Skip default values for local scope array that doesn't have initializ…

…ed values
fruits-lab · Jul 7, 2024 · 6e743d3 · 6e743d3
1 parent a8b9e37
commit 6e743d3
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Showing 3 changed files with 18 additions and 7 deletions.
diff --git a/src/llvm_ir_generator.cpp b/src/llvm_ir_generator.cpp
@@ -201,13 +201,15 @@ void LLVMIRGenerator::Visit(const ArrDeclNode& arr_decl) {
   auto arr_decl_type = dynamic_cast<ArrType*>(arr_decl.type.get());
   // This vector stores the initialize values for an array.
   std::vector<llvm::Constant*> arr_elems{};
-  for (auto i = std::size_t{0}, e = arr_decl_type->len(); i < e; ++i) {
-    if (i < arr_decl.init_list.size()) {
+  for (auto i = std::size_t{0}, e = arr_decl_type->len(),
+            init_len = arr_decl.init_list.size();
+       i < e; ++i) {
+    if (i < init_len) {
       auto& arr_init = arr_decl.init_list.at(i);
       arr_init->Accept(*this);
     }
 
-    if (i < arr_decl.init_list.size()) {
+    if (i < init_len) {
       auto init_val = val_recorder.ValOfPrevExpr();
       if (arr_decl.is_global) {
         auto const_val = llvm::dyn_cast<llvm::Constant>(init_val);
@@ -222,7 +224,9 @@ void LLVMIRGenerator::Visit(const ArrDeclNode& arr_decl) {
       auto zero = llvm::ConstantInt::get(builder_.getInt32Ty(), 0, true);
       if (arr_decl.is_global) {
         arr_elems.push_back(zero);
-      } else {
+      } else if (!arr_decl.is_global && 0 < init_len) {
+        // If array is in local scope and its initialized values is not
+        // declared, then compiler will not set element values to 0.
         auto res_addr = builder_.CreateConstInBoundsGEP2_32(
             type, id_to_val.at(arr_decl.id), 0, i);
         builder_.CreateStore(zero, res_addr);

diff --git a/src/qbe_ir_generator.cpp b/src/qbe_ir_generator.cpp
@@ -257,8 +257,14 @@ void QbeIrGenerator::Visit(const ArrDeclNode& arr_decl) {
                 arr_decl.type->size());
     id_to_num[arr_decl.id] = base_addr_num;
 
-    for (auto i = std::size_t{0}, e = arr_type->len(); i < e; ++i) {
-      if (i < arr_decl.init_list.size()) {
+    // NOTE: Compiler will not set elements to 0 if `init_len` is 0.
+    // 6.7.9 Initialization
+    // 10. If an object that has automatic storage duration is not initialized
+    // explicitly, its value is indeterminate.
+    for (auto i = std::size_t{0}, e = arr_type->len(),
+              init_len = arr_decl.init_list.size();
+         i < e && 0 < init_len; ++i) {
+      if (i < init_len) {
         auto& arr_init = arr_decl.init_list.at(i);
         arr_init->Accept(*this);
       }
@@ -271,7 +277,7 @@ void QbeIrGenerator::Visit(const ArrDeclNode& arr_decl) {
       WriteInstr_("{} =l add {}, {}", FuncScopeTemp{res_addr_num},
                   FuncScopeTemp{base_addr_num}, FuncScopeTemp{offset});
 
-      if (i < arr_decl.init_list.size()) {
+      if (i < init_len) {
         int init_val_num = num_recorder.NumOfPrevExpr();
         WriteInstr_("storew {}, {}", FuncScopeTemp{init_val_num},
                     FuncScopeTemp{res_addr_num});

diff --git a/test/codegen/array.c b/test/codegen/array.c
@@ -22,5 +22,6 @@ int main() {
   __builtin_print(e[1]);
   __builtin_print(e[2]);
 
+  int f[2];
   return 0;
 }