Emit GC info into a COMDAT section

[MichalStrehovsky]

Saves 3.14% on BasicWebApi. 🤯

We run linker with COMDAT folding enabled. Generating this data into COMDAT foldable section allows linker to deduplicate these.

Cc @dotnet/ilc-contrib

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Issue Details

---

Saves 3.14% on BasicWebApi. 🤯

We run linker with COMDAT folding enabled. Generating this data into COMDAT foldable section allows linker to deduplicate these.

Cc @dotnet/ilc-contrib

Author:	MichalStrehovsky
Assignees:	-
Labels:	area-NativeAOT-coreclr
Milestone:	-

[agocke]

It looks like LLVM defines a number of different SelectionKinds for COMDAT sections, but I can't see where define any. The LLVM ones look like this:

any
exactMatch
largest
noDuplicates
sameSize

[agocke]

Reason I'm asking is because it sounds like we would probably only want "any" or "noDuplicates"

[MichalStrehovsky]

It appears to do the right thing on Windows, but I haven't checked Linux (hoped I would see it in the CI results).

If it doesn't work on Linux, it's likely because of #77491. I'd back out the non-Windows portion of this change and file a bug to reap this benefit outside Windows in 8.0 (it would be more work than just a one-liner that I randomly tried).

[MichalStrehovsky]

Yeah, looks like it doesn't work on Linux.

Before:

/datadisks/disk1/work/A211088F/p/nativeaottest.sh /datadisks/disk1/work/A211088F/w/B4D609BE/e/nativeaot/SmokeTests/HardwareIntrinsics/X64Baseline/ X64Baseline.dll ''
****************************************************
* Size test *
* Size of the executable is 1,916 kB *
****************************************************

After:

/datadisks/disk1/work/A63308E9/p/nativeaottest.sh /datadisks/disk1/work/A63308E9/w/AEDE099C/e/nativeaot/SmokeTests/HardwareIntrinsics/X64Baseline/ X64Baseline.dll ''
****************************************************
* Size test *
* Size of the executable is 1,916 kB *
****************************************************

For comparison, on Windows:

call C:\h\w\B28909D9\p\nativeaottest.cmd C:\h\w\B28909D9\w\C78F0A4F\e\nativeaot\SmokeTests\HardwareIntrinsics\X64Baseline\ X64Baseline.dll ****************************************************
* Size test *
* Size of the executable is 1,835 kB *
****************************************************

call C:\h\w\B6CE09A2\p\nativeaottest.cmd C:\h\w\B6CE09A2\w\A54E08F8\e\nativeaot\SmokeTests\HardwareIntrinsics\X64Baseline\ X64Baseline.dll ****************************************************
* Size test *
* Size of the executable is 1,809 kB *
****************************************************

[MichalStrehovsky]

Rolled back the Linux portion and file #83375 for tracking :(.

[MichalStrehovsky]

/azp run runtime-extra-platforms

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[am11]

Rolled back the Linux portion and file #83375 for tracking :(.

Seems like -Wl,--icf=all and -Wl,--icf=safe etc. do shave off 10.24 KB from hello-world-app on linux-arm64 (using daily build). Unfortunately, bfd linker doesn't support --icf option yet, but LLVM linker -fuse-ld=lld (activated by -p:UseLlvmLinker=true) and Gold linker -fuse-ld=gold support it. Gold linker is better on linux anyway; it's faster, consumes less memory and generates slimmer apps (hello-world-app published with ld.bfd (the default) on linux-arm64: 1918984 bytes while ld.gold: 1853632 bytes). We should probably prefer gold anyway, when it's available.

Introspection in Microsoft.NETCore.Native.Unix.targets will look like this:

<![CDATA[ Testing: if ! cc -fuse-ld=gold -Wl,--icf=all 2>&1 | grep -q unrecognized; then echo "yes, gold linker is available" fi]]>
<ExecCommand="! &quot;$(CppLinker)&quot; -fuse-ld=gold -Wl,--icf=all 2&gt;&amp;1 | grep -q unrecognized"Condition="'$(UseLlvmLinker)' != 'true' and '$(_targetOS)' == 'linux'"IgnoreExitCode="true"StandardOutputImportance="Low">
<OutputTaskParameter="ExitCode"PropertyName="_GoldLinkerSupportedExitCode" />
</Exec>
<ItemGroup>
<LinkerArgInclude="-fuse-ld=gold"Condition="'$(_GoldLinkerSupportedExitCode)' == '0'" />
<LinkerArgInclude="-Wl,--icf=all"Condition="'$(_GoldLinkerSupportedExitCode)' == '0' or '$(UseLlvmLinker)' == 'true'" />
</ItemGroup>

[MichalStrehovsky]

Seems like -Wl,--icf=all and -Wl,--icf=safe etc. do shave off 10.24 KB from hello-world-app on linux-arm64 (using daily build)

Thanks for testing it out! I think this is only folding the native code within the executable. We don't generate code into separate sections unless -p:IlcFoldIdenticalMethodBodies=true was specified. This optimization produces non-sensical callstacks so we're unlikely to enable it by default. It's a good addition, but it's unlikely to help more than those 10 kB even for larger apps.

Looks like what we want is -Wl,--icf-data. But it doesn't seem to have an effect, likely due to #77491 (we mess up something in object writing).

Introspection in Microsoft.NETCore.Native.Unix.targets will look like this:

Is this something you'd be willing to contribute? I just tried -fuse-ld=gold on BasicMinimalApi on Ubuntu 18 and just this one switch without anything else reduced the size from 11.5 MB to 10.8 MB (🤯).

[am11]

This optimization produces non-sensical callstacks so we're unlikely to enable it by default. It's a good addition, but it's unlikely to help more than those 10 kB even for larger apps.

Ah, I haven't looked too closely. I found the difference only with -p:UseLlvmLinker=true (fuse-ld=lld which is selected by default in NativeAOT BuildIntegration on FreeBSD). From the binutils ticket:

The lld's safe ICF works with an LLVM feature. Since mid-2018, LLVM emits a
`.llvm_addrsig` section to all object files by default. That section contains
symbol indices whose addresses are taken. Using this table, lld can merge
functions more aggressively than gold can do.

Perhaps we would see more differences with lld if we start emitting this section? Other linkers apparently will optimize using this section in the future and we will automatically opt-in without additional work (bfd: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105625, mold: rui314/mold#484).

Is this something you'd be willing to contribute? I just tried -fuse-ld=gold on BasicMinimalApi on Ubuntu 18 and just this one switch without anything else reduced the size from 11.5 MB to 10.8 MB (🤯).

Sounds great, I will take a look. 🙂

[MichalStrehovsky]

@dotnet/ilc-contrib could someone have a look? This is a good saving on Windows. We'll have to investigate how to translate this to Linux but that's tracked in #83375. It's likely object writer work around COMDATs.

[jkotas]

LGTM!