[clang] Support -fpass-plugin + -fthinlto-index together (PR #183525)

[Nuri Amari via cfe-commits]

https://github.com/NuriAmari created https://github.com/llvm/llvm-project/pull/183525

Without this change, passing -fthinlto-index causes -fpass-plugin arguments to be ignored. We want to be able to use plugins with distributed thin-lto, so add support for this.

>From cfe82951e17fd541e9eb436161a827fc0032dc70 Mon Sep 17 00:00:00 2001
From: Nuri Amari <nuriamari at fb.com>
Date: Tue, 24 Feb 2026 14:34:51 -0800
Subject: [PATCH] Support -fpass-plugin + -fthinlto-index together

Without this change, passing -fthinlto-index causes -fpass-plugin
arguments to be ignored. We want to be able to use plugins
with distributed thin-lto, so add support for this.
---
 clang/lib/CodeGen/BackendUtil.cpp             |  1 +
 .../distributed-thin-lto/pass-plugin.ll       | 25 +++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 clang/test/CodeGen/distributed-thin-lto/pass-plugin.ll

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 94257fb96fc7f..9b7c7cef8a5d3 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -1382,6 +1382,7 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
   Conf.RemarksFormat = CGOpts.OptRecordFormat;
   Conf.SplitDwarfFile = CGOpts.SplitDwarfFile;
   Conf.SplitDwarfOutput = CGOpts.SplitDwarfOutput;
+  Conf.PassPlugins = CGOpts.PassPlugins;
   switch (Action) {
   case Backend_EmitNothing:
     Conf.PreCodeGenModuleHook = [](size_t Task, const llvm::Module &Mod) {
diff --git a/clang/test/CodeGen/distributed-thin-lto/pass-plugin.ll b/clang/test/CodeGen/distributed-thin-lto/pass-plugin.ll
new file mode 100644
index 0000000000000..fef5f1f3c2c0c
--- /dev/null
+++ b/clang/test/CodeGen/distributed-thin-lto/pass-plugin.ll
@@ -0,0 +1,25 @@
+; REQUIRES: x86-registered-target, plugins, examples
+
+;; Validate that -fpass-plugin works for distributed ThinLTO backends.
+
+; RUN: opt -thinlto-bc -o %t.o %s
+
+; RUN: llvm-lto2 run -thinlto-distributed-indexes %t.o \
+; RUN:   -o %t2.index \
+; RUN:   -r=%t.o,main,px
+
+; RUN: %clang_cc1 -triple x86_64-grtev4-linux-gnu \
+; RUN:   -O2 -emit-obj -fthinlto-index=%t.o.thinlto.bc \
+; RUN:   -fpass-plugin=%llvmshlibdir/Bye%pluginext \
+; RUN:   -mllvm -wave-goodbye \
+; RUN:   -o %t.native.o -x ir %t.o 2>&1 | FileCheck %s
+
+; CHECK: Bye: main
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-grtev4-linux-gnu"
+
+define i32 @main() {
+entry:
+  ret i32 0
+}