runtime: store incremented PC in result of runtime.Callers

In 1.11 we stored "return addresses" in the result of runtime.Callers.
I changed that behavior in CL 152537 to store an address in the call
instruction itself. This CL reverts that part of 152537.

The change in 152537 was made because we now store pcs of inline marks
in the result of runtime.Callers as well. This CL will now store the
address of the inline mark + 1 in the results of runtime.Callers, so
that the subsequent -1 done in CallersFrames will pick out the correct
inline mark instruction.

This CL means that the results of runtime.Callers can be passed to
runtime.FuncForPC as they were before. There are a bunch of packages
in the wild that take the results of runtime.Callers, subtract 1, and
then call FuncForPC. This CL keeps that pattern working as it did in
1.11.

The changes to runtime/pprof in this CL are exactly a revert of the
changes to that package in 152537 (except the locForPC comment).

Update #29582

Change-Id: I04d232000fb482f0f0ff6277f8d7b9c72e97eb48
Reviewed-on: https://go-review.googlesource.com/c/156657
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

5 files changed, 31 insertions, 9 deletions

diff --git a/src/runtime/pprof/proto.go b/src/runtime/pprof/proto.go
index 7621fe2134..7864dd79ad 100644
--- a/src/runtime/pprof/proto.go
+++ b/src/runtime/pprof/proto.go
@@ -208,7 +208,7 @@ func (b *profileBuilder) pbMapping(tag int, id, base, limit, offset uint64, file
 }
 
 // locForPC returns the location ID for addr.
-// addr must a PC which is part of a call or the PC of an inline marker. This returns the location of the call.
+// addr must a return PC or 1 + the PC of an inline marker. This returns the location of the corresponding call.
 // It may emit to b.pb, so there must be no message encoding in progress.
 func (b *profileBuilder) locForPC(addr uintptr) uint64 {
 	id := uint64(b.locs[addr])
@@ -236,7 +236,7 @@ func (b *profileBuilder) locForPC(addr uintptr) uint64 {
 	if frame.PC == 0 {
 		// If we failed to resolve the frame, at least make up
 		// a reasonable call PC. This mostly happens in tests.
-		frame.PC = addr
+		frame.PC = addr - 1
 	}
 
 	// We can't write out functions while in the middle of the
@@ -403,7 +403,16 @@ func (b *profileBuilder) build() {
 		}
 
 		locs = locs[:0]
-		for _, addr := range e.stk {
+		for i, addr := range e.stk {
+			// Addresses from stack traces point to the
+			// next instruction after each call, except
+			// for the leaf, which points to where the
+			// signal occurred. locForPC expects return
+			// PCs, so increment the leaf address to look
+			// like a return PC.
+			if i == 0 {
+				addr++
+			}
 			l := b.locForPC(addr)
 			if l == 0 { // runtime.goexit
 				continue
diff --git a/src/runtime/pprof/proto_test.go b/src/runtime/pprof/proto_test.go
index 9b2de5f644..4452d51231 100644
--- a/src/runtime/pprof/proto_test.go
+++ b/src/runtime/pprof/proto_test.go
@@ -133,11 +133,11 @@ func TestConvertCPUProfile(t *testing.T) {
 	samples := []*profile.Sample{
 		{Value: []int64{20, 20 * 2000 * 1000}, Location: []*profile.Location{
 			{ID: 1, Mapping: map1, Address: addr1},
-			{ID: 2, Mapping: map1, Address: addr1 + 2},
+			{ID: 2, Mapping: map1, Address: addr1 + 1},
 		}},
 		{Value: []int64{40, 40 * 2000 * 1000}, Location: []*profile.Location{
 			{ID: 3, Mapping: map2, Address: addr2},
-			{ID: 4, Mapping: map2, Address: addr2 + 2},
+			{ID: 4, Mapping: map2, Address: addr2 + 1},
 		}},
 	}
 	checkProfile(t, p, period, periodType, sampleType, samples, "")
diff --git a/src/runtime/pprof/protomem_test.go b/src/runtime/pprof/protomem_test.go
index 65ef4edf8f..471b1ae9c3 100644
--- a/src/runtime/pprof/protomem_test.go
+++ b/src/runtime/pprof/protomem_test.go
@@ -14,7 +14,11 @@ import (
 func TestConvertMemProfile(t *testing.T) {
 	addr1, addr2, map1, map2 := testPCs(t)
 
-	a1, a2 := uintptr(addr1), uintptr(addr2)
+	// MemProfileRecord stacks are return PCs, so add one to the
+	// addresses recorded in the "profile". The proto profile
+	// locations are call PCs, so conversion will subtract one
+	// from these and get back to addr1 and addr2.
+	a1, a2 := uintptr(addr1)+1, uintptr(addr2)+1
 	rate := int64(512 * 1024)
 	rec := []runtime.MemProfileRecord{
 		{AllocBytes: 4096, FreeBytes: 1024, AllocObjects: 4, FreeObjects: 1, Stack0: [32]uintptr{a1, a2}},
diff --git a/src/runtime/symtab.go b/src/runtime/symtab.go
index 0fd4330944..245a7e6b01 100644
--- a/src/runtime/symtab.go
+++ b/src/runtime/symtab.go
@@ -87,6 +87,13 @@ func (ci *Frames) Next() (frame Frame, more bool) {
 		}
 		f := funcInfo._Func()
 		entry := f.Entry()
+		if pc > entry {
+			// We store the pc of the start of the instruction following
+			// the instruction in question (the call or the inline mark).
+			// This is done for historical reasons, and to make FuncForPC
+			// work correctly for entries in the result of runtime.Callers.
+			pc--
+		}
 		name := funcname(funcInfo)
 		file, line := funcline1(funcInfo, pc, false)
 		if inldata := funcdata(funcInfo, _FUNCDATA_InlTree); inldata != nil {
diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index 9b7fafcad7..a536fb2a71 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -344,8 +344,9 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		}
 
 		if pcbuf != nil {
+			pc := frame.pc
 			// backup to CALL instruction to read inlining info (same logic as below)
-			tracepc := frame.pc
+			tracepc := pc
 			if (n > 0 || flags&_TraceTrap == 0) && frame.pc > f.entry && !waspanic {
 				tracepc--
 			}
@@ -363,12 +364,13 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 					} else if skip > 0 {
 						skip--
 					} else if n < max {
-						(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = tracepc
+						(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
 						n++
 					}
 					lastFuncID = inltree[ix].funcID
 					// Back up to an instruction in the "caller".
 					tracepc = frame.fn.entry + uintptr(inltree[ix].parentPc)
+					pc = tracepc + 1
 				}
 			}
 			// Record the main frame.
@@ -377,7 +379,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 			} else if skip > 0 {
 				skip--
 			} else if n < max {
-				(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = tracepc
+				(*[1 << 20]uintptr)(unsafe.Pointer(pcbuf))[n] = pc
 				n++
 			}
 			lastFuncID = f.funcID