From afd691c579198387c874512ef1c75db651dba9bd Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@golang.org>
Date: Thu, 13 Feb 2020 08:34:41 -0800
Subject: [PATCH] runtime: special case interface hashing for pointers
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Interfaces often contain pointers. Implement a fast path for this case.

name                   old time/op  new time/op  delta
MapInterfaceString-16  21.4ns ±19%  20.5ns ±10%     ~     (p=0.361 n=10+10)
MapInterfacePtr-16     25.8ns ± 8%  17.3ns ± 7%  -33.11%  (p=0.000 n=10+9)

Fixes #37086

Change-Id: Ice52820e6259a3edeafcbbbeb25b1e363bef00d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/219338
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
---
 src/runtime/alg.go | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/runtime/alg.go b/src/runtime/alg.go
index d8789b4b5f..5a0656513d 100644
--- a/src/runtime/alg.go
+++ b/src/runtime/alg.go
@@ -160,7 +160,15 @@ func nilinterhash(p unsafe.Pointer, h uintptr) uintptr {
 // maps generated by reflect.MapOf (reflect_typehash, below).
 func typehash(t *_type, p unsafe.Pointer, h uintptr) uintptr {
 	if t.tflag&tflagRegularMemory != 0 {
-		return memhash(p, h, t.size)
+		// Handle ptr sizes specially, see issue 37086.
+		switch t.size {
+		case 4:
+			return memhash32(p, h)
+		case 8:
+			return memhash64(p, h)
+		default:
+			return memhash(p, h, t.size)
+		}
 	}
 	switch t.kind & kindMask {
 	case kindFloat32: