[LLD][MachO] Option to emit separate cstring sections #158720
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@llvm/pr-subscribers-lld-macho @llvm/pr-subscribers-lld Author: Ellis Hoag (ellishg) ChangesAdd the The reason this is useful is because strings in different sections might have different access patterns at runtime. By splitting these strings into separate sections, we may reduce the number of page faults during startup. For example, the ObjC runtime accesses all strings in Full diff: https://github.com/llvm/llvm-project/pull/158720.diff 9 Files Affected:
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For reference, code emitting the |
lld/MachO/MapFile.cpp
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I don't think performance is a concern here, since the number of entries in this map is not large.
Although I don't believe using this map impacts the determinism of the output, could you double-check that?
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Thanks for pointing out the iteration order. Instead I decided to store cstring sections in a map whose order is deterministic and use a map to point to the indices in the vector.
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| firstTLVDataSection = nullptr; | ||
| tar = nullptr; | ||
| memset(&in, 0, sizeof(in)); | ||
| in = InStruct(); |
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I think adding a vector and map to this struct made it no longer trivially copyable. This seems like a better way to clear it anyway.
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Maybe a little bit off-topic, but could we consider installing string tail merging for |
I wasn't aware of string tail merging. Thanks for sharing! I think it's definitely worth pursuing if it gives a size win. Do ObjC methods tend to have shares suffixes? I would assume they might share prefixes if two methods are the same but with different number of arguments. |
I just check a large binary and it seems |
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I realized this broke the builds. I'll try to land a fix shortly. https://lab.llvm.org/buildbot/#/builders/190/builds/27994 |
Fix a test added in #158720. I had accidentally required arm64 when the test was using x86_64.
Fix a test added in llvm/llvm-project#158720. I had accidentally required arm64 when the test was using x86_64.
@nocchijiang I just published #161262 to tail merge cstring sections and it gives a nice win! Thanks again for pointing this out! |
I forgot to add a release note for llvm/llvm-project#158720 so I'll add it here.
I forgot to add a release note for llvm#158720 so I'll add it here.
Add the flag `--tail-merge-strings` to enable tail merging of cstrings.
For example, if we have strings `mystring\0` and `ring\0`, we could
place `mystring\0` at address `0x1000` and `ring\0` at address `0x1004`
and have them share the same underlying data.
It turns out that many ObjC method names can be tail merged. For
example, `error:` and `doFoo:error:`. On a large iOS binary, we saw
nearly a 15% size improvement in the `__TEXT__objc_methname` section and
negligible impact on link time.
```
$ bloaty --domain=vm merged.o.stripped -- base.o.stripped
VM SIZE
--------------
+95% +5.85Ki [__TEXT]
-2.4% -239Ki __TEXT,__cstring
-14.5% -710Ki __TEXT,__objc_methname
-1.0% -944Ki TOTAL
```
Tail merging for MachO was originally removed in
7c269db.
The previous implementation used `StringTableBuilder`, but that was
removed in
4308f03
to ensure deduplicated strings are aligned correctly. This
implementation ensures that tail merged strings are also aligned
correctly.
Special thanks to nocchijiang for pointing this out in
#158720 (comment).
Depends on #161253.
Add the flag `--tail-merge-strings` to enable tail merging of cstrings.
For example, if we have strings `mystring\0` and `ring\0`, we could
place `mystring\0` at address `0x1000` and `ring\0` at address `0x1004`
and have them share the same underlying data.
It turns out that many ObjC method names can be tail merged. For
example, `error:` and `doFoo:error:`. On a large iOS binary, we saw
nearly a 15% size improvement in the `__TEXT__objc_methname` section and
negligible impact on link time.
```
$ bloaty --domain=vm merged.o.stripped -- base.o.stripped
VM SIZE
--------------
+95% +5.85Ki [__TEXT]
-2.4% -239Ki __TEXT,__cstring
-14.5% -710Ki __TEXT,__objc_methname
-1.0% -944Ki TOTAL
```
Tail merging for MachO was originally removed in
llvm/llvm-project@7c269db.
The previous implementation used `StringTableBuilder`, but that was
removed in
llvm/llvm-project@4308f03
to ensure deduplicated strings are aligned correctly. This
implementation ensures that tail merged strings are also aligned
correctly.
Special thanks to nocchijiang for pointing this out in
llvm/llvm-project#158720 (comment).
Depends on llvm/llvm-project#161253.
4 participants
Add the
--{no-}separate-cstring-literal-sectionsoption to emit cstring literals into sections defined by their section name. This allows for changes like swiftlang/swift#84300 and swiftlang/swift#84236 to actually have an affect. The default behavior has not changed.The reason this is useful is because strings in different sections might have different access patterns at runtime. By splitting these strings into separate sections, we may reduce the number of page faults during startup. For example, the ObjC runtime accesses all strings in
__objc_classnamebefore main.