[llvm] [Exegesis] Do not assume the size and layout of the assembled snippet (PR #79636)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-tools-llvm-exegesis

Author: Min-Yih Hsu (mshockwave)

<details>
<summary>Changes</summary>

Currently llvm-exegesis assumes that there will only be 3 symbols in the snippet object, in which the benchmarking function 'foo' is always the last symbol.

These assumptions do not hold for object file formats of other targets we support downstream. I think it would be more ideal to generalize this part of the logics into a simple search on all symbols, as proposed by this patch.

---
Full diff: https://github.com/llvm/llvm-project/pull/79636.diff


1 Files Affected:

- (modified) llvm/tools/llvm-exegesis/lib/Assembler.cpp (+12-5) 


``````````diff
diff --git a/llvm/tools/llvm-exegesis/lib/Assembler.cpp b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
index 9f03a4e3a5a6ff..366971ed6a546d 100644
--- a/llvm/tools/llvm-exegesis/lib/Assembler.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
@@ -365,11 +365,18 @@ Expected<ExecutableFunction> ExecutableFunction::create(
 
   auto SymbolSizes = object::computeSymbolSizes(*ObjectFileHolder.getBinary());
   // Get the size of the function that we want to call into (with the name of
-  // FunctionID). This should always be the third symbol returned by
-  // calculateSymbolSizes.
-  assert(SymbolSizes.size() == 3);
-  assert(cantFail(std::get<0>(SymbolSizes[2]).getName()) == FunctionID);
-  uintptr_t CodeSize = std::get<1>(SymbolSizes[2]);
+  // FunctionID).
+  auto SymbolIt = llvm::find_if(SymbolSizes, [&](const auto &Pair) {
+    auto SymbolName = Pair.first.getName();
+    if (SymbolName)
+      return *SymbolName == FunctionID;
+    // Suppress the error.
+    llvm::consumeError(SymbolName.takeError());
+    return false;
+  });
+  assert(SymbolIt != SymbolSizes.end() &&
+         "Cannot find the symbol for FunctionID");
+  uintptr_t CodeSize = SymbolIt->second;
 
   auto EJITOrErr = orc::LLJITBuilder().create();
   if (!EJITOrErr)

``````````

</details>


https://github.com/llvm/llvm-project/pull/79636