From 7b62e984d941c753e7fb20f8b59a49acf62c88a7 Mon Sep 17 00:00:00 2001
From: Michael Munday <mike.munday@ibm.com>
Date: Sun, 31 Mar 2019 15:23:07 +0100
Subject: [PATCH] runtime: always mask shift amount regardless of architecture

Currently the shift amount is only masked on x86. Change it so it
is masked on all architectures. In the worst case we generate a
couple of extra instructions to perform the masking and in the best
case we can elide overflow checks.

This particular shift could also be replaced with a rotate
instruction during optimization which would remove both the masking
instructions and overflow checks on all architectures.

Fixes #31165.

Change-Id: I16b7a8800b4ba8813dc83735dfc59564e661d3b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/170122
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
---
 src/runtime/map.go | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/runtime/map.go b/src/runtime/map.go
index 1282a12193..d2ff19336f 100644
--- a/src/runtime/map.go
+++ b/src/runtime/map.go
@@ -181,10 +181,8 @@ type hiter struct {
 
 // bucketShift returns 1<<b, optimized for code generation.
 func bucketShift(b uint8) uintptr {
-	if sys.GoarchAmd64|sys.GoarchAmd64p32|sys.Goarch386 != 0 {
-		b &= sys.PtrSize*8 - 1 // help x86 archs remove shift overflow checks
-	}
-	return uintptr(1) << b
+	// Masking the shift amount allows overflow checks to be elided.
+	return uintptr(1) << (b & (sys.PtrSize*8 - 1))
 }
 
 // bucketMask returns 1<<b - 1, optimized for code generation.
-- 
2.48.1