runtime: document implicit fields at the end of _func

Currently these are only implicitly documented by the code that
generates and consumes them. Add explicit documentation.

Change-Id: I25a1294f04dc11785242643bec83370c66ff7a20
Reviewed-on: https://go-review.googlesource.com/c/go/+/427954
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 44dcfcca82..f9bdb8e236 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -873,6 +873,28 @@ type _func struct {
 	flag      funcFlag
 	_         [1]byte // pad
 	nfuncdata uint8   // must be last, must end on a uint32-aligned boundary
+
+	// The end of the struct is followed immediately by two variable-length
+	// arrays that reference the pcdata and funcdata locations for this
+	// function.
+
+	// pcdata contains the offset into moduledata.pctab for the start of
+	// that index's table. e.g.,
+	// &moduledata.pctab[_func.pcdata[_PCDATA_UnsafePoint]] is the start of
+	// the unsafe point table.
+	//
+	// An offset of 0 indicates that there is no table.
+	//
+	// pcdata [npcdata]uint32
+
+	// funcdata contains the offset past moduledata.gofunc which contains a
+	// pointer to that index's funcdata. e.g.,
+	// *(moduledata.gofunc +  _func.funcdata[_FUNCDATA_ArgsPointerMaps]) is
+	// the argument pointer map.
+	//
+	// An offset of ^uint32(0) indicates that there is no entry.
+	//
+	// funcdata [nfuncdata]uint32
 }
 
 // Pseudo-Func that is returned for PCs that occur in inlined code.