cmd/compile, runtime: add metadata for argument printing in traceback

Currently, when the runtime printing a stack track (at panic, or
when runtime.Stack is called), it prints the function arguments
as words in memory. With a register-based calling convention,
the layout of argument area of the memory changes, so the
printing also needs to change. In particular, the memory order
and the syntax order of the arguments may differ. To address
that, this CL lets the compiler to emit some metadata about the
memory layout of the arguments, and the runtime will use this
information to print arguments in syntax order.

Previously we print the memory contents of the results along with
the arguments. The results are likely uninitialized when the
traceback is taken, so that information is rarely useful. Also,
with a register-based calling convention the results may not
have corresponding locations in memory. This CL changes it to not
print results.

Previously the runtime simply prints the memory contents as
pointer-sized words. With a register-based calling convention,
as the layout changes, arguments that were packed in one word
may no longer be in one word. Also, as the spill slots are not
always initialized, it is possible that some part of a word
contains useful informationwhile the rest contains garbage.
Instead of letting the runtime recreating the ABI0 layout and
print them as words, we now print each component separately.
Aggregate-typed argument/component is surrounded by "{}".

For example, for a function

F(int, [3]byte, byte) int

when called as F(1, [3]byte{2, 3, 4}, 5), it used to print

F(0x1, 0x5040302, 0xXXXXXXXX) // assuming little endian, 0xXXXXXXXX is uninitilized result

Now prints

F(0x1, {0x2, 0x3, 0x4}, 0x5).

Note: the liveness tracking of the spill splots has not been
implemented in this CL. Currently the runtime just assumes all
the slots are live and print them all.

Increase binary sizes by ~1.5%.

                     old          new
hello (println)    1171328      1187712 (+1.4%)
hello (fmt)        1877024      1901600 (+1.3%)
cmd/compile       22326928     22662800 (+1.5%)
cmd/go            13505024     13726208 (+1.6%)

Updates #40724.

Change-Id: I351e0bf497f99bdbb3f91df2fb17e3c2c5c316dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/304470
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 78 insertions, 11 deletions

diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index 0969af1a21..167d51c452 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -457,17 +457,8 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 					name = "panic"
 				}
 				print(name, "(")
-				argp := (*[100]uintptr)(unsafe.Pointer(frame.argp))
-				for i := uintptr(0); i < frame.arglen/sys.PtrSize; i++ {
-					if i >= 10 {
-						print(", ...")
-						break
-					}
-					if i != 0 {
-						print(", ")
-					}
-					print(hex(argp[i]))
-				}
+				argp := unsafe.Pointer(frame.argp)
+				printArgs(f, argp)
 				print(")\n")
 				print("\t", file, ":", line)
 				if frame.pc > f.entry {
@@ -579,6 +570,82 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	return n
 }
 
+// printArgs prints function arguments in traceback.
+func printArgs(f funcInfo, argp unsafe.Pointer) {
+	// The "instruction" of argument printing is encoded in _FUNCDATA_ArgInfo.
+	// See cmd/compile/internal/ssagen.emitArgInfo for the description of the
+	// encoding.
+	// These constants need to be in sync with the compiler.
+	const (
+		_endSeq         = 0xff
+		_startAgg       = 0xfe
+		_endAgg         = 0xfd
+		_dotdotdot      = 0xfc
+		_offsetTooLarge = 0xfb
+	)
+
+	const (
+		limit    = 10                       // print no more than 10 args/components
+		maxDepth = 5                        // no more than 5 layers of nesting
+		maxLen   = (maxDepth*3+2)*limit + 1 // max length of _FUNCDATA_ArgInfo (see the compiler side for reasoning)
+	)
+
+	p := (*[maxLen]uint8)(funcdata(f, _FUNCDATA_ArgInfo))
+	if p == nil {
+		return
+	}
+
+	print1 := func(off, sz uint8) {
+		x := readUnaligned64(add(argp, uintptr(off)))
+		// mask out irrelavant bits
+		if sz < 8 {
+			shift := 64 - sz*8
+			if sys.BigEndian {
+				x = x >> shift
+			} else {
+				x = x << shift >> shift
+			}
+		}
+		print(hex(x))
+	}
+
+	start := true
+	printcomma := func() {
+		if !start {
+			print(", ")
+		}
+	}
+	pi := 0
+printloop:
+	for {
+		o := p[pi]
+		pi++
+		switch o {
+		case _endSeq:
+			break printloop
+		case _startAgg:
+			printcomma()
+			print("{")
+			start = true
+			continue
+		case _endAgg:
+			print("}")
+		case _dotdotdot:
+			printcomma()
+			print("...")
+		case _offsetTooLarge:
+			printcomma()
+			print("_")
+		default:
+			printcomma()
+			sz := p[pi]
+			pi++
+			print1(o, sz)
+		}
+		start = false
+	}
+}
+
 // reflectMethodValue is a partial duplicate of reflect.makeFuncImpl
 // and reflect.methodValue.
 type reflectMethodValue struct {