Commit 32ea6f54e5

Andrew Kelley <superjoe30@gmail.com>
2018-01-12 08:12:11
*WIP* proof of concept error return traces
1 parent 7ec7838
src/all_types.hpp
@@ -1205,6 +1205,7 @@ struct FnTableEntry {
     uint32_t alignstack_value;
 
     ZigList<FnExport> export_list;
+    bool calls_errorable_function;
 };
 
 uint32_t fn_table_entry_hash(FnTableEntry*);
@@ -1530,6 +1531,7 @@ struct CodeGen {
     FnTableEntry *panic_fn;
     LLVMValueRef cur_ret_ptr;
     LLVMValueRef cur_fn_val;
+    LLVMValueRef cur_err_ret_trace_val;
     bool c_want_stdint;
     bool c_want_stdbool;
     AstNode *root_export_decl;
src/analyze.cpp
@@ -869,7 +869,7 @@ static const char *calling_convention_fn_type_str(CallingConvention cc) {
     zig_unreachable();
 }
 
-static TypeTableEntry *get_ptr_to_stack_trace_type(CodeGen *g) {
+TypeTableEntry *get_ptr_to_stack_trace_type(CodeGen *g) {
     if (g->stack_trace_type == nullptr) {
         ConstExprValue *stack_trace_type_val = get_builtin_value(g, "StackTrace");
         assert(stack_trace_type_val->type->id == TypeTableEntryIdMetaType);
@@ -1191,6 +1191,9 @@ static TypeTableEntry *analyze_fn_type(CodeGen *g, AstNode *proto_node, Scope *c
         }
 
         TypeTableEntry *type_entry = analyze_type_expr(g, child_scope, param_node->data.param_decl.type);
+        if (type_is_invalid(type_entry)) {
+            return g->builtin_types.entry_invalid;
+        }
         if (fn_type_id.cc != CallingConventionUnspecified) {
             type_ensure_zero_bits_known(g, type_entry);
             if (!type_has_bits(type_entry)) {
@@ -2586,6 +2589,7 @@ static void wrong_panic_prototype(CodeGen *g, AstNode *proto_node, TypeTableEntr
 }
 
 static void typecheck_panic_fn(CodeGen *g, FnTableEntry *panic_fn) {
+    return; // TODO
     AstNode *proto_node = panic_fn->proto_node;
     assert(proto_node->type == NodeTypeFnProto);
     TypeTableEntry *fn_type = panic_fn->type_entry;
src/analyze.hpp
@@ -187,6 +187,7 @@ void add_fn_export(CodeGen *g, FnTableEntry *fn_table_entry, Buf *symbol_name, G
 
 
 ConstExprValue *get_builtin_value(CodeGen *codegen, const char *name);
+TypeTableEntry *get_ptr_to_stack_trace_type(CodeGen *g);
 
 
 #endif
src/codegen.cpp
@@ -22,6 +22,8 @@
 #include <stdio.h>
 #include <errno.h>
 
+static const size_t stack_trace_ptr_count = 31;
+
 static void init_darwin_native(CodeGen *g) {
     char *osx_target = getenv("MACOSX_DEPLOYMENT_TARGET");
     char *ios_target = getenv("IPHONEOS_DEPLOYMENT_TARGET");
@@ -867,16 +869,24 @@ static LLVMValueRef get_panic_msg_ptr_val(CodeGen *g, PanicMsgId msg_id) {
     return LLVMConstBitCast(val->global_refs->llvm_global, LLVMPointerType(str_type->type_ref, 0));
 }
 
-static void gen_panic(CodeGen *g, LLVMValueRef msg_arg) {
+static void gen_panic(CodeGen *g, LLVMValueRef msg_arg, LLVMValueRef stack_trace_arg) {
     assert(g->panic_fn != nullptr);
     LLVMValueRef fn_val = fn_llvm_value(g, g->panic_fn);
     LLVMCallConv llvm_cc = get_llvm_cc(g, g->panic_fn->type_entry->data.fn.fn_type_id.cc);
-    ZigLLVMBuildCall(g->builder, fn_val, &msg_arg, 1, llvm_cc, ZigLLVM_FnInlineAuto, "");
+    if (stack_trace_arg == nullptr) {
+        TypeTableEntry *ptr_to_stack_trace_type = get_ptr_to_stack_trace_type(g);
+        stack_trace_arg = LLVMConstNull(ptr_to_stack_trace_type->type_ref);
+    }
+    LLVMValueRef args[] = {
+        msg_arg,
+        stack_trace_arg,
+    };
+    ZigLLVMBuildCall(g->builder, fn_val, args, 2, llvm_cc, ZigLLVM_FnInlineAuto, "");
     LLVMBuildUnreachable(g->builder);
 }
 
