compiler: document defer frame field dependency

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

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