From da878dc0776b56c1e2c251857a40415a35e9535a Mon Sep 17 00:00:00 2001
From: Jacob Young <jacobly0@users.noreply.github.com>
Date: Fri, 5 May 2023 14:56:10 -0400
Subject: [PATCH 1/2] AstGen: fix branch on undefined

`isAlwaysVoid` was being called with the undefined tag added by
`addOne`, causing non-deterministic behavior failures with release
builds of the compiler.  Prevents the following random failure:

    test/behavior/defer.zig:120:40: error: expected type 'error{One}', found 'void'
---
 src/AstGen.zig | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/AstGen.zig b/src/AstGen.zig
index 2568b89980..000151a2cc 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -2905,7 +2905,10 @@ fn deferStmt(
     const sub_scope = if (!have_err_code) &defer_gen.base else blk: {
         try gz.addDbgBlockBegin();
         const ident_name = try gz.astgen.identAsString(payload_token);
-        remapped_err_code = @intCast(u32, try gz.astgen.instructions.addOne(gz.astgen.gpa));
+        remapped_err_code = @intCast(Zir.Inst.Index, gz.astgen.instructions.len);
+        // Use a placeholder tag of .as to allow querying things that depend on the tag,
+        // but undefined data to prevent querying of data.bin.
+        try gz.astgen.instructions.append(gz.astgen.gpa, .{ .tag = .as, .data = undefined });
         const remapped_err_code_ref = Zir.indexToRef(remapped_err_code);
         local_val_scope = .{
             .parent = &defer_gen.base,

From df4849c4f5aafc7eb80fd7b2299b49162fa2aa79 Mon Sep 17 00:00:00 2001
From: Jacob Young <jacobly0@users.noreply.github.com>
Date: Fri, 5 May 2023 16:07:27 -0400
Subject: [PATCH 2/2] AstGen: cleanup previous fix

Allocating an extended tag is much cleaner and easier to reason about
than reusing an existing tag.  The previous `.data = undefined` was a
clear indication that we don't have any data to store, and so might as
well store an extended tag in that space almost for free.
---
 src/AstGen.zig    | 11 ++++++++---
 src/Sema.zig      |  1 +
 src/Zir.zig       |  7 +++++--
 src/print_zir.zig |  1 +
 4 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/AstGen.zig b/src/AstGen.zig
index 000151a2cc..749e3d28c4 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -2906,9 +2906,14 @@ fn deferStmt(
         try gz.addDbgBlockBegin();
         const ident_name = try gz.astgen.identAsString(payload_token);
         remapped_err_code = @intCast(Zir.Inst.Index, gz.astgen.instructions.len);
-        // Use a placeholder tag of .as to allow querying things that depend on the tag,
-        // but undefined data to prevent querying of data.bin.
-        try gz.astgen.instructions.append(gz.astgen.gpa, .{ .tag = .as, .data = undefined });
+        try gz.astgen.instructions.append(gz.astgen.gpa, .{
+            .tag = .extended,
+            .data = .{ .extended = .{
+                .opcode = .errdefer_err_code,
+                .small = undefined,
+                .operand = undefined,
+            } },
+        });
         const remapped_err_code_ref = Zir.indexToRef(remapped_err_code);
         local_val_scope = .{
             .parent = &defer_gen.base,
diff --git a/src/Sema.zig b/src/Sema.zig
index 13f4d684a1..b2387dfe12 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -1198,6 +1198,7 @@ fn analyzeBodyInner(
                         i += 1;
                         continue;
                     },
+                    .errdefer_err_code => unreachable, // never appears in a body
                 };
             },
 
diff --git a/src/Zir.zig b/src/Zir.zig
index 51c90c61cb..7f9681e533 100644
--- a/src/Zir.zig
+++ b/src/Zir.zig
@@ -1979,7 +1979,7 @@ pub const Inst = struct {
         /// `operand` is `src_node: i32`.
         breakpoint,
         /// Implements the `@select` builtin.
-        /// operand` is payload index to `Select`.
+        /// `operand` is payload index to `Select`.
         select,
         /// Implement builtin `@errToInt`.
         /// `operand` is payload index to `UnNode`.
@@ -1999,7 +1999,7 @@ pub const Inst = struct {
         /// `operand` is payload index to `Cmpxchg`.
         cmpxchg,
         /// Implement the builtin `@addrSpaceCast`
-        /// `Operand` is payload index to `BinNode`. `lhs` is dest type, `rhs` is operand.
+        /// `operand` is payload index to `BinNode`. `lhs` is dest type, `rhs` is operand.
         addrspace_cast,
         /// Implement builtin `@cVaArg`.
         /// `operand` is payload index to `BinNode`.
@@ -2031,6 +2031,9 @@ pub const Inst = struct {
         /// Implements the `@inComptime` builtin.
         /// `operand` is `src_node: i32`.
         in_comptime,
+        /// Used as a placeholder for the capture of an `errdefer`.
+        /// This is replaced by Sema with the captured value.
+        errdefer_err_code,
 
         pub const InstData = struct {
             opcode: Extended,
diff --git a/src/print_zir.zig b/src/print_zir.zig
index 4e4f13641c..f228adc7ea 100644
--- a/src/print_zir.zig
+++ b/src/print_zir.zig
@@ -467,6 +467,7 @@ const Writer = struct {
             .breakpoint,
             .c_va_start,
             .in_comptime,
+            .errdefer_err_code,
             => try self.writeExtNode(stream, extended),
 
             .builtin_src => {