 static void gen_debug_safety_crash(CodeGen *g, PanicMsgId msg_id) {
-    gen_panic(g, get_panic_msg_ptr_val(g, msg_id));
+    gen_panic(g, get_panic_msg_ptr_val(g, msg_id), nullptr);
 }
 
 static LLVMValueRef get_memcpy_fn_val(CodeGen *g) {
@@ -956,7 +966,11 @@ static LLVMValueRef get_safety_crash_err_fn(CodeGen *g) {
     LLVMValueRef offset_buf_ptr = LLVMConstInBoundsGEP(global_array, offset_ptr_indices, 2);
 
     Buf *fn_name = get_mangled_name(g, buf_create_from_str("__zig_fail_unwrap"), false);
-    LLVMTypeRef fn_type_ref = LLVMFunctionType(LLVMVoidType(), &g->err_tag_type->type_ref, 1, false);
+    LLVMTypeRef arg_types[] = {
+        g->err_tag_type->type_ref,
+        g->ptr_to_stack_trace_type->type_ref,
+    };
+    LLVMTypeRef fn_type_ref = LLVMFunctionType(LLVMVoidType(), arg_types, 2, false);
     LLVMValueRef fn_val = LLVMAddFunction(g->module, buf_ptr(fn_name), fn_type_ref);
     addLLVMFnAttr(fn_val, "noreturn");
     addLLVMFnAttr(fn_val, "cold");
@@ -1008,7 +1022,7 @@ static LLVMValueRef get_safety_crash_err_fn(CodeGen *g) {
     LLVMValueRef global_slice_len_field_ptr = LLVMBuildStructGEP(g->builder, global_slice, slice_len_index, "");
     gen_store(g, full_buf_len, global_slice_len_field_ptr, u8_ptr_type);
 
-    gen_panic(g, global_slice);
+    gen_panic(g, global_slice, LLVMGetParam(fn_val, 1));
 
     LLVMPositionBuilderAtEnd(g->builder, prev_block);
     LLVMSetCurrentDebugLocation(g->builder, prev_debug_location);
@@ -1019,7 +1033,16 @@ static LLVMValueRef get_safety_crash_err_fn(CodeGen *g) {
 
