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start: Avoid string concatenation in inline asm. #21056

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merged 4 commits into from
Aug 16, 2024
Merged

start: Avoid string concatenation in inline asm. #21056

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ab9628d
start: Avoid concatenating strings for inline asm.
alexrp Aug 12, 2024
2bf244c
llvm: Always omit the frame pointer for naked functions.
alexrp Aug 13, 2024
acad2fa
llvm: Remove the aarch64-windows @llvm.dbg.value() workaround.
alexrp Aug 13, 2024
7a8e86e
llvm: Don't emit extra debug instructions for `dbg_var_val` in naked …
alexrp Aug 13, 2024
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43 changes: 17 additions & 26 deletions lib/std/start.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -232,6 +232,16 @@ fn _start() callconv(.Naked) noreturn {
);
}

// Move this to the riscv prong below when this is resolved: https://github.com/ziglang/zig/issues/20918
if (builtin.cpu.arch.isRISCV() and builtin.zig_backend != .stage2_riscv64) asm volatile (
\\ .weak __global_pointer$
\\ .hidden __global_pointer$
\\ .option push
\\ .option norelax
\\ lla gp, __global_pointer$
\\ .option pop
);

// Note that we maintain a very low level of trust with regards to ABI guarantees at this point.
// We will redundantly align the stack, clear the link register, etc. While e.g. the Linux
// kernel is usually good about upholding the ABI guarantees, the same cannot be said of dynamic
Expand Down Expand Up @@ -275,24 +285,19 @@ fn _start() callconv(.Naked) noreturn {
\\ and sp, #-16
\\ b %[posixCallMainAndExit]
,
// zig fmt: off
.csky =>
if (builtin.position_independent_code)
// The CSKY ABI assumes that `gb` is set to the address of the GOT in order for
// position-independent code to work. We depend on this in `std.os.linux.start_pie`
// to locate `_DYNAMIC` as well.
\\ grs t0, 1f
\\ 1:
\\ lrw gb, 1b@GOTPC
\\ addu gb, t0
else ""
++
// The CSKY ABI assumes that `gb` is set to the address of the GOT in order for
// position-independent code to work. We depend on this in `std.os.linux.start_pie`
// to locate `_DYNAMIC` as well.
\\ grs t0, 1f
\\ 1:
\\ lrw gb, 1b@GOTPC
\\ addu gb, t0
\\ movi lr, 0
\\ mov a0, sp
\\ andi sp, sp, -8
\\ jmpi %[posixCallMainAndExit]
,
// zig fmt: on
.hexagon =>
// r29 = SP, r30 = FP
\\ r30 = #0
Expand All @@ -308,27 +313,13 @@ fn _start() callconv(.Naked) noreturn {
\\ bstrins.d $sp, $zero, 3, 0
\\ b %[posixCallMainAndExit]
,
// zig fmt: off
.riscv32, .riscv64 =>
// The self-hosted riscv64 backend is not able to assemble this yet.
if (builtin.zig_backend != .stage2_riscv64)
// The RISC-V ELF ABI assumes that `gp` is set to the value of `__global_pointer$` at
// startup in order for GP relaxation to work, even in static builds.
\\ .weak __global_pointer$
\\ .hidden __global_pointer$
\\ .option push
\\ .option norelax
\\ lla gp, __global_pointer$
\\ .option pop
else ""
++
\\ li s0, 0
\\ li ra, 0
\\ mv a0, sp
\\ andi sp, sp, -16
\\ tail %[posixCallMainAndExit]@plt
,
// zig fmt: on
.m68k =>
// Note that the - 8 is needed because pc in the jsr instruction points into the middle
// of the jsr instruction. (The lea is 6 bytes, the jsr is 4 bytes.)
Expand Down
39 changes: 20 additions & 19 deletions src/codegen/llvm.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -2909,7 +2909,17 @@ pub const Object = struct {
function_index.setAlignment(resolved.alignment.toLlvm(), &o.builder);

// Function attributes that are independent of analysis results of the function body.
try o.addCommonFnAttributes(&attributes, owner_mod);
try o.addCommonFnAttributes(
&attributes,
owner_mod,
// Some backends don't respect the `naked` attribute in `TargetFrameLowering::hasFP()`,
// so for these backends, LLVM will happily emit code that accesses the stack through
// the frame pointer. This is nonsensical since what the `naked` attribute does is
// suppress generation of the prologue and epilogue, and the prologue is where the
// frame pointer normally gets set up. At time of writing, this is the case for at
// least x86 and RISC-V.
owner_mod.omit_frame_pointer or fn_info.cc == .Naked,
);

if (fn_info.return_type == .noreturn_type) try attributes.addFnAttr(.noreturn, &o.builder);

