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std.debug: consider FP-based unwinding on hexagon and powerpc safe #25454

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Oct 4, 2025
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std.debug: consider FP-based unwinding on hexagon and powerpc safe #25454

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20 changes: 16 additions & 4 deletions lib/std/debug.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -697,7 +697,7 @@ pub fn writeCurrentStackTrace(options: StackUnwindOptions, writer: *Writer, tty_
var printed_any_frame = false;
while (true) switch (it.next()) {
.switch_to_fp => |unwind_error| {
if (StackIterator.fp_unwind_is_safe) continue; // no need to even warn
if (StackIterator.abi_requires_backchain or StackIterator.fp_unwind_is_safe) continue; // no need to even warn
const module_name = di.getModuleName(di_gpa, unwind_error.address) catch "???";
const caption: []const u8 = switch (unwind_error.err) {
error.MissingDebugInfo => "unwind info unavailable",
Expand Down Expand Up @@ -855,10 +855,22 @@ const StackIterator = union(enum) {
}
}

/// On aarch64-macos, Apple mandate that the frame pointer is always used.
/// TODO: are there any other architectures with guarantees like this?
/// <https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Respect-the-purpose-of-specific-CPU-registers>
const fp_unwind_is_safe = builtin.cpu.arch == .aarch64 and builtin.os.tag.isDarwin();

/// On some architectures, we can do FP unwinding even with `-fomit-frame-pointer` due to ABI
/// requirements. Typically this is because the ABI requires a backchain regardless of whether
/// codegen actually uses a frame pointer for stack frame access.
const abi_requires_backchain = switch (builtin.cpu.arch) {
.hexagon,
.powerpc,
.powerpcle,
.powerpc64,
.powerpc64le,
=> true,
else => false,
};

/// Whether the current unwind strategy is allowed given `allow_unsafe`.
fn stratOk(it: *const StackIterator, allow_unsafe: bool) bool {
return switch (it.*) {
Expand All @@ -867,7 +879,7 @@ const StackIterator = union(enum) {
// immediately regardless of anything. But FPs could also be omitted from a different
// linked object, so it's not guaranteed to be safe, unless the target specifically
// requires it.
.fp => !builtin.omit_frame_pointer and (fp_unwind_is_safe or allow_unsafe),
.fp => abi_requires_backchain or (!builtin.omit_frame_pointer and (fp_unwind_is_safe or allow_unsafe)),
};
}

Expand Down