 static void gen_debug_safety_crash_for_err(CodeGen *g, LLVMValueRef err_val) {
     LLVMValueRef safety_crash_err_fn = get_safety_crash_err_fn(g);
-    ZigLLVMBuildCall(g->builder, safety_crash_err_fn, &err_val, 1, get_llvm_cc(g, CallingConventionUnspecified),
+    LLVMValueRef err_ret_trace_val = g->cur_err_ret_trace_val;
+    if (err_ret_trace_val == nullptr) {
+        TypeTableEntry *ptr_to_stack_trace_type = get_ptr_to_stack_trace_type(g);
+        err_ret_trace_val = LLVMConstNull(ptr_to_stack_trace_type->type_ref);
+    }
+    LLVMValueRef args[] = {
+        err_val,
+        err_ret_trace_val,
+    };
+    ZigLLVMBuildCall(g->builder, safety_crash_err_fn, args, 2, get_llvm_cc(g, CallingConventionUnspecified),
         ZigLLVM_FnInlineAuto, "");
     LLVMBuildUnreachable(g->builder);
 }
@@ -1299,6 +1322,50 @@ static LLVMValueRef ir_llvm_value(CodeGen *g, IrInstruction *instruction) {
 static LLVMValueRef ir_render_return(CodeGen *g, IrExecutable *executable, IrInstructionReturn *return_instruction) {
     LLVMValueRef value = ir_llvm_value(g, return_instruction->value);
     TypeTableEntry *return_type = return_instruction->value->value.type;
+
+    bool is_err_return = false;
+    if (return_type->id == TypeTableEntryIdErrorUnion) {
+        if (return_instruction->value->value.special == ConstValSpecialStatic) {
+            is_err_return = return_instruction->value->value.data.x_err_union.err != nullptr;
+        } else if (return_instruction->value->value.special == ConstValSpecialRuntime) {
+            is_err_return = return_instruction->value->value.data.rh_error_union == RuntimeHintErrorUnionError;
+            // TODO: emit a branch to check if the return value is an error
+        }
+    } else if (return_type->id == TypeTableEntryIdPureError) {
+        is_err_return = true;
+    }
+    if (is_err_return) {
+        LLVMTypeRef usize_type_ref = g->builtin_types.entry_usize->type_ref;
+
+        // stack_trace.instruction_addresses[stack_trace.index % stack_trace_ptr_count] = @instructionPointer();
+        // stack_trace.index += 1;
+
+        LLVMBasicBlockRef return_block = LLVMAppendBasicBlock(g->cur_fn_val, "ReturnError");
+
+        LLVMValueRef block_address = LLVMBlockAddress(g->cur_fn_val, return_block);
+        size_t index_field_index = g->stack_trace_type->data.structure.fields[0].gen_index;
+        LLVMValueRef index_field_ptr = LLVMBuildStructGEP(g->builder, g->cur_err_ret_trace_val, (unsigned)index_field_index, "");
+        size_t addresses_field_index = g->stack_trace_type->data.structure.fields[1].gen_index;
+        LLVMValueRef addresses_field_ptr = LLVMBuildStructGEP(g->builder, g->cur_err_ret_trace_val, (unsigned)addresses_field_index, "");
+
+        // stack_trace.instruction_addresses[stack_trace.index % stack_trace_ptr_count] = @instructionPointer();
+        LLVMValueRef index_val = gen_load_untyped(g, index_field_ptr, 0, false, "");
+        LLVMValueRef modded_val = LLVMBuildURem(g->builder, index_val, LLVMConstInt(usize_type_ref, stack_trace_ptr_count, false), "");
+        LLVMValueRef address_indices[] = {
+            LLVMConstNull(usize_type_ref),
+            modded_val,
+        };
+        LLVMValueRef address_slot = LLVMBuildInBoundsGEP(g->builder, addresses_field_ptr, address_indices, 2, "");
+        LLVMValueRef address_value = LLVMBuildPtrToInt(g->builder, block_address, usize_type_ref, "");
+        gen_store_untyped(g, address_value, address_slot, 0, false);
+
+        // stack_trace.index += 1;
+        LLVMValueRef index_plus_one_val = LLVMBuildAdd(g->builder, index_val, LLVMConstInt(usize_type_ref, 1, false), "");
+        gen_store_untyped(g, index_plus_one_val, index_field_ptr, 0, false);
+
+        LLVMBuildBr(g->builder, return_block);
+        LLVMPositionBuilderAtEnd(g->builder, return_block);
+    }
     if (handle_is_ptr(return_type)) {
         if (calling_convention_does_first_arg_return(g->cur_fn->type_entry->data.fn.fn_type_id.cc)) {
             assert(g->cur_ret_ptr);
@@ -2353,7 +2420,7 @@ static LLVMValueRef ir_render_call(CodeGen *g, IrExecutable *executable, IrInstr
         }
     }
     if (last_arg_err_ret_stack) {
-        gen_param_values[gen_param_index] = LLVMGetUndef(g->ptr_to_stack_trace_type->type_ref);
+        gen_param_values[gen_param_index] = g->cur_err_ret_trace_val;
         gen_param_index += 1;
     }
 
@@ -3482,7 +3549,7 @@ static LLVMValueRef ir_render_container_init_list(CodeGen *g, IrExecutable *exec
 }
 
 static LLVMValueRef ir_render_panic(CodeGen *g, IrExecutable *executable, IrInstructionPanic *instruction) {
-    gen_panic(g, ir_llvm_value(g, instruction->msg));
+    gen_panic(g, ir_llvm_value(g, instruction->msg), nullptr);
     return nullptr;
 }
 
