From 47c9e139aed3e6d24ee980828835fb03229272ff Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@golang.org>
Date: Wed, 23 Mar 2016 10:20:44 -0700
Subject: cmd/compile: extend prove pass to handle constant comparisons
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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>
---
 src/encoding/binary/binary.go | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

(limited to 'src/encoding/binary/binary.go')

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)
-- 
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