From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Tue, 19 Jul 2016 00:05:54 +0000 (-0700)
Subject: cmd/vet: allow large shifts of constants
X-Git-Tag: go1.8beta1~1779
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=794442375d87e57d012bab2d7424575f6cdff018;p=gostls13.git

cmd/vet: allow large shifts of constants

Large shifts of constants are frequently
used for fancy 32/64 bit detection.

This removes 14 false positives from the
standard library.

Updates #11041

Change-Id: Ib39346e5c161da04c38a6a3067932ef43bf74f2d
Reviewed-on: https://go-review.googlesource.com/27155
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
---

diff --git a/src/cmd/vet/shift.go b/src/cmd/vet/shift.go
index 8c038b4bdd..55f3ea369f 100644
--- a/src/cmd/vet/shift.go
+++ b/src/cmd/vet/shift.go
@@ -41,6 +41,12 @@ func checkShift(f *File, node ast.Node) {
 // checkLongShift checks if shift or shift-assign operations shift by more than
 // the length of the underlying variable.
 func checkLongShift(f *File, node ast.Node, x, y ast.Expr) {
+	if f.pkg.types[x].Value != nil {
+		// Ignore shifts of constants.
+		// These are frequently used for bit-twiddling tricks
+		// like ^uint(0) >> 63 for 32/64 bit detection and compatibility.
+		return
+	}
 	v := f.pkg.types[y].Value
 	if v == nil {
 		return
diff --git a/src/cmd/vet/testdata/shift.go b/src/cmd/vet/testdata/shift.go
index 6624f09cc1..99acaadf6d 100644
--- a/src/cmd/vet/testdata/shift.go
+++ b/src/cmd/vet/testdata/shift.go
@@ -75,4 +75,6 @@ func ShiftTest() {
 	_ = p >> 32 // ERROR "p might be too small for shift of 32"
 	p <<= 32    // ERROR "p might be too small for shift of 32"
 	p >>= 32    // ERROR "p might be too small for shift of 32"
+
+	const oneIf64Bit = ^uint(0) >> 63 // allow large shifts of constants; they are used for 32/64 bit compatibility tricks
 }