@@ -4501,7 +4568,8 @@ static void do_code_gen(CodeGen *g) {
         LLVMValueRef fn = fn_llvm_value(g, fn_table_entry);
         g->cur_fn = fn_table_entry;
         g->cur_fn_val = fn;
-        if (handle_is_ptr(fn_table_entry->type_entry->data.fn.fn_type_id.return_type)) {
+        TypeTableEntry *return_type = fn_table_entry->type_entry->data.fn.fn_type_id.return_type;
+        if (handle_is_ptr(return_type)) {
             g->cur_ret_ptr = LLVMGetParam(fn, 0);
         } else {
             g->cur_ret_ptr = nullptr;
@@ -4510,6 +4578,18 @@ static void do_code_gen(CodeGen *g) {
         build_all_basic_blocks(g, fn_table_entry);
         clear_debug_source_node(g);
 
+        if (return_type->id == TypeTableEntryIdPureError || return_type->id == TypeTableEntryIdErrorUnion) {
+            g->cur_err_ret_trace_val = LLVMGetParam(fn, LLVMCountParamTypes(fn_table_entry->type_entry->data.fn.raw_type_ref) - 1);
+        } else if (fn_table_entry->calls_errorable_function) {
+            g->cur_err_ret_trace_val = build_alloca(g, g->stack_trace_type, "error_return_trace", get_abi_alignment(g, g->stack_trace_type));
+            size_t index_field_index = g->stack_trace_type->data.structure.fields[0].gen_index;
+            LLVMValueRef index_field_ptr = LLVMBuildStructGEP(g->builder, g->cur_err_ret_trace_val, (unsigned)index_field_index, "");
+            TypeTableEntry *usize = g->builtin_types.entry_usize;
+            gen_store_untyped(g, LLVMConstNull(usize->type_ref), index_field_ptr, 0, false);
+        } else {
+            g->cur_err_ret_trace_val = nullptr;
+        }
+
         // allocate temporary stack data
         for (size_t alloca_i = 0; alloca_i < fn_table_entry->alloca_list.length; alloca_i += 1) {
             IrInstruction *instruction = fn_table_entry->alloca_list.at(alloca_i);
@@ -5096,12 +5176,11 @@ static void define_builtin_compile_vars(CodeGen *g) {
     os_path_join(g->cache_dir, buf_create_from_str(builtin_zig_basename), builtin_zig_path);
     Buf *contents = buf_alloc();
 
-    buf_append_str(contents,
+    buf_appendf(contents,
         "pub const StackTrace = struct {\n"
         "    index: usize,\n"
-        "    instruction_addresses: [31]usize,\n"
-        "};\n\n"
-    );
+        "    instruction_addresses: [%" ZIG_PRI_usize "]usize,\n"
+        "};\n\n", stack_trace_ptr_count);
 
     const char *cur_os = nullptr;
     {
@@ -5266,6 +5345,7 @@ static void define_builtin_compile_vars(CodeGen *g) {
     g->root_package->package_table.put(buf_create_from_str("builtin"), g->compile_var_package);
     g->std_package->package_table.put(buf_create_from_str("builtin"), g->compile_var_package);
     g->compile_var_import = add_source_file(g, g->compile_var_package, abs_full_path, contents);
+    scan_import(g, g->compile_var_import);
 }
 
 static void init(CodeGen *g) {
src/ir.cpp
@@ -10043,9 +10043,21 @@ static TypeTableEntry *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCall *cal
         TypeTableEntry *return_type = impl_fn->type_entry->data.fn.fn_type_id.return_type;
         ir_add_alloca(ira, new_call_instruction, return_type);
 
+        if (return_type->id == TypeTableEntryIdPureError || return_type->id == TypeTableEntryIdErrorUnion) {
+            parent_fn_entry->calls_errorable_function = true;
+        }
+
         return ir_finish_anal(ira, return_type);
     }
 
+    FnTableEntry *parent_fn_entry = exec_fn_entry(ira->new_irb.exec);
+    assert(fn_type_id->return_type != nullptr);
+    assert(parent_fn_entry != nullptr);
+    if (fn_type_id->return_type->id == TypeTableEntryIdPureError || fn_type_id->return_type->id == TypeTableEntryIdErrorUnion) {
+        parent_fn_entry->calls_errorable_function = true;
+    }
+
+
     IrInstruction **casted_args = allocate<IrInstruction *>(call_param_count);
     size_t next_arg_index = 0;
     if (first_arg_ptr) {
std/debug/index.zig
@@ -37,10 +37,16 @@ fn getStderrStream() -> %&io.OutStream {
     }
 }
 
