From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Tue, 21 Sep 2021 20:48:23 +0000 (-0700)
Subject: runtime: change funcinl sentinel value from 0 to ^0
X-Git-Tag: go1.18beta1~1166
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=6c163e5ac9cdb258566f1287de7915a2fd30a0de;p=gostls13.git

runtime: change funcinl sentinel value from 0 to ^0

_func and funcinl are type-punned.
We distinguish them at runtime by inspecting the first word.

Prior to this change, we used 0 as the sentinel value
that means that a Func is a funcinl.
That worked because _func's first word is the functions' entry PC,
and 0 is not a valid PC. I plan to make *_func's entry PC relative
to the containing module. As a result, 0 will be a valid value,
for the first function in the module.

Switch to ^0 as the new sentinel value, which is neither a valid
entry PC nor a valid PC offset.

Change-Id: I4c718523a083ed6edd57767c3548640681993522
Reviewed-on: https://go-review.googlesource.com/c/go/+/351459
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
---

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 04bd5cb887..442042eb16 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -879,8 +879,8 @@ type _func struct {
 // A *Func can be either a *_func or a *funcinl, and they are distinguished
 // by the first uintptr.
 type funcinl struct {
-	zero  uintptr // set to 0 to distinguish from _func
-	entry uintptr // entry of the real (the "outermost") frame.
+	ones  uintptr // set to ^0 to distinguish from _func
+	entry uintptr // entry of the real (the "outermost") frame
 	name  string
 	file  string
 	line  int
diff --git a/src/runtime/symtab.go b/src/runtime/symtab.go
index 6236643ceb..0f4ad5ab95 100644
--- a/src/runtime/symtab.go
+++ b/src/runtime/symtab.go
@@ -650,6 +650,7 @@ func FuncForPC(pc uintptr) *Func {
 			name := funcnameFromNameoff(f, inltree[ix].func_)
 			file, line := funcline(f, pc)
 			fi := &funcinl{
+				ones:  ^uintptr(0),
 				entry: f.entry, // entry of the real (the outermost) function.
 				name:  name,
 				file:  file,
@@ -667,7 +668,7 @@ func (f *Func) Name() string {
 		return ""
 	}
 	fn := f.raw()
-	if fn.entry == 0 { // inlined version
+	if fn.isInlined() { // inlined version
 		fi := (*funcinl)(unsafe.Pointer(fn))
 		return fi.name
 	}
@@ -677,7 +678,7 @@ func (f *Func) Name() string {
 // Entry returns the entry address of the function.
 func (f *Func) Entry() uintptr {
 	fn := f.raw()
-	if fn.entry == 0 { // inlined version
+	if fn.isInlined() { // inlined version
 		fi := (*funcinl)(unsafe.Pointer(fn))
 		return fi.entry
 	}
@@ -690,7 +691,7 @@ func (f *Func) Entry() uintptr {
 // counter within f.
 func (f *Func) FileLine(pc uintptr) (file string, line int) {
 	fn := f.raw()
-	if fn.entry == 0 { // inlined version
+	if fn.isInlined() { // inlined version
 		fi := (*funcinl)(unsafe.Pointer(fn))
 		return fi.file, fi.line
 	}
@@ -728,6 +729,11 @@ func (f funcInfo) _Func() *Func {
 	return (*Func)(unsafe.Pointer(f._func))
 }
 
+// isInlined reports whether f should be re-interpreted as a *funcinl.
+func (f *_func) isInlined() bool {
+	return f.entry == ^uintptr(0) // see comment for funcinl.ones
+}
+
 // findfunc looks up function metadata for a PC.
 //
 // It is nosplit because it's part of the isgoexception