cmd/compile: don't use nilcheck information until the next block

When nilcheck runs, the values in a block are not in any particular
order.  So any facts derived from examining the blocks shouldn't be
used until we reach the next block.

This is suboptimal as it won't eliminate nil checks within a block.
But it's probably a better fix for now as it is a much smaller change
than other strategies for fixing this bug.

nilptr3.go changes are mostly because for this pattern:
  _ = *p
  _ = *p
either nil check is fine to keep, and this CL changes which one
the compiler tends to keep.
There are a few regressions from code like this:
  _ = *p
  f()
  _ = *p
For this pattern, after this CL we issue 2 nil checks instead of one.
(For the curious, this happens because intra-block nil check
 elimination now falls to CSE, not nilcheck proper.  The former
 pattern has two nil checks with the same store argument.  The latter
 pattern has two nil checks with different store arguments.)

Fixes #18725

Change-Id: I3721b494c8bc9ba1142dc5c4361ea55c66920ac8
Reviewed-on: https://go-review.googlesource.com/35485
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 18 insertions, 18 deletions

diff --git a/test/nilptr3.go b/test/nilptr3.go
index 8fdae8c075..c681cba50c 100644
--- a/test/nilptr3.go
+++ b/test/nilptr3.go
@@ -40,23 +40,23 @@ var (
 )
 
 func f1() {
-	_ = *intp // ERROR "generated nil check"
+	_ = *intp // ERROR "removed nil check"
 
 	// This one should be removed but the block copy needs
 	// to be turned into its own pseudo-op in order to see
 	// the indirect.
-	_ = *arrayp // ERROR "generated nil check"
+	_ = *arrayp // ERROR "removed nil check"
 
 	// 0-byte indirect doesn't suffice.
 	// we don't registerize globals, so there are no removed.* nil checks.
-	_ = *array0p // ERROR "generated nil check"
 	_ = *array0p // ERROR "removed nil check"
+	_ = *array0p // ERROR "generated nil check"
 
-	_ = *intp    // ERROR "removed nil check"
+	_ = *intp    // ERROR "generated nil check"
 	_ = *arrayp  // ERROR "removed nil check"
 	_ = *structp // ERROR "generated nil check"
 	_ = *emptyp  // ERROR "generated nil check"
-	_ = *arrayp  // ERROR "removed nil check"
+	_ = *arrayp  // ERROR "generated nil check"
 }
 
 func f2() {
@@ -71,15 +71,15 @@ func f2() {
 		empty1p    *Empty1
 	)
 
-	_ = *intp       // ERROR "generated nil check"
-	_ = *arrayp     // ERROR "generated nil check"
-	_ = *array0p    // ERROR "generated nil check"
-	_ = *array0p    // ERROR "removed.* nil check"
 	_ = *intp       // ERROR "removed.* nil check"
 	_ = *arrayp     // ERROR "removed.* nil check"
+	_ = *array0p    // ERROR "removed.* nil check"
+	_ = *array0p    // ERROR "generated nil check"
+	_ = *intp       // ERROR "generated nil check"
+	_ = *arrayp     // ERROR "removed.* nil check"
 	_ = *structp    // ERROR "generated nil check"
 	_ = *emptyp     // ERROR "generated nil check"
-	_ = *arrayp     // ERROR "removed.* nil check"
+	_ = *arrayp     // ERROR "generated nil check"
 	_ = *bigarrayp  // ERROR "generated nil check" ARM removed nil check before indirect!!
 	_ = *bigstructp // ERROR "generated nil check"
 	_ = *empty1p    // ERROR "generated nil check"
@@ -122,16 +122,16 @@ func f3(x *[10000]int) {
 	// x wasn't going to change across the function call.
 	// But it's a little complex to do and in practice doesn't
 	// matter enough.
-	_ = x[9999] // ERROR "removed nil check"
+	_ = x[9999] // ERROR "generated nil check" // TODO: fix
 }
 
 func f3a() {
 	x := fx10k()
 	y := fx10k()
 	z := fx10k()
-	_ = &x[9] // ERROR "generated nil check"
-	y = z
 	_ = &x[9] // ERROR "removed.* nil check"
+	y = z
+	_ = &x[9] // ERROR "generated nil check"
 	x = y
 	_ = &x[9] // ERROR "generated nil check"
 }
@@ -139,11 +139,11 @@ func f3a() {
 func f3b() {
 	x := fx10k()
 	y := fx10k()
-	_ = &x[9] // ERROR "generated nil check"
+	_ = &x[9] // ERROR "removed.* nil check"
 	y = x
 	_ = &x[9] // ERROR "removed.* nil check"
 	x = y
-	_ = &x[9] // ERROR "removed.* nil check"
+	_ = &x[9] // ERROR "generated nil check"
 }
 
 func fx10() *[10]int
@@ -179,15 +179,15 @@ func f4(x *[10]int) {
 	_ = x[9] // ERROR "generated nil check"  // bug would like to remove before indirect
 
 	fx10()
-	_ = x[9] // ERROR "removed nil check"
+	_ = x[9] // ERROR "generated nil check"  // TODO: fix
 
 	x = fx10()
 	y := fx10()
-	_ = &x[9] // ERROR "generated nil check"
+	_ = &x[9] // ERROR "removed[a-z ]* nil check"
 	y = x
 	_ = &x[9] // ERROR "removed[a-z ]* nil check"
 	x = y
-	_ = &x[9] // ERROR "removed[a-z ]* nil check"
+	_ = &x[9] // ERROR "generated nil check"
 }
 
 func f5(p *float32, q *float64, r *float32, s *float64) float64 {