[lld] [lld][symbol-ordering] Strip out llvm suffixes for orderfile (PR #102458)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-elf

Author: Egor Pasko (pasko)

<details>
<summary>Changes</summary>

When Chrome DSO on Android is built with ThinLTO a large number of symbols acquire unique suffixes. A renamed symbol looks like this:
```
_ZL13_uhash_removeP10UHashtable8UElement.llvm.4477738147604602593
```

This makes it difficult to provide a precise --symbol-ordering-file because the build does not have control over these unique names.

One workaround is to rebuild with symbol ordering file after inspecting symbols in the first build. Chrome build infrastructure does not scale well when it has to deal with such additional build steps. It also significantly increases build times for release binaries, mostly doing duplicate work.

Instead, remove such suffix from each symbol before matching it against the ordering file.

Similar stripping is done in FunctionSamples::getCanonicalFnName().

This should help fixing http://crbug.com/356360613.

---
Full diff: https://github.com/llvm/llvm-project/pull/102458.diff


1 Files Affected:

- (modified) lld/ELF/Writer.cpp (+16-1) 


``````````diff
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 4c0b4df5bea17..08bafd85f1e98 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -1061,6 +1061,20 @@ static void maybeShuffle(DenseMap<const InputSectionBase *, int> &order) {
   }
 }
 
+/// Indicator of a suffix inserted by LTO. Each symbol is matched against the
+/// symbol ordering file after stripping out such a suffix. This is similar to
+/// canonicalization in SampleProf.h.
+static constexpr const char *LLVMSuffix = ".llvm.";
+
+// Strips out suffixes commonly added by optimization passes.
+static StringRef getCanonicalSymbolName(StringRef symbolName) {
+  StringRef ret(symbolName);
+  auto it = ret.rfind(LLVMSuffix);
+  if (it != StringRef::npos)
+    ret = ret.substr(0, it);
+  return ret;
+}
+
 // Builds section order for handling --symbol-ordering-file.
 static DenseMap<const InputSectionBase *, int> buildSectionOrder() {
   DenseMap<const InputSectionBase *, int> sectionOrder;
@@ -1086,7 +1100,8 @@ static DenseMap<const InputSectionBase *, int> buildSectionOrder() {
 
   // Build a map from sections to their priorities.
   auto addSym = [&](Symbol &sym) {
-    auto it = symbolOrder.find(CachedHashStringRef(sym.getName()));
+    auto it = symbolOrder.find(
+        CachedHashStringRef(getCanonicalSymbolName(sym.getName())));
     if (it == symbolOrder.end())
       return;
     SymbolOrderEntry &ent = it->second;

``````````

</details>


https://github.com/llvm/llvm-project/pull/102458