From 735647d92e839f9ac3a91864a2c34263338a35e6 Mon Sep 17 00:00:00 2001
From: Matthew Dempsky <mdempsky@google.com>
Date: Mon, 8 Mar 2021 15:36:28 -0800
Subject: [PATCH] runtime: add alignment info to sizeclasses.go comments

I was curious about the minimum possible alignment for each size class
and the minimum size to guarantee any particular alignment (e.g., to
know at what class size you can start assuming heap bits are byte- or
word-aligned).

Change-Id: I205b750286e8914986533c4f60712c420c3e63e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/299909
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
---
 src/runtime/mksizeclasses.go |  26 ++++++-
 src/runtime/sizeclasses.go   | 145 +++++++++++++++++++----------------
 2 files changed, 101 insertions(+), 70 deletions(-)

diff --git a/src/runtime/mksizeclasses.go b/src/runtime/mksizeclasses.go
index b4a117d343..8b9bbe01e6 100644
--- a/src/runtime/mksizeclasses.go
+++ b/src/runtime/mksizeclasses.go
@@ -37,6 +37,7 @@ import (
 	"go/format"
 	"io"
 	"log"
+	"math/bits"
 	"os"
 )
 
@@ -242,8 +243,9 @@ nextk:
 }
 
 func printComment(w io.Writer, classes []class) {
-	fmt.Fprintf(w, "// %-5s  %-9s  %-10s  %-7s  %-10s  %-9s\n", "class", "bytes/obj", "bytes/span", "objects", "tail waste", "max waste")
+	fmt.Fprintf(w, "// %-5s  %-9s  %-10s  %-7s  %-10s  %-9s  %-9s\n", "class", "bytes/obj", "bytes/span", "objects", "tail waste", "max waste", "min align")
 	prevSize := 0
+	var minAligns [32]int
 	for i, c := range classes {
 		if i == 0 {
 			continue
@@ -252,8 +254,28 @@ func printComment(w io.Writer, classes []class) {
 		objects := spanSize / c.size
 		tailWaste := spanSize - c.size*(spanSize/c.size)
 		maxWaste := float64((c.size-prevSize-1)*objects+tailWaste) / float64(spanSize)
+		alignBits := bits.TrailingZeros(uint(c.size))
+		for i := range minAligns {
+			if i > alignBits {
+				minAligns[i] = 0
+			} else if minAligns[i] == 0 {
+				minAligns[i] = c.size
+			}
+		}
 		prevSize = c.size
-		fmt.Fprintf(w, "// %5d  %9d  %10d  %7d  %10d  %8.2f%%\n", i, c.size, spanSize, objects, tailWaste, 100*maxWaste)
+		fmt.Fprintf(w, "// %5d  %9d  %10d  %7d  %10d  %8.2f%%  %9d\n", i, c.size, spanSize, objects, tailWaste, 100*maxWaste, 1<<alignBits)
+	}
+	fmt.Fprintf(w, "\n")
+
+	fmt.Fprintf(w, "// %-9s  %-4s  %-12s\n", "alignment", "bits", "min obj size")
+	for bits, size := range minAligns {
+		if size == 0 {
+			break
+		}
+		if bits+1 < len(minAligns) && size == minAligns[bits+1] {
+			continue
+		}
+		fmt.Fprintf(w, "// %9d  %4d  %12d\n", 1<<bits, bits, size)
 	}
 	fmt.Fprintf(w, "\n")
 }
diff --git a/src/runtime/sizeclasses.go b/src/runtime/sizeclasses.go
index c5521ce1bd..d71ceeab7b 100644
--- a/src/runtime/sizeclasses.go
+++ b/src/runtime/sizeclasses.go
@@ -3,74 +3,83 @@
 
