cmd/compile: enable stack maps everywhere except unsafe points

This modifies issafepoint in liveness analysis to report almost every
operation as a safe point. There are four things we don't mark as
safe-points:

1. Runtime code (other than at calls).

2. go:nosplit functions (other than at calls).

3. Instructions between the load of the write barrier-enabled flag and
   the write.

4. Instructions leading up to a uintptr -> unsafe.Pointer conversion.

We'll optimize this in later CLs:

name        old time/op       new time/op       delta
Template          185ms ± 2%        190ms ± 2%   +2.95%  (p=0.000 n=10+10)
Unicode          96.3ms ± 3%       96.4ms ± 1%     ~     (p=0.905 n=10+9)
GoTypes           658ms ± 0%        669ms ± 1%   +1.72%  (p=0.000 n=10+9)
Compiler          3.14s ± 1%        3.18s ± 1%   +1.56%  (p=0.000 n=9+10)
SSA               7.41s ± 2%        7.59s ± 1%   +2.48%  (p=0.000 n=9+10)
Flate             126ms ± 1%        128ms ± 1%   +2.08%  (p=0.000 n=10+10)
GoParser          153ms ± 1%        157ms ± 2%   +2.38%  (p=0.000 n=10+10)
Reflect           437ms ± 1%        442ms ± 1%   +0.98%  (p=0.001 n=10+10)
Tar               178ms ± 1%        179ms ± 1%   +0.67%  (p=0.035 n=10+9)
XML               223ms ± 1%        229ms ± 1%   +2.58%  (p=0.000 n=10+10)
[Geo mean]        394ms             401ms        +1.75%

No effect on binary size because we're not yet emitting these extra
safe points.

For #24543.

Change-Id: I16a1eebb9183cad7cef9d53c0fd21a973cad6859
Reviewed-on: https://go-review.googlesource.com/109348
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/writebarrier.go

@@ -111,6 +111,7 @@ func writebarrier(f *Func) {
 		// order values in store order
 		b.Values = storeOrder(b.Values, sset, storeNumber)
 
+		firstSplit := true
 	again:
 		// find the start and end of the last contiguous WB store sequence.
 		// a branch will be inserted there. values after it will be moved
@@ -268,6 +269,23 @@ func writebarrier(f *Func) {
 			w.Block = bEnd
 		}
 
+		// Preemption is unsafe between loading the write
+		// barrier-enabled flag and performing the write
+		// because that would allow a GC phase transition,
+		// which would invalidate the flag. Remember the
+		// conditional block so liveness analysis can disable
+		// safe-points. This is somewhat subtle because we're
+		// splitting b bottom-up.
+		if firstSplit {
+			// Add b itself.
+			b.Func.WBLoads = append(b.Func.WBLoads, b)
+			firstSplit = false
+		} else {
+			// We've already split b, so we just pushed a
+			// write barrier test into bEnd.
+			b.Func.WBLoads = append(b.Func.WBLoads, bEnd)
+		}
+
 		// if we have more stores in this block, do this block again
 		if nWBops > 0 {
 			goto again