug builds would produce full debug info.
> Since all bots set symbol_level=1, this might help with build speed
> on debug bots.  For people who set this locally, it'll also speed up
> gdb startup time for target chrome from 39s to 13s and time from
> `run` in gdb to the program actually starting from 255s to 35s.
> 
> *: clang always writes stack debug info to both .o and .dwo files, and lets
> the linker link them into the executable, so -g1 -gsplit-dwarf would make
> no sense. This was used as justification to make -gsplit-dwarf imply -g2,
> as it otherwise wouldn't have an effect.
> 
> BUG=374952
> 
> Review-Url: https://codereview.chromium.org/2770933009
> Cr-Commit-Position: refs/heads/master@{#459790}
> Committed:
https://chromium.googlesource.com/chromium/src/+/7b26c51270cebd5de672a155b34f...

TBR=dpranke@chromium.org,piman@chromium.org,tansell@chromium.org,thakis@chrom...
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=374952

Review-Url: https://codereview.chromium.org/2776193002
Cr-Commit-Position: refs/heads/master@{#459800}
Committed:
https://chromium.googlesource.com/chromium/src/+/745ab003bddd9043058e1f19db83...
==========

[commit-bot: I haz the power, 2017-03-27 16:31:40]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/745ab003bddd9043058e1f19db83...