From 469e2434fd7b174026fbc932cfdf780e818e11e9 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Tue, 2 Jun 2026 14:13:36 -0700
Subject: [PATCH] compiler: document defer frame field dependency

---
 compiler/defer.go    | 2 ++
 src/runtime/panic.go | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/compiler/defer.go b/compiler/defer.go
index 8863bfd87..b0bc3a967 100644
--- a/compiler/defer.go
+++ b/compiler/defer.go
@@ -69,6 +69,8 @@ func (b *builder) deferInitFunc() {
 		// in the setjmp-like inline assembly.
 		deferFrameType := b.getLLVMRuntimeType("deferFrame")
 		b.deferFrame = b.CreateAlloca(deferFrameType, "deferframe.buf")
+		// The field index must match the DeferPtr field in runtime.deferFrame,
+		// defined in src/runtime/panic.go.
 		b.deferPtr = b.CreateInBoundsGEP(deferFrameType, b.deferFrame, []llvm.Value{
 			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
 			llvm.ConstInt(b.ctx.Int32Type(), 6, false), // DeferPtr field
diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index cdd323267..f28cb75a4 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -31,8 +31,8 @@ func panicStrategy() uint8
 // DeferFrame is a stack allocated object that stores information for the
 // current "defer frame", which is used in functions that use the `defer`
 // keyword.
-// The compiler knows about the JumpPC struct offset, so it should not be moved
-// without also updating compiler/defer.go.
+// The compiler knows about the JumpPC struct offset and the DeferPtr field
+// index, so they should not be moved without also updating compiler/defer.go.
 type deferFrame struct {
 	JumpSP     unsafe.Pointer                 // stack pointer to return to
 	JumpPC     unsafe.Pointer                 // pc to return to