cmd/compile: extend prove pass to handle constant comparisons

Find comparisons to constants and propagate that information
down the dominator tree.  Use it to resolve other constant
comparisons on the same variable.

So if we know x >= 7, then a x > 4 condition must return true.

This change allows us to use "_ = b[7]" hints to eliminate bounds checks.

Fixes #14900

Change-Id: Idbf230bd5b7da43de3ecb48706e21cf01bf812f7
Reviewed-on: https://go-review.googlesource.com/21008
Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>

1 files changed, 12 insertions, 12 deletions

diff --git a/src/encoding/binary/binary.go b/src/encoding/binary/binary.go
index 225ecd7d7a..ada5768695 100644
--- a/src/encoding/binary/binary.go
+++ b/src/encoding/binary/binary.go
@@ -49,23 +49,23 @@ var BigEndian bigEndian
 type littleEndian struct{}
 
 func (littleEndian) Uint16(b []byte) uint16 {
-	b = b[:2:len(b)] // bounds check hint to compiler; see golang.org/issue/14808
+	_ = b[1] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint16(b[0]) | uint16(b[1])<<8
 }
 
 func (littleEndian) PutUint16(b []byte, v uint16) {
-	b = b[:2:len(b)] // early bounds check to guarantee safety of writes below
+	_ = b[1] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v)
 	b[1] = byte(v >> 8)
 }
 
 func (littleEndian) Uint32(b []byte) uint32 {
-	b = b[:4:len(b)] // bounds check hint to compiler; see golang.org/issue/14808
+	_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
 }
 
 func (littleEndian) PutUint32(b []byte, v uint32) {
-	b = b[:4:len(b)] // early bounds check to guarantee safety of writes below
+	_ = b[3] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v)
 	b[1] = byte(v >> 8)
 	b[2] = byte(v >> 16)
@@ -73,13 +73,13 @@ func (littleEndian) PutUint32(b []byte, v uint32) {
 }
 
 func (littleEndian) Uint64(b []byte) uint64 {
-	b = b[:8:len(b)] // bounds check hint to compiler; see golang.org/issue/14808
+	_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
 		uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
 }
 
 func (littleEndian) PutUint64(b []byte, v uint64) {
-	b = b[:8:len(b)] // early bounds check to guarantee safety of writes below
+	_ = b[7] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v)
 	b[1] = byte(v >> 8)
 	b[2] = byte(v >> 16)
@@ -97,23 +97,23 @@ func (littleEndian) GoString() string { return "binary.LittleEndian" }
 type bigEndian struct{}
 
 func (bigEndian) Uint16(b []byte) uint16 {
-	b = b[:2:len(b)] // bounds check hint to compiler; see golang.org/issue/14808
+	_ = b[1] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint16(b[1]) | uint16(b[0])<<8
 }
 
 func (bigEndian) PutUint16(b []byte, v uint16) {
-	b = b[:2:len(b)] // early bounds check to guarantee safety of writes below
+	_ = b[1] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v >> 8)
 	b[1] = byte(v)
 }
 
 func (bigEndian) Uint32(b []byte) uint32 {
-	b = b[:4:len(b)] // bounds check hint to compiler; see golang.org/issue/14808
+	_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
 }
 
 func (bigEndian) PutUint32(b []byte, v uint32) {
-	b = b[:4:len(b)] // early bounds check to guarantee safety of writes below
+	_ = b[3] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v >> 24)
 	b[1] = byte(v >> 16)
 	b[2] = byte(v >> 8)
@@ -121,13 +121,13 @@ func (bigEndian) PutUint32(b []byte, v uint32) {
 }
 
 func (bigEndian) Uint64(b []byte) uint64 {
-	b = b[:8:len(b)] // bounds check hint to compiler; see golang.org/issue/14808
+	_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint64(b[7]) | uint64(b[6])<<8 | uint64(b[5])<<16 | uint64(b[4])<<24 |
 		uint64(b[3])<<32 | uint64(b[2])<<40 | uint64(b[1])<<48 | uint64(b[0])<<56
 }
 
 func (bigEndian) PutUint64(b []byte, v uint64) {
-	b = b[:8:len(b)] // early bounds check to guarantee safety of writes below
+	_ = b[7] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v >> 56)
 	b[1] = byte(v >> 48)
 	b[2] = byte(v >> 40)