handle zero-length array pattern

Author: sebpop

clang/lib/CodeGen/CGExpr.cpp

@@ -4624,13 +4624,13 @@ void CodeGenFunction::EmitArrayBoundsConstraints(const ArraySubscriptExpr *E,
     return;
 
   llvm::APInt ArraySize = CAT->getSize();
-  if (ArraySize == 0)
-    return;
 
   // Don't generate assumes for flexible array member pattern.
-  // Arrays of size 1 in structs are often used as placeholders for
-  // variable-length data (pre-C99 flexible array member idiom.)
-  if (ArraySize == 1) {
+  // Size-1 arrays: "struct { int len; char data[1]; }" (pre-C99 idiom.)
+  // Zero-length arrays: "struct { int len; char data[0]; }" (GCC extension
+  // https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html)
+  // Both patterns use arrays as placeholders for variable-length data.
+  if (ArraySize == 0 || ArraySize == 1) {
     if (const auto *ME = dyn_cast<MemberExpr>(Base->IgnoreParenImpCasts())) {
       if (const auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl())) {
         const RecordDecl *RD = FD->getParent();
@@ -4640,7 +4640,7 @@ void CodeGenFunction::EmitArrayBoundsConstraints(const ArraySubscriptExpr *E,
         for (const auto *Field : RD->fields())
           LastField = Field;
         if (LastField == FD)
-          // This is a size-1 array as the last field in a struct.
+          // This is a zero-length or size-1 array as the last field.
           // Likely a flexible array member pattern - skip assumes.
           return;
       }

clang/test/CodeGen/array-bounds-constraints-safety.c

@@ -6,6 +6,19 @@
 // intentionally accesses out-of-bounds for performance or compatibility.
 // This test verifies that bounds constraints are only applied to safe cases.
 
+// CHECK-LABEL: define {{.*}} @test_zero_length_array
+struct ZeroLengthData {
+    int count;
+    int items[0];  // GNU C extension: zero-length array
+};
+
+int test_zero_length_array(struct ZeroLengthData *d, int i) {
+  // CHECK-NOT: call void @llvm.assume
+  // Zero-length array as last field should not generate bounds constraints.
+  // See https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
+  return d->items[i];
+}
+
 // CHECK-LABEL: define {{.*}} @test_flexible_array_member
 struct Data {
     int count;