[llvm] [LTO] Avoid assert fail on failed pass plugin load (PR #96691)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lto

Author: Joel E. Denny (jdenny-ornl)

<details>
<summary>Changes</summary>

Without this patch, passing -load-pass-plugin=nonexistent.so to llvm-lto2 produces a backtrace because LTOBackend.cpp does not handle the error correctly:

```
Failed to load passes from 'nonexistant.so'. Request ignored.
Expected<T> must be checked before access or destruction.
Unchecked Expected<T> contained error:
Could not load library 'nonexistant.so': nonexistant.so: cannot open shared object file: No such file or directoryPLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
```

Any tool using `lto::Config::PassPlugins` should suffer similarly.

Based on the message "Request ignored" and the continue statement in the implementation, this patch assumes the intention was not to cause the program to fail.

---
Full diff: https://github.com/llvm/llvm-project/pull/96691.diff


2 Files Affected:

- (modified) llvm/lib/LTO/LTOBackend.cpp (+3-2) 
- (modified) llvm/test/Feature/load_plugin_error.ll (+16-3) 


``````````diff
diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index 84a69d9ff1a1f..7931108c0ec99 100644
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -192,8 +192,9 @@ static void RegisterPassPlugins(ArrayRef<std::string> PassPlugins,
   for (auto &PluginFN : PassPlugins) {
     auto PassPlugin = PassPlugin::Load(PluginFN);
     if (!PassPlugin) {
-      errs() << "Failed to load passes from '" << PluginFN
-             << "'. Request ignored.\n";
+      errs() << "Failed to load passes from plugin '" << PluginFN
+             << "' (request ignored): " << toString(PassPlugin.takeError())
+             <<  "\n";
       continue;
     }
 
diff --git a/llvm/test/Feature/load_plugin_error.ll b/llvm/test/Feature/load_plugin_error.ll
index 24a7cce5d8d39..2b860a901ee41 100644
--- a/llvm/test/Feature/load_plugin_error.ll
+++ b/llvm/test/Feature/load_plugin_error.ll
@@ -1,5 +1,18 @@
-; REQUIRES: plugins, examples
+; REQUIRES: plugins
 ; UNSUPPORTED: target={{.*windows.*}}
 
-; RUN: not opt < %s -load-pass-plugin=%t/nonexistant.so -disable-output 2>&1 | FileCheck %s
-; CHECK: Could not load library {{.*}}nonexistant.so
+; RUN: not opt < %s -load-pass-plugin=%t/nonexistent.so -disable-output 2>&1 | FileCheck %s
+
+; RUN: opt %s -o %t.o
+; RUN: llvm-lto2 run -load-pass-plugin=%t/nonexistent.so %t.o -o %t \
+; RUN:     -r %t.o,test 2>&1 | \
+; RUN:   FileCheck %s
+
+; CHECK: Could not load library {{.*}}nonexistent.so
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @test() {
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/96691