[RFC][clang][BPF] Make trivial uninit var value to be 0

clang/include/clang/Basic/LangOptions.h

@@ -653,6 +653,13 @@ class LangOptions : public LangOptionsBase {
   void set##Name(Type Value) { Name = static_cast<unsigned>(Value); }
 #include "clang/Basic/LangOptions.def"
 
+  unsigned getTrivialAutoVarInitMaxSize() const {
+    return TrivialAutoVarInitMaxSize;
+  }
+  void setTrivialAutoVarInitMaxSize(unsigned max_size) {
+    TrivialAutoVarInitMaxSize = max_size;
+  }
+
   /// Are we compiling a module?
   bool isCompilingModule() const {
     return getCompilingModule() != CMK_None;

clang/lib/Basic/Targets/BPF.cpp

@@ -89,6 +89,18 @@ void BPFTargetInfo::fillValidCPUList(SmallVectorImpl<StringRef> &Values) const {
   Values.append(std::begin(ValidCPUNames), std::end(ValidCPUNames));
 }
 
+void BPFTargetInfo::adjust(DiagnosticsEngine &Diags, LangOptions &Opts) {
+  TargetInfo::adjust(Diags, Opts);
+
+  if (Opts.getTrivialAutoVarInit() ==
+      LangOptions::TrivialAutoVarInitKind::Uninitialized) {
+    Opts.setTrivialAutoVarInit(LangOptions::TrivialAutoVarInitKind::Zero);
+    // Set the maximum auto init size to be 8 to avoid potential regression
+    // e.g. for xdp programs where ip/tcp header size is more than 8.
+    Opts.setTrivialAutoVarInitMaxSize(8);
+  }
+}
+
 llvm::SmallVector<Builtin::InfosShard>
 BPFTargetInfo::getTargetBuiltins() const {
   return {{&BuiltinStrings, BuiltinInfos}};

clang/lib/Basic/Targets/BPF.h

@@ -60,6 +60,8 @@ class LLVM_LIBRARY_VISIBILITY BPFTargetInfo : public TargetInfo {
 
   llvm::SmallVector<Builtin::InfosShard> getTargetBuiltins() const override;
 
+  void adjust(DiagnosticsEngine &Diags, LangOptions &Opts) override;
+
   std::string_view getClobbers() const override { return ""; }
 
   BuiltinVaListKind getBuiltinVaListKind() const override {

clang/test/CodeGen/bpf-auto-var-init.c

@@ -0,0 +1,24 @@
+// REQUIRES: bpf-registered-target
+// RUN: %clang_cc1 -triple bpf -O2 -emit-llvm %s -o - | FileCheck %s
+
+int foo1() {
+  int val;
+  return val;
+}
+// CHECK: ret i32 0
+
+struct val_t {
+  int val;
+};
+
+int foo3() {
+  struct val_t v;
+  return v.val;
+}
+// CHECK: ret i32 0
+
+int foo4() {
+  int val = 5;
+  return val;
+}
+// CHECK: ret i32 5