[llvm] [BOLT] Support instrumentation hook via DT_FINI_ARRAY (PR #67348)

[Job Noorman via llvm-commits]

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@@ -1276,6 +1280,60 @@ void RewriteInstance::discoverFileObjects() {
   registerFragments();
 }
 
+Error RewriteInstance::discoverFiniAddress() {
+  // If FiniFunctionAddress is already set, we got if from DT_FINI. We use
+  // DT_FINI instead of DT_FINI_ARRAY if it's available.
+  if (BC->FiniFunctionAddress)
+    return Error::success();
+
+  if (BC->IsStaticExecutable)
+    return Error::success();
+
+  if (!BC->FiniArrayAddress || !BC->FiniArraySize) {
+    return createStringError(
+        std::errc::not_supported,
+        "Instrumentation needs either DT_FINI or DT_FINI_ARRAY");
+  }
+
+  if (*BC->FiniArraySize < BC->AsmInfo->getCodePointerSize()) {
+    return createStringError(std::errc::not_supported,
+                             "Need at least 1 DT_FINI_ARRAY slot");
+  }
+
+  ErrorOr<BinarySection &> FiniArraySection =
+      BC->getSectionForAddress(*BC->FiniArrayAddress);
+  if (auto EC = FiniArraySection.getError())
+    return errorCodeToError(EC);
+
+  BC->FiniArraySection = &*FiniArraySection;
+  const Relocation *FiniFunctionReloc =
+      FiniArraySection->getDynamicRelocationAt(0);
+
+  if (!FiniFunctionReloc) {
+    return createStringError(std::errc::not_supported,
----------------
mtvec wrote:

> Maybe I'm loosing smth but for static relocation you might just use symbol, not the addend?

The problem is that we don't have an `MCSymbol` for the fini address: we get its value by performing a symbol lookup using the linker after the instrumentation library is loaded (see [here](https://github.com/llvm/llvm-project/blob/f9149a34d9b4ea0a13e83befea6c1b1bcf95b1c4/bolt/lib/RuntimeLibs/InstrumentationRuntimeLibrary.cpp#L212)).

https://github.com/llvm/llvm-project/pull/67348