Set TrivialAutoVarInitMaxSize to 8

This is to prevent auto init for large data structures, e.g.
for xdp prog, iphdr or tcphdr they all more than 8 bytes.
If auto init is not really needed, auto init might cause
performance regression for those programs.

Author: Yonghong Song

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

@@ -93,8 +93,12 @@ void BPFTargetInfo::adjust(DiagnosticsEngine &Diags, LangOptions &Opts) {
   TargetInfo::adjust(Diags, Opts);
 
   if (Opts.getTrivialAutoVarInit() ==
-      LangOptions::TrivialAutoVarInitKind::Uninitialized)
+      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>

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

@@ -7,12 +7,6 @@ int foo1() {
 }
 // CHECK: ret i32 0
 
-int foo2() {
-  int val[4];
-  return val[2];
-}
-// CHECK: ret i32 0
-
 struct val_t {
   int val;
 };