Expand Down Expand Up @@ -2956,13 +2966,14 @@ pub const Object = struct {
o: *Object,
attributes: *Builder.FunctionAttributes.Wip,
owner_mod: *Package.Module,
omit_frame_pointer: bool,
) Allocator.Error!void {
const comp = o.pt.zcu.comp;

if (!owner_mod.red_zone) {
try attributes.addFnAttr(.noredzone, &o.builder);
}
if (owner_mod.omit_frame_pointer) {
if (omit_frame_pointer) {
try attributes.addFnAttr(.{ .string = .{
.kind = try o.builder.string("frame-pointer"),
.value = try o.builder.string("none"),
Expand Down Expand Up @@ -4528,7 +4539,7 @@ pub const Object = struct {

var attributes: Builder.FunctionAttributes.Wip = .{};
defer attributes.deinit(&o.builder);
try o.addCommonFnAttributes(&attributes, zcu.root_mod);
try o.addCommonFnAttributes(&attributes, zcu.root_mod, zcu.root_mod.omit_frame_pointer);

function_index.setLinkage(.internal, &o.builder);
function_index.setCallConv(.fastcc, &o.builder);
Expand Down Expand Up @@ -4557,7 +4568,7 @@ pub const Object = struct {

var attributes: Builder.FunctionAttributes.Wip = .{};
defer attributes.deinit(&o.builder);
try o.addCommonFnAttributes(&attributes, zcu.root_mod);
try o.addCommonFnAttributes(&attributes, zcu.root_mod, zcu.root_mod.omit_frame_pointer);

function_index.setLinkage(.internal, &o.builder);
function_index.setCallConv(.fastcc, &o.builder);
Expand Down Expand Up @@ -6709,8 +6720,6 @@ pub const FuncGen = struct {
const operand_ty = self.typeOf(pl_op.operand);
const name = self.air.nullTerminatedString(pl_op.payload);

if (needDbgVarWorkaround(o)) return .none;

const debug_local_var = try o.builder.debugLocalVar(
try o.builder.metadataString(name),
self.file,
Expand All @@ -6734,7 +6743,10 @@ pub const FuncGen = struct {
},
"",
);
} else if (owner_mod.optimize_mode == .Debug) {
} else if (owner_mod.optimize_mode == .Debug and !self.is_naked) {
// We avoid taking this path for naked functions because there's no guarantee that such
// functions even have a valid stack pointer, making the `alloca` + `store` unsafe.

const alignment = operand_ty.abiAlignment(pt).toLlvm();
const alloca = try self.buildAlloca(operand.typeOfWip(&self.wip), alignment);
_ = try self.wip.store(.normal, operand, alloca, alignment);
Expand Down Expand Up @@ -8815,7 +8827,6 @@ pub const FuncGen = struct {
if (self.is_naked) return arg_val;

const inst_ty = self.typeOfIndex(inst);
if (needDbgVarWorkaround(o)) return arg_val;

const name = self.air.instructions.items(.data)[@intFromEnum(inst)].arg.name;
if (name == .none) return arg_val;
Expand Down Expand Up @@ -9676,7 +9687,7 @@ pub const FuncGen = struct {

var attributes: Builder.FunctionAttributes.Wip = .{};
defer attributes.deinit(&o.builder);
try o.addCommonFnAttributes(&attributes, zcu.root_mod);
try o.addCommonFnAttributes(&attributes, zcu.root_mod, zcu.root_mod.omit_frame_pointer);

function_index.setLinkage(.internal, &o.builder);
function_index.setCallConv(.fastcc, &o.builder);
Expand Down Expand Up @@ -11820,16 +11831,6 @@ const optional_layout_version = 3;

const lt_errors_fn_name = "__zig_lt_errors_len";

/// Without this workaround, LLVM crashes with "unknown codeview register H1"
/// https://github.com/llvm/llvm-project/issues/56484
fn needDbgVarWorkaround(o: *Object) bool {
const target = o.pt.zcu.getTarget();
if (target.os.tag == .windows and target.cpu.arch == .aarch64) {
return true;
}
return false;
}

fn compilerRtIntBits(bits: u16) u16 {
inline for (.{ 32, 64, 128 }) |b| {
if (bits <= b) {
Expand Down