 package runtime
 
-// class  bytes/obj  bytes/span  objects  tail waste  max waste
-//     1          8        8192     1024           0     87.50%
-//     2         16        8192      512           0     43.75%
-//     3         24        8192      341           8     29.24%
-//     4         32        8192      256           0     21.88%
-//     5         48        8192      170          32     31.52%
-//     6         64        8192      128           0     23.44%
-//     7         80        8192      102          32     19.07%
-//     8         96        8192       85          32     15.95%
-//     9        112        8192       73          16     13.56%
-//    10        128        8192       64           0     11.72%
-//    11        144        8192       56         128     11.82%
-//    12        160        8192       51          32      9.73%
-//    13        176        8192       46          96      9.59%
-//    14        192        8192       42         128      9.25%
-//    15        208        8192       39          80      8.12%
-//    16        224        8192       36         128      8.15%
-//    17        240        8192       34          32      6.62%
-//    18        256        8192       32           0      5.86%
-//    19        288        8192       28         128     12.16%
-//    20        320        8192       25         192     11.80%
-//    21        352        8192       23          96      9.88%
-//    22        384        8192       21         128      9.51%
-//    23        416        8192       19         288     10.71%
-//    24        448        8192       18         128      8.37%
-//    25        480        8192       17          32      6.82%
-//    26        512        8192       16           0      6.05%
-//    27        576        8192       14         128     12.33%
-//    28        640        8192       12         512     15.48%
-//    29        704        8192       11         448     13.93%
-//    30        768        8192       10         512     13.94%
-//    31        896        8192        9         128     15.52%
-//    32       1024        8192        8           0     12.40%
-//    33       1152        8192        7         128     12.41%
-//    34       1280        8192        6         512     15.55%
-//    35       1408       16384       11         896     14.00%
-//    36       1536        8192        5         512     14.00%
-//    37       1792       16384        9         256     15.57%
-//    38       2048        8192        4           0     12.45%
-//    39       2304       16384        7         256     12.46%
-//    40       2688        8192        3         128     15.59%
-//    41       3072       24576        8           0     12.47%
-//    42       3200       16384        5         384      6.22%
-//    43       3456       24576        7         384      8.83%
-//    44       4096        8192        2           0     15.60%
-//    45       4864       24576        5         256     16.65%
-//    46       5376       16384        3         256     10.92%
-//    47       6144       24576        4           0     12.48%
-//    48       6528       32768        5         128      6.23%
-//    49       6784       40960        6         256      4.36%
-//    50       6912       49152        7         768      3.37%
-//    51       8192        8192        1           0     15.61%
-//    52       9472       57344        6         512     14.28%
-//    53       9728       49152        5         512      3.64%
-//    54      10240       40960        4           0      4.99%
-//    55      10880       32768        3         128      6.24%
-//    56      12288       24576        2           0     11.45%
-//    57      13568       40960        3         256      9.99%
-//    58      14336       57344        4           0      5.35%
-//    59      16384       16384        1           0     12.49%
-//    60      18432       73728        4           0     11.11%
-//    61      19072       57344        3         128      3.57%
-//    62      20480       40960        2           0      6.87%
-//    63      21760       65536        3         256      6.25%
-//    64      24576       24576        1           0     11.45%
-//    65      27264       81920        3         128     10.00%
-//    66      28672       57344        2           0      4.91%
-//    67      32768       32768        1           0     12.50%
+// class  bytes/obj  bytes/span  objects  tail waste  max waste  min align
+//     1          8        8192     1024           0     87.50%          8
+//     2         16        8192      512           0     43.75%         16
+//     3         24        8192      341           8     29.24%          8
+//     4         32        8192      256           0     21.88%         32
+//     5         48        8192      170          32     31.52%         16
+//     6         64        8192      128           0     23.44%         64
+//     7         80        8192      102          32     19.07%         16
+//     8         96        8192       85          32     15.95%         32
+//     9        112        8192       73          16     13.56%         16
+//    10        128        8192       64           0     11.72%        128
+//    11        144        8192       56         128     11.82%         16
+//    12        160        8192       51          32      9.73%         32
+//    13        176        8192       46          96      9.59%         16
+//    14        192        8192       42         128      9.25%         64
+//    15        208        8192       39          80      8.12%         16
+//    16        224        8192       36         128      8.15%         32
+//    17        240        8192       34          32      6.62%         16
+//    18        256        8192       32           0      5.86%        256
+//    19        288        8192       28         128     12.16%         32
+//    20        320        8192       25         192     11.80%         64
+//    21        352        8192       23          96      9.88%         32
+//    22        384        8192       21         128      9.51%        128
+//    23        416        8192       19         288     10.71%         32
+//    24        448        8192       18         128      8.37%         64
+//    25        480        8192       17          32      6.82%         32
+//    26        512        8192       16           0      6.05%        512
+//    27        576        8192       14         128     12.33%         64
+//    28        640        8192       12         512     15.48%        128
+//    29        704        8192       11         448     13.93%         64
+//    30        768        8192       10         512     13.94%        256
+//    31        896        8192        9         128     15.52%        128
+//    32       1024        8192        8           0     12.40%       1024
+//    33       1152        8192        7         128     12.41%        128
+//    34       1280        8192        6         512     15.55%        256
+//    35       1408       16384       11         896     14.00%        128
+//    36       1536        8192        5         512     14.00%        512
+//    37       1792       16384        9         256     15.57%        256
+//    38       2048        8192        4           0     12.45%       2048
+//    39       2304       16384        7         256     12.46%        256
+//    40       2688        8192        3         128     15.59%        128
+//    41       3072       24576        8           0     12.47%       1024
+//    42       3200       16384        5         384      6.22%        128
+//    43       3456       24576        7         384      8.83%        128
+//    44       4096        8192        2           0     15.60%       4096
+//    45       4864       24576        5         256     16.65%        256
+//    46       5376       16384        3         256     10.92%        256
+//    47       6144       24576        4           0     12.48%       2048
+//    48       6528       32768        5         128      6.23%        128
+//    49       6784       40960        6         256      4.36%        128
+//    50       6912       49152        7         768      3.37%        256
+//    51       8192        8192        1           0     15.61%       8192
+//    52       9472       57344        6         512     14.28%        256
+//    53       9728       49152        5         512      3.64%        512
+//    54      10240       40960        4           0      4.99%       2048
+//    55      10880       32768        3         128      6.24%        128
+//    56      12288       24576        2           0     11.45%       4096
+//    57      13568       40960        3         256      9.99%        256
+//    58      14336       57344        4           0      5.35%       2048
+//    59      16384       16384        1           0     12.49%      16384
+//    60      18432       73728        4           0     11.11%       2048
+//    61      19072       57344        3         128      3.57%        128
+//    62      20480       40960        2           0      6.87%       4096
+//    63      21760       65536        3         256      6.25%        256
+//    64      24576       24576        1           0     11.45%       8192
+//    65      27264       81920        3         128     10.00%        128
+//    66      28672       57344        2           0      4.91%       4096
+//    67      32768       32768        1           0     12.50%      32768
+
+// alignment  bits  min obj size
+//         8     3             8
+//        16     4            32
+//        32     5           256
+//        64     6           512
+//       128     7           768
+//      4096    12         28672
+//     32768    15         32768
 
 const (
 	_MaxSmallSize   = 32768