[flang][driver] do not crash when fc1 process multiple files (#138875)

This is a fix for the issue https://github.com/llvm/llvm-project/issues/137126
that turned out to be a driver issue.

FrontendActions has a loop to process multiple input files and `flang -fc1`
accept multiple files, but the semantic, lowering, and llvm codegen
actions were not re-entrant, and crash or weird behaviors occurred
when processing multiple files with `-fc1`. 

This patch makes the actions reentrant by cleaning-up the contexts/modules
if needed on entry.

5 files changed, 23 insertions, 3 deletions

diff --git a/flang/include/flang/Frontend/CompilerInstance.h b/flang/include/flang/Frontend/CompilerInstance.h
index e37ef5e..4ad95c9 100644
--- a/flang/include/flang/Frontend/CompilerInstance.h
+++ b/flang/include/flang/Frontend/CompilerInstance.h
@@ -147,6 +147,12 @@ public:
   /// @name Semantic analysis
   /// {
 
+  Fortran::semantics::SemanticsContext &createNewSemanticsContext() {
+    semaContext =
+        getInvocation().getSemanticsCtx(*allCookedSources, getTargetMachine());
+    return *semaContext;
+  }
+
   Fortran::semantics::SemanticsContext &getSemanticsContext() {
     return *semaContext;
   }
diff --git a/flang/lib/Frontend/CompilerInstance.cpp b/flang/lib/Frontend/CompilerInstance.cpp
index f7ed969..cbd2c58 100644
--- a/flang/lib/Frontend/CompilerInstance.cpp
+++ b/flang/lib/Frontend/CompilerInstance.cpp
@@ -162,8 +162,6 @@ bool CompilerInstance::executeAction(FrontendAction &act) {
   allSources->set_encoding(invoc.getFortranOpts().encoding);
   if (!setUpTargetMachine())
     return false;
-  // Create the semantics context
-  semaContext = invoc.getSemanticsCtx(*allCookedSources, getTargetMachine());
   // Set options controlling lowering to FIR.
   invoc.setLoweringOptions();
 
diff --git a/flang/lib/Frontend/FrontendAction.cpp b/flang/lib/Frontend/FrontendAction.cpp
index ab77d14..d178fd6 100644
--- a/flang/lib/Frontend/FrontendAction.cpp
+++ b/flang/lib/Frontend/FrontendAction.cpp
@@ -183,7 +183,7 @@ bool FrontendAction::runSemanticChecks() {
 
   // Transfer any pending non-fatal messages from parsing to semantics
   // so that they are merged and all printed in order.
-  auto &semanticsCtx{ci.getSemanticsContext()};
+  auto &semanticsCtx{ci.createNewSemanticsContext()};
   semanticsCtx.messages().Annex(std::move(ci.getParsing().messages()));
   semanticsCtx.set_debugModuleWriter(ci.getInvocation().getDebugModuleDir());
 
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
index c1f47b1..e5a15c5 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -171,6 +171,10 @@ static void addDependentLibs(mlir::ModuleOp mlirModule, CompilerInstance &ci) {
 }
 
 bool CodeGenAction::beginSourceFileAction() {
+  // Delete previous LLVM module depending on old context before making a new
+  // one.
+  if (llvmModule)
+    llvmModule.reset(nullptr);
   llvmCtx = std::make_unique<llvm::LLVMContext>();
   CompilerInstance &ci = this->getInstance();
   mlir::DefaultTimingManager &timingMgr = ci.getTimingManager();
@@ -197,6 +201,9 @@ bool CodeGenAction::beginSourceFileAction() {
     return true;
   }
 
+  // Reset MLIR module if it was set before overriding the old context.
+  if (mlirModule)
+    mlirModule = mlir::OwningOpRef<mlir::ModuleOp>(nullptr);
   // Load the MLIR dialects required by Flang
   mlirCtx = std::make_unique<mlir::MLIRContext>();
   fir::support::loadDialects(*mlirCtx);
diff --git a/flang/test/Driver/multiple-fc1-input.f90 b/flang/test/Driver/multiple-fc1-input.f90
new file mode 100644
index 0000000..57f7c5e
--- /dev/null
+++ b/flang/test/Driver/multiple-fc1-input.f90
@@ -0,0 +1,9 @@
+! Test that flang -fc1 can be called with several input files without
+! crashing.
+! Regression tests for: https://github.com/llvm/llvm-project/issues/137126
+
+! RUN: %flang_fc1 -emit-fir %s %s -o - | FileCheck %s
+subroutine foo()
+end subroutine
+! CHECK: func @_QPfoo() 
+! CHECK: func @_QPfoo()