Commit `c23a5ccd19`

Alex Rønne Petersen <alex@alexrp.com>

2025-10-07 09:28:43

std.debug.Dwarf.SelfUnwinder: skip caching rules for unsupported registers

For unwinding purposes, we don't care about unsupported registers. Yet because we added these rules to the cache entry, we'd later try to evaluate them and thus fail the unwind attempt for no good reason. They'd also take up cache rule slots that would be better spent on actually relevant registers. Note that any attempt to read unsupported registers during unwinding will still fail the unwind attempt as expected.

master

1 parent 6893e7f

Changed files (1)

lib

std

debug

Dwarf

SelfUnwinder.zig

@@ -15,14 +15,14 @@ cfi_vm: Dwarf.Unwind.VirtualMachine,
 expr_vm: Dwarf.expression.StackMachine(.{ .call_frame_context = true }),
 
 pub const CacheEntry = struct {
-    const max_regs = 32;
+    const max_rules = 32;
 
     pc: usize,
     cie: *const Dwarf.Unwind.CommonInformationEntry,
     cfa_rule: Dwarf.Unwind.VirtualMachine.CfaRule,
     num_rules: u8,
-    rules_regs: [max_regs]u16,
-    rules: [max_regs]Dwarf.Unwind.VirtualMachine.RegisterRule,
+    rules_regs: [max_rules]u16,
+    rules: [max_rules]Dwarf.Unwind.VirtualMachine.RegisterRule,
 
     pub fn find(entries: []const CacheEntry, pc: usize) ?*const CacheEntry {
         assert(pc != 0);
@@ -108,22 +108,30 @@ pub fn computeRules(
 
     unwinder.cfi_vm.reset();
     const row = try unwinder.cfi_vm.runTo(gpa, pc_vaddr, cie, &fde, @sizeOf(usize), native_endian);
-    const cols = unwinder.cfi_vm.rowColumns(&row);
-
-    if (cols.len > CacheEntry.max_regs) return error.UnsupportedDebugInfo;
 
     var entry: CacheEntry = .{
         .pc = unwinder.pc,
         .cie = cie,
         .cfa_rule = row.cfa,
-        .num_rules = @intCast(cols.len),
+        .num_rules = undefined,
         .rules_regs = undefined,
         .rules = undefined,
     };
-    for (cols, 0..) |col, i| {
+    var i: usize = 0;
+    for (unwinder.cfi_vm.rowColumns(&row)) |col| {
+        if (i == CacheEntry.max_rules) return error.UnsupportedDebugInfo;
+
+        _ = unwinder.cpu_state.dwarfRegisterBytes(col.register) catch |err| switch (err) {
+            // Reading an unsupported register during unwinding will result in an error, so there is
+            // no point wasting a rule slot in the cache entry for it.
+            error.UnsupportedRegister => continue,
+            error.InvalidRegister => return error.InvalidDebugInfo,
+        };
         entry.rules_regs[i] = col.register;
         entry.rules[i] = col.rule;
+        i += 1;
     }
+    entry.num_rules = @intCast(i);
     return entry;
 }

Commit c23a5ccd19

Commit `c23a5ccd19`