+/// Tries to print the current stack trace to stderr, unbuffered, and ignores any error returned.
+pub fn dumpCurrentStackTrace() {
+    const stderr = getStderrStream() catch return;
+    writeCurrentStackTrace(stderr, global_allocator, stderr_file.isTty(), 1) catch return;
+}
+
 /// Tries to print a stack trace to stderr, unbuffered, and ignores any error returned.
-pub fn dumpStackTrace() {
+pub fn dumpStackTrace(stack_trace: &builtin.StackTrace) {
     const stderr = getStderrStream() catch return;
-    writeStackTrace(stderr, global_allocator, stderr_file.isTty(), 1) catch return;
+    writeStackTrace(stack_trace, stderr, global_allocator, stderr_file.isTty()) catch return;
 }
 
 /// This function invokes undefined behavior when `ok` is `false`.
@@ -88,7 +94,7 @@ pub fn panic(comptime format: []const u8, args: ...) -> noreturn {
 
     const stderr = getStderrStream() catch os.abort();
     stderr.print(format ++ "\n", args) catch os.abort();
-    writeStackTrace(stderr, global_allocator, stderr_file.isTty(), 1) catch os.abort();
+    writeCurrentStackTrace(stderr, global_allocator, stderr_file.isTty(), 1) catch os.abort();
 
     os.abort();
 }
@@ -101,7 +107,103 @@ const RESET = "\x1b[0m";
 error PathNotFound;
 error InvalidDebugInfo;
 
