zig

commit 7ee6e7d61edcd19c594cb40fdb64703f58e32994 (tree)
parent a3508c4b3a66b15eaa543768b1ea7157f5e5f75b
Author: Alex Rønne Petersen <alex@alexrp.com>
Date:   Wed, 27 May 2026 07:25:29 +0200

std.debug.Dwarf.Unwind.VM: deal with negative offsets in unsigned CFI insns

The DWARF spec has CFI instructions that take signed offsets, but at least LLVM
always emits the unsigned variants, even for e.g. `.cfi_def_cfa_offset -4`.

Diffstat:
M
lib/std/debug/Dwarf/Unwind/VirtualMachine.zig
 | 
16
+++++++++++-----

1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/lib/std/debug/Dwarf/Unwind/VirtualMachine.zig b/lib/std/debug/Dwarf/Unwind/VirtualMachine.zig
@@ -247,10 +247,15 @@ fn evalInstructions(
                     .val_expr => |len| .{ .val_expression = try takeExprBlock(&fr, len) },
                 };
             },
-            .def_cfa => |cfa| vm.current_row.cfa = .{ .reg_off = .{
-                .register = cfa.register,
-                .offset = @intCast(cfa.offset),
-            } },
+            .def_cfa => |cfa| vm.current_row.cfa = .{
+                .reg_off = .{
+                    .register = cfa.register,
+                    // Unfortunately, LLVM emits negative CFI directives as their unsigned variants
+                    // rather than the signed variants that DWARF has for exactly that purpose, hence
+                    // `@bitCast` instead of `@intCast`.
+                    .offset = @bitCast(cfa.offset),
+                },
+            },
             .def_cfa_sf => |cfa| vm.current_row.cfa = .{ .reg_off = .{
                 .register = cfa.register,
                 .offset = cfa.offset_sf * cie.data_alignment_factor,
@@ -272,7 +277,8 @@ fn evalInstructions(
             },
             .def_cfa_offset => |offset| switch (vm.current_row.cfa) {
                 .none, .expression => return error.InvalidOperation,
-                .reg_off => |*ro| ro.offset = @intCast(offset),
+                // See the comment for `def_cfa` above.
+                .reg_off => |*ro| ro.offset = @bitCast(offset),
             },
             .def_cfa_offset_sf => |offset_sf| switch (vm.current_row.cfa) {
                 .none, .expression => return error.InvalidOperation,