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link.MachO.UnwindInfo: reproduce lld's comment

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it explains why zero is used instead of saturation
This commit is contained in:
Andrew Kelley 2025-02-25 11:18:42 -08:00
parent 743623bc54
commit 466ce48e4c
1 changed files with 12 additions and 7 deletions
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19
src/link/MachO/UnwindInfo.zig
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@ -68,13 +68,18 @@ pub fn generate(info: *UnwindInfo, macho_file: *MachO) !void {
for (info.records.items) |ref| {
const rec = ref.getUnwindRecord(macho_file);
if (rec.getFde(macho_file)) |fde| {
// The unwinder will start looking for a matching CFI at the offset we specify here; it
// isn't actually an offset to the exact CFI for this record. A consequence of this is
// that if the offset doesn't fit in 24 bits, we can just leave it as zero so the
// unwinder starts searching at the beginning of the section.
if (std.math.cast(u24, fde.out_offset)) |off| {
rec.enc.setDwarfSectionOffset(off);
}
// The unwinder will look for the DWARF entry starting at the hint,
// assuming the hint points to a valid CFI record start. If it
// fails to find the record, it proceeds in a linear search through
// the contiguous CFI records from the hint until the end of the
// section. Ideally, in the case where the offset is too large to
// be encoded, we would instead encode the largest possible offset
// to a valid CFI record, but since we don't keep track of that,
// just encode zero -- the start of the section is always the start
// of a CFI record.
const hint = std.math.cast(u24, fde.out_offset) orelse 0;
rec.enc.setDwarfSectionOffset(hint);
if (fde.getLsdaAtom(macho_file)) |lsda| {
rec.lsda = lsda.atom_index;
rec.lsda_offset = fde.lsda_offset;