-pub fn writeStackTrace(out_stream: &io.OutStream, allocator: &mem.Allocator, tty_color: bool,
+pub fn writeStackTrace(st_addrs: &builtin.StackTrace, out_stream: &io.OutStream, allocator: &mem.Allocator, tty_color: bool) -> %void {
+    switch (builtin.object_format) {
+        builtin.ObjectFormat.elf => {
+            var stack_trace = ElfStackTrace {
+                .self_exe_file = undefined,
+                .elf = undefined,
+                .debug_info = undefined,
+                .debug_abbrev = undefined,
+                .debug_str = undefined,
+                .debug_line = undefined,
+                .debug_ranges = null,
+                .abbrev_table_list = ArrayList(AbbrevTableHeader).init(allocator),
+                .compile_unit_list = ArrayList(CompileUnit).init(allocator),
+            };
+            const st = &stack_trace;
+            st.self_exe_file = try os.openSelfExe();
+            defer st.self_exe_file.close();
+
+            try st.elf.openFile(allocator, &st.self_exe_file);
+            defer st.elf.close();
+
+            st.debug_info = (try st.elf.findSection(".debug_info")) ?? return error.MissingDebugInfo;
+            st.debug_abbrev = (try st.elf.findSection(".debug_abbrev")) ?? return error.MissingDebugInfo;
+            st.debug_str = (try st.elf.findSection(".debug_str")) ?? return error.MissingDebugInfo;
+            st.debug_line = (try st.elf.findSection(".debug_line")) ?? return error.MissingDebugInfo;
+            st.debug_ranges = (try st.elf.findSection(".debug_ranges"));
+            try scanAllCompileUnits(st);
+
+            var ignored_count: usize = 0;
+
+            var frame_index: usize = undefined;
+            var frames_left: usize = undefined;
+            if (st_addrs.index < st_addrs.instruction_addresses.len) {
+                frame_index = 0;
+                frames_left = st_addrs.index;
+            } else {
+                frame_index = (st_addrs.index + 1) % st_addrs.instruction_addresses.len;
+                frames_left = st_addrs.instruction_addresses.len;
+            }
+
+            while (frames_left != 0) : ({frames_left -= 1; frame_index = (frame_index + 1) % st_addrs.instruction_addresses.len;}) {
+                const return_address = st_addrs.instruction_addresses[frame_index];
+
+                // TODO we really should be able to convert @sizeOf(usize) * 2 to a string literal
+                // at compile time. I'll call it issue #313
+                const ptr_hex = if (@sizeOf(usize) == 4) "0x{x8}" else "0x{x16}";
+
+                const compile_unit = findCompileUnit(st, return_address) catch {
+                    try out_stream.print("???:?:?: " ++ DIM ++ ptr_hex ++ " in ??? (???)" ++ RESET ++ "\n    ???\n\n",
+                        return_address);
+                    continue;
+                };
+                const compile_unit_name = try compile_unit.die.getAttrString(st, DW.AT_name);
+                if (getLineNumberInfo(st, compile_unit, usize(return_address) - 1)) |line_info| {
+                    defer line_info.deinit();
+                    try out_stream.print(WHITE ++ "{}:{}:{}" ++ RESET ++ ": " ++
+                        DIM ++ ptr_hex ++ " in ??? ({})" ++ RESET ++ "\n",
+                        line_info.file_name, line_info.line, line_info.column,
+                        return_address, compile_unit_name);
+                    if (printLineFromFile(st.allocator(), out_stream, line_info)) {
+                        if (line_info.column == 0) {
+                            try out_stream.write("\n");
+                        } else {
+                            {var col_i: usize = 1; while (col_i < line_info.column) : (col_i += 1) {
+                                try out_stream.writeByte(' ');
+                            }}
+                            try out_stream.write(GREEN ++ "^" ++ RESET ++ "\n");
+                        }
+                    } else |err| switch (err) {
+                        error.EndOfFile, error.PathNotFound => {},
+                        else => return err,
+                    }
+                } else |err| switch (err) {
+                    error.MissingDebugInfo, error.InvalidDebugInfo => {
+                        try out_stream.print(ptr_hex ++ " in ??? ({})\n",
+                            return_address, compile_unit_name);
+                    },
+                    else => return err,
+                }
+            }
+        },
+        builtin.ObjectFormat.coff => {
+            try out_stream.write("(stack trace unavailable for COFF object format)\n");
+        },
+        builtin.ObjectFormat.macho => {
+            try out_stream.write("(stack trace unavailable for Mach-O object format)\n");
+        },
+        builtin.ObjectFormat.wasm => {
+            try out_stream.write("(stack trace unavailable for WASM object format)\n");
+        },
+        builtin.ObjectFormat.unknown => {
+            try out_stream.write("(stack trace unavailable for unknown object format)\n");
+        },
+    }
+}
+
+pub fn writeCurrentStackTrace(out_stream: &io.OutStream, allocator: &mem.Allocator, tty_color: bool,
     ignore_frame_count: usize) -> %void
 {
     switch (builtin.object_format) {
std/special/compiler_rt/index.zig
@@ -74,7 +74,7 @@ const __udivmoddi4 = @import("udivmoddi4.zig").__udivmoddi4;
 
 // Avoid dragging in the debug safety mechanisms into this .o file,
 // unless we're trying to test this file.
-pub coldcc fn panic(msg: []const u8) -> noreturn {
+pub coldcc fn panic(msg: []const u8, error_return_trace: ?&builtin.StackTrace) -> noreturn {
     if (is_test) {
         @import("std").debug.panic("{}", msg);
     } else {
std/special/build_runner.zig
@@ -112,7 +112,7 @@ pub fn main() -> %void {
     }
 
     builder.setInstallPrefix(prefix);
-    root.build(&builder);
+    root.build(&builder) catch unreachable;
 
     if (builder.validateUserInputDidItFail())
         return usageAndErr(&builder, true, try stderr_stream);
@@ -129,7 +129,7 @@ fn usage(builder: &Builder, already_ran_build: bool, out_stream: &io.OutStream)
     // run the build script to collect the options
     if (!already_ran_build) {
         builder.setInstallPrefix(null);
-        root.build(builder);
+        root.build(builder) catch unreachable;
     }
 
     // This usage text has to be synchronized with src/main.cpp
std/special/builtin.zig
@@ -5,7 +5,7 @@ const builtin = @import("builtin");
 
