cmd/gc, runtime: make GODEBUG=gcdead=1 mode work with liveness

Trying to make GODEBUG=gcdead=1 work with liveness
and in particular ambiguously live variables.

1. In the liveness computation, mark all ambiguously live
variables as live for the entire function, except the entry.
They are zeroed directly after entry, and we need them not
to be poisoned thereafter.

2. In the liveness computation, compute liveness (and deadness)
for all parameters, not just pointer-containing parameters.
Otherwise gcdead poisons untracked scalar parameters and results.

3. Fix liveness debugging print for -live=2 to use correct bitmaps.
(Was not updated for compaction during compaction CL.)

4. Correct varkill during map literal initialization.
Was killing the map itself instead of the inserted value temp.

5. Disable aggressive varkill cleanup for call arguments if
the call appears in a defer or go statement.

6. In the garbage collector, avoid bug scanning empty
strings. An empty string is two zeros. The multiword
code only looked at the first zero and then interpreted
the next two bits in the bitmap as an ordinary word bitmap.
For a string the bits are 11 00, so if a live string was zero
length with a 0 base pointer, the poisoning code treated
the length as an ordinary word with code 00, meaning it
needed poisoning, turning the string into a poison-length
string with base pointer 0. By the same logic I believe that
a live nil slice (bits 11 01 00) will have its cap poisoned.
Always scan full multiword struct.

7. In the runtime, treat both poison words (PoisonGC and
PoisonStack) as invalid pointers that warrant crashes.

Manual testing as follows:

- Create a script called gcdead on your PATH containing:

        #!/bin/bash
        GODEBUG=gcdead=1 GOGC=10 GOTRACEBACK=2 exec "$@"
- Now you can build a test and then run 'gcdead ./foo.test'.
- More importantly, you can run 'go test -short -exec gcdead std'
   to run all the tests.

Fixes issue 7676.

While here, enable the precise scanning of slices, since that was
disabled due to bugs like these. That now works, both with and
without gcdead.

Fixes issue 7549.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/83410044

1 files changed, 33 insertions, 6 deletions

diff --git a/test/live.go b/test/live.go
index 43b3c3e4c..21d3e6a5f 100644
--- a/test/live.go
+++ b/test/live.go
@@ -23,20 +23,24 @@ func f2(b bool) {
 }
 
 func f3(b bool) {
-	print(0)
+	// Because x and y are ambiguously live, they appear
+	// live throughout the function, to avoid being poisoned
+	// in GODEBUG=gcdead=1 mode.
+
+	print(0) // ERROR "live at call to printint: x y$"
 	if b == false {
-		print(0) // nothing live here
+		print(0) // ERROR "live at call to printint: x y$"
 		return
 	}
 
 	if b {
 		var x *int
-		print(&x) // ERROR "live at call to printpointer: x$"
-		print(&x) // ERROR "live at call to printpointer: x$"
+		print(&x) // ERROR "live at call to printpointer: x y$"
+		print(&x) // ERROR "live at call to printpointer: x y$"
 	} else {
 		var y *int
-		print(&y) // ERROR "live at call to printpointer: y$"
-		print(&y) // ERROR "live at call to printpointer: y$"
+		print(&y) // ERROR "live at call to printpointer: x y$"
+		print(&y) // ERROR "live at call to printpointer: x y$"
 	}
 	print(0) // ERROR "live at call to printint: x y$" "x \(type \*int\) is ambiguously live" "y \(type \*int\) is ambiguously live"
 }
@@ -371,6 +375,29 @@ func f27(b bool) {
 	}
 	call27(func() {x++}) // ERROR "live at call to call27: autotmp_[0-9]+$"
 	call27(func() {x++}) // ERROR "live at call to call27: autotmp_[0-9]+$"
+	println()
+}
+
+// but defer does escape to later execution in the function
+
+func f27defer(b bool) {
+	x := 0
+	if b {
+		defer call27(func() {x++}) // ERROR "live at call to deferproc: autotmp_[0-9]+$" "live at call to deferreturn: autotmp_[0-9]+$"
+	}
+	defer call27(func() {x++}) // ERROR "live at call to deferproc: autotmp_[0-9]+ autotmp_[0-9]+$" "live at call to deferreturn: autotmp_[0-9]+ autotmp_[0-9]+$" "ambiguously live"
+	println() // ERROR "live at call to printnl: autotmp_[0-9]+ autotmp_[0-9]+$"
+} // ERROR "live at call to deferreturn: autotmp_[0-9]+ autotmp_[0-9]+$"
+
+// and newproc (go) escapes to the heap
+
+func f27go(b bool) {
+	x := 0
+	if b {
+		go call27(func() {x++}) // ERROR "live at call to new: &x" "live at call to newproc: &x$"
+	}
+	go call27(func() {x++}) // ERROR "live at call to new: &x"
+	println()
 }
 
 //go:noescape