Add retval builtin

README.md

@@ -61,6 +61,7 @@ Variables:
 - `nsecs` - Nanosecond timestamp
 - `stack` - Kernel stack trace
 - `arg0`, `arg1`, ... etc. - Arguments to the function being traced
+- `retval` - Return value from function being traced
 
 Functions:
 - `quantize(int n)` - produce a log2 histogram of values of `n`

src/codegen_llvm.cpp

@@ -62,6 +62,14 @@ void CodegenLLVM::visit(Builtin &builtin)
     b_.CreateProbeRead(dst, b_.getInt64(8), src);
     expr_ = b_.CreateLoad(dst);
   }
+  else if (builtin.ident == "retval")
+  {
+    AllocaInst *dst = b_.CreateAllocaBPF();
+    int offset = arch::ret_offset() * sizeof(uintptr_t);
+    Value *src = b_.CreateGEP(ctx_, b_.getInt64(offset));
+    b_.CreateProbeRead(dst, b_.getInt64(8), src);
+    expr_ = b_.CreateLoad(dst);
+  }
   else
   {
     abort();

src/semantic_analyser.cpp

@@ -19,7 +19,8 @@ void SemanticAnalyser::visit(Builtin &builtin)
       builtin.ident == "pid" ||
       builtin.ident == "tid" ||
       builtin.ident == "uid" ||
-      builtin.ident == "gid") {
+      builtin.ident == "gid" ||
+      builtin.ident == "retval") {
     type_ = Type::integer;
   }
   else if (builtin.ident == "stack")