 // Avoid dragging in the debug safety mechanisms into this .o file,
 // unless we're trying to test this file.
-pub coldcc fn panic(msg: []const u8) -> noreturn {
+pub coldcc fn panic(msg: []const u8, error_return_trace: ?&builtin.StackTrace) -> noreturn {
     if (builtin.is_test) {
         @import("std").debug.panic("{}", msg);
     } else {
std/special/panic.zig
@@ -4,14 +4,20 @@
 // have to be added in the compiler.
 
 const builtin = @import("builtin");
+const std = @import("std");
 
-pub coldcc fn panic(msg: []const u8) -> noreturn {
+pub coldcc fn panic(msg: []const u8, error_return_trace: ?&builtin.StackTrace) -> noreturn {
     switch (builtin.os) {
         // TODO: fix panic in zen.
         builtin.Os.freestanding, builtin.Os.zen => {
             while (true) {}
         },
         else => {
+            if (error_return_trace) |trace| {
+                std.debug.warn("{}\n", msg);
+                std.debug.dumpStackTrace(trace);
+                @import("std").debug.panic("");
+            }
             @import("std").debug.panic("{}", msg);
         },
     }
std/build.zig
@@ -721,11 +721,9 @@ pub const Builder = struct {
         return error.FileNotFound;
     }
 
-    pub fn exec(self: &Builder, argv: []const []const u8) -> []u8 {
+    pub fn exec(self: &Builder, argv: []const []const u8) -> %[]u8 {
         const max_output_size = 100 * 1024;
-        const result = os.ChildProcess.exec(self.allocator, argv, null, null, max_output_size) catch |err| {
-            std.debug.panic("Unable to spawn {}: {}", argv[0], @errorName(err));
-        };
+        const result = try os.ChildProcess.exec(self.allocator, argv, null, null, max_output_size);
         switch (result.term) {
             os.ChildProcess.Term.Exited => |code| {
                 if (code != 0) {
build.zig
@@ -10,7 +10,7 @@ const ArrayList = std.ArrayList;
 const Buffer = std.Buffer;
 const io = std.io;
 
-pub fn build(b: &Builder) {
+pub fn build(b: &Builder) -> %void {
     const mode = b.standardReleaseOptions();
 
     var docgen_exe = b.addExecutable("docgen", "doc/docgen.zig");
@@ -36,7 +36,7 @@ pub fn build(b: &Builder) {
     const test_step = b.step("test", "Run all the tests");
 
     // find the stage0 build artifacts because we're going to re-use config.h and zig_cpp library
-    const build_info = b.exec([][]const u8{b.zig_exe, "BUILD_INFO"});
+    const build_info = try b.exec([][]const u8{b.zig_exe, "BUILD_INFO"});
     var index: usize = 0;
     const cmake_binary_dir = nextValue(&index, build_info);
     const cxx_compiler = nextValue(&index, build_info);
@@ -68,7 +68,7 @@ pub fn build(b: &Builder) {
     dependOnLib(exe, llvm);
 
     if (exe.target.getOs() == builtin.Os.linux) {
-        const libstdcxx_path_padded = b.exec([][]const u8{cxx_compiler, "-print-file-name=libstdc++.a"});
+        const libstdcxx_path_padded = try b.exec([][]const u8{cxx_compiler, "-print-file-name=libstdc++.a"});
         const libstdcxx_path = ??mem.split(libstdcxx_path_padded, "\r\n").next();
         exe.addObjectFile(libstdcxx_path);
 
@@ -155,9 +155,9 @@ const LibraryDep = struct {
 };
 
 fn findLLVM(b: &Builder, llvm_config_exe: []const u8) -> %LibraryDep {
-    const libs_output = b.exec([][]const u8{llvm_config_exe, "--libs", "--system-libs"});
-    const includes_output = b.exec([][]const u8{llvm_config_exe, "--includedir"});
-    const libdir_output = b.exec([][]const u8{llvm_config_exe, "--libdir"});
+    const libs_output = try b.exec([][]const u8{llvm_config_exe, "--libs", "--system-libs"});
+    const includes_output = try b.exec([][]const u8{llvm_config_exe, "--includedir"});
+    const libdir_output = try b.exec([][]const u8{llvm_config_exe, "--libdir"});
 
     var result = LibraryDep {
         .libs = ArrayList([]const u8).init(b.allocator),