cmd/compile: remove duplicate nilchecks

Mark nil check operations as faulting if their arg is zero.
This lets the late nilcheck pass remove duplicates.

Fixes #17242.

Change-Id: I4c9938d8a5a1e43edd85b4a66f0b34004860bcd9
Reviewed-on: https://go-review.googlesource.com/29952
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/test/nilptr3.go b/test/nilptr3.go
index a97873b97c..b965cd262d 100644
--- a/test/nilptr3.go
+++ b/test/nilptr3.go
@@ -233,3 +233,17 @@ func c1() {
 	var x Struct
 	func() { x.m() }() // ERROR "removed nil check"
 }
+
+type SS struct {
+	x byte
+}
+
+type TT struct {
+	SS
+}
+
+func f(t *TT) *byte {
+	// See issue 17242.
+	s := &t.SS  // ERROR "removed nil check"
+	return &s.x // ERROR "generated nil check"
+}