feat: 表达式作为值 — if/else 和 block 可产生值

src/codegen/codegen.c

@@ -451,6 +451,48 @@ static LLVMValueRef codegen_expr(CgCtx* ctx, AstNode* node) {
         return LLVMBuildLoad2(ctx->builder, elem_load_ty, elem_ptr, "arr_load");
     }
 
+    // 块表达式: { stmt*; expr } → 最后表达式的值
+    case AST_BLOCK: {
+        LLVMValueRef result = NULL;
+        for (size_t i = 0; i < node->as.block.stmt_count; i++) {
+            AstNode* stmt = node->as.block.stmts[i];
+            bool is_last = (i == node->as.block.stmt_count - 1);
+            if (is_last && stmt->kind == AST_EXPR_STMT && node->type.kind != TYPE_VOID) {
+                result = codegen_expr(ctx, stmt->as.expr_stmt.expr);
+            } else {
+                codegen_stmt(ctx, stmt);
+            }
+        }
+        return result;
+    }
+
+    // if 表达式: if cond { a } else { b }
+    case AST_IF_STMT: {
+        if (node->type.kind == TYPE_VOID) { codegen_stmt(ctx, node); return NULL; }
+        LLVMValueRef cond_val = codegen_expr(ctx, node->as.if_stmt.cond);
+        if (!cond_val) return NULL;
+        LLVMTypeRef res_ty = type_info_to_llvm(ctx, &node->type);
+        LLVMValueRef alloca = LLVMBuildAlloca(ctx->builder, res_ty, "if_res");
+        LLVMValueRef func = LLVMGetBasicBlockParent(LLVMGetInsertBlock(ctx->builder));
+        LLVMBasicBlockRef then_bb = LLVMAppendBasicBlockInContext(ctx->context, func, "then");
+        LLVMBasicBlockRef else_bb = LLVMAppendBasicBlockInContext(ctx->context, func, "else");
+        LLVMBasicBlockRef merge_bb = LLVMAppendBasicBlockInContext(ctx->context, func, "if_merge");
+        LLVMBuildCondBr(ctx->builder, cond_val, then_bb, else_bb);
+
+        LLVMPositionBuilderAtEnd(ctx->builder, then_bb);
+        LLVMValueRef then_val = codegen_expr(ctx, node->as.if_stmt.then_block);
+        if (then_val) LLVMBuildStore(ctx->builder, then_val, alloca);
+        LLVMBuildBr(ctx->builder, merge_bb);
+
+        LLVMPositionBuilderAtEnd(ctx->builder, else_bb);
+        LLVMValueRef else_val = codegen_expr(ctx, node->as.if_stmt.else_block);
+        if (else_val) LLVMBuildStore(ctx->builder, else_val, alloca);
+        LLVMBuildBr(ctx->builder, merge_bb);
+
+        LLVMPositionBuilderAtEnd(ctx->builder, merge_bb);
+        return LLVMBuildLoad2(ctx->builder, res_ty, alloca, "if_val");
+    }
+
     default:
         return NULL;
     }