cmd/asm, cmd/link, runtime: introduce FuncInfo flag bits

The runtime traceback code has its own definition of which functions
mark the top frame of a stack, separate from the TOPFRAME bits that
exist in the assembly and are passed along in DWARF information.
It's error-prone and redundant to have two different sources of truth.
This CL provides the actual TOPFRAME bits to the runtime, so that
the runtime can use those bits instead of reinventing its own category.

This CL also adds a new bit, SPWRITE, which marks functions that
write directly to SP (anything but adding and subtracting constants).
Such functions must stop a traceback, because the traceback has no
way to rederive the SP on entry. Again, the runtime has its own definition
which is mostly correct, but also missing some functions. During ordinary
goroutine context switches, such functions do not appear on the stack,
so the incompleteness in the runtime usually doesn't matter.
But profiling signals can arrive at any moment, and the runtime may
crash during traceback if it attempts to unwind an SP-writing frame
and gets out-of-sync with the actual stack. The runtime contains code
to try to detect likely candidates but again it is incomplete.
Deriving the SPWRITE bit automatically from the actual assembly code
provides the complete truth, and passing it to the runtime lets the
runtime use it.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I227f53b23ac5b3dabfcc5e8ee3f00df4e113cf58
Reviewed-on: https://go-review.googlesource.com/c/go/+/288800
Trust: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>

3 files changed, 5 insertions, 0 deletions

diff --git a/src/cmd/asm/internal/asm/endtoend_test.go b/src/cmd/asm/internal/asm/endtoend_test.go
index 7472507caf..a4153f3af1 100644
--- a/src/cmd/asm/internal/asm/endtoend_test.go
+++ b/src/cmd/asm/internal/asm/endtoend_test.go
@@ -36,6 +36,7 @@ func testEndToEnd(t *testing.T, goarch, file string) {
 	var ok bool
 	testOut = new(bytes.Buffer) // The assembler writes test output to this buffer.
 	ctxt.Bso = bufio.NewWriter(os.Stdout)
+	ctxt.IsAsm = true
 	defer ctxt.Bso.Flush()
 	failed := false
 	ctxt.DiagFunc = func(format string, args ...interface{}) {
@@ -278,6 +279,7 @@ func testErrors(t *testing.T, goarch, file string) {
 	var ok bool
 	testOut = new(bytes.Buffer) // The assembler writes test output to this buffer.
 	ctxt.Bso = bufio.NewWriter(os.Stdout)
+	ctxt.IsAsm = true
 	defer ctxt.Bso.Flush()
 	failed := false
 	var errBuf bytes.Buffer
diff --git a/src/cmd/asm/internal/flags/flags.go b/src/cmd/asm/internal/flags/flags.go
index 1335860315..dd947c7b5b 100644
--- a/src/cmd/asm/internal/flags/flags.go
+++ b/src/cmd/asm/internal/flags/flags.go
@@ -32,11 +32,13 @@ var (
 	D        MultiFlag
 	I        MultiFlag
 	PrintOut int
+	DebugV   bool
 )
 
 func init() {
 	flag.Var(&D, "D", "predefined symbol with optional simple value -D=identifier=value; can be set multiple times")
 	flag.Var(&I, "I", "include directory; can be set multiple times")
+	flag.BoolVar(&DebugV, "v", false, "print debug output")
 	objabi.AddVersionFlag() // -V
 	objabi.Flagcount("S", "print assembly and machine code", &PrintOut)
 }
diff --git a/src/cmd/asm/main.go b/src/cmd/asm/main.go
index 31636e3045..98618a67ef 100644
--- a/src/cmd/asm/main.go
+++ b/src/cmd/asm/main.go
@@ -36,6 +36,7 @@ func main() {
 
 	ctxt := obj.Linknew(architecture.LinkArch)
 	ctxt.Debugasm = flags.PrintOut
+	ctxt.Debugvlog = flags.DebugV
 	ctxt.Flag_dynlink = *flags.Dynlink
 	ctxt.Flag_linkshared = *flags.Linkshared
 	ctxt.Flag_shared = *flags.Shared || *flags.Dynlink