runtime: add 24 byte allocation size class

This CL introduces a 24 byte allocation size class which
fits 3 pointers on 64 bit and 6 pointers on 32 bit architectures.

Notably this new size class fits a slice header on 64 bit
architectures exactly while previously a 32 byte size class
would have been used for allocating a slice header on the heap.

The main complexity added with this CL is that heapBitsSetType
needs to handle objects that aren't 16-byte aligned but contain
more than a single pointer on 64-bit architectures.

Due to having a non 16 byte aligned size class on 32 bit a
h.shift of 2 is now possible which means a heap bitmap byte might
only be partially written. Due to this already having been
possible on 64 bit before the heap bitmap code only needed
minor adjustments for 32 bit doublecheck code paths.

Note that this CL changes the slice capacity allocated by append
for slice growth to a target capacity of 17 to 24 bytes.

On 64 bit architectures the capacity of the slice returned by
append([]byte{}, make([]byte, 24)...)) is 32 bytes before and
24 bytes after this CL. Depending on allocation patterns of the
specific Go program this can increase the number of total
alloctions as subsequent appends to the slice can trigger slice
growth earlier than before. On the other side if the slice is
never appended to again above its capacity this will lower heap
usage by 8 bytes.

This CL changes the set of size classes reported in the
runtime.MemStats.BySize array due to it being limited to a
total of 61 size classes. The new 24 byte size class is now
included and the 20480 byte size class is not included anymore.

Fixes #8885

name                      old time/op       new time/op       delta
Template                        196ms ± 3%        194ms ± 2%    ~     (p=0.247 n=10+10)
Unicode                        85.6ms ±16%       88.1ms ± 1%    ~     (p=0.165 n=10+10)
GoTypes                         673ms ± 2%        668ms ± 2%    ~     (p=0.258 n=9+9)
Compiler                        3.14s ± 6%        3.08s ± 1%    ~     (p=0.243 n=10+9)
SSA                             6.82s ± 1%        6.76s ± 1%  -0.87%  (p=0.006 n=9+10)
Flate                           128ms ± 7%        127ms ± 3%    ~     (p=0.739 n=10+10)
GoParser                        154ms ± 3%        153ms ± 4%    ~     (p=0.730 n=9+9)
Reflect                         404ms ± 1%        412ms ± 4%  +1.99%  (p=0.022 n=9+10)
Tar                             172ms ± 4%        170ms ± 4%    ~     (p=0.065 n=10+9)
XML                             231ms ± 4%        230ms ± 3%    ~     (p=0.912 n=10+10)
LinkCompiler                    341ms ± 1%        339ms ± 1%    ~     (p=0.243 n=9+10)
ExternalLinkCompiler            1.72s ± 1%        1.72s ± 1%    ~     (p=0.661 n=9+10)
LinkWithoutDebugCompiler        221ms ± 2%        221ms ± 2%    ~     (p=0.529 n=10+10)
StdCmd                          18.4s ± 3%        18.2s ± 1%    ~     (p=0.515 n=10+8)

name                      old user-time/op  new user-time/op  delta
Template                        238ms ± 4%        243ms ± 6%    ~     (p=0.661 n=9+10)
Unicode                         116ms ± 6%        113ms ± 3%  -3.37%  (p=0.035 n=9+10)
GoTypes                         854ms ± 2%        848ms ± 2%    ~     (p=0.604 n=9+10)
Compiler                        4.10s ± 1%        4.11s ± 1%    ~     (p=0.481 n=8+9)
SSA                             9.49s ± 1%        9.41s ± 1%  -0.92%  (p=0.001 n=9+10)
Flate                           149ms ± 6%        151ms ± 7%    ~     (p=0.481 n=10+10)
GoParser                        189ms ± 2%        190ms ± 2%    ~     (p=0.497 n=9+10)
Reflect                         511ms ± 2%        508ms ± 2%    ~     (p=0.211 n=9+10)
Tar                             215ms ± 4%        212ms ± 3%    ~     (p=0.105 n=10+10)
XML                             288ms ± 2%        288ms ± 2%    ~     (p=0.971 n=10+10)
LinkCompiler                    559ms ± 4%        557ms ± 1%    ~     (p=0.968 n=9+10)
ExternalLinkCompiler            1.78s ± 1%        1.77s ± 1%    ~     (p=0.055 n=8+10)
LinkWithoutDebugCompiler        245ms ± 3%        245ms ± 2%    ~     (p=0.684 n=10+10)

name                      old alloc/op      new alloc/op      delta
Template                       34.8MB ± 0%       34.4MB ± 0%  -0.95%  (p=0.000 n=9+10)
Unicode                        28.6MB ± 0%       28.3MB ± 0%  -0.95%  (p=0.000 n=10+10)
GoTypes                         115MB ± 0%        114MB ± 0%  -1.02%  (p=0.000 n=10+9)
Compiler                        554MB ± 0%        549MB ± 0%  -0.86%  (p=0.000 n=9+10)
SSA                            1.28GB ± 0%       1.27GB ± 0%  -0.83%  (p=0.000 n=10+10)
Flate                          21.8MB ± 0%       21.6MB ± 0%  -0.87%  (p=0.000 n=8+10)
GoParser                       26.7MB ± 0%       26.4MB ± 0%  -0.97%  (p=0.000 n=10+9)
Reflect                        75.0MB ± 0%       74.1MB ± 0%  -1.18%  (p=0.000 n=10+10)
Tar                            32.6MB ± 0%       32.3MB ± 0%  -0.94%  (p=0.000 n=10+7)
XML                            41.5MB ± 0%       41.2MB ± 0%  -0.90%  (p=0.000 n=10+8)
LinkCompiler                    105MB ± 0%        104MB ± 0%  -0.94%  (p=0.000 n=10+10)
ExternalLinkCompiler            153MB ± 0%        152MB ± 0%  -0.69%  (p=0.000 n=10+10)
LinkWithoutDebugCompiler       63.7MB ± 0%       63.6MB ± 0%  -0.13%  (p=0.000 n=10+10)

name                      old allocs/op     new allocs/op     delta
Template                         336k ± 0%         336k ± 0%  +0.02%  (p=0.002 n=10+10)
Unicode                          332k ± 0%         332k ± 0%    ~     (p=0.447 n=10+10)
GoTypes                         1.16M ± 0%        1.16M ± 0%  +0.01%  (p=0.001 n=10+10)
Compiler                        4.92M ± 0%        4.92M ± 0%  +0.01%  (p=0.000 n=10+10)
SSA                             11.9M ± 0%        11.9M ± 0%  +0.02%  (p=0.000 n=9+10)
Flate                            214k ± 0%         214k ± 0%  +0.02%  (p=0.032 n=10+8)
GoParser                         270k ± 0%         270k ± 0%  +0.02%  (p=0.004 n=10+9)
Reflect                          877k ± 0%         877k ± 0%  +0.01%  (p=0.000 n=10+10)
Tar                              313k ± 0%         313k ± 0%    ~     (p=0.075 n=9+10)
XML                              387k ± 0%         387k ± 0%  +0.02%  (p=0.007 n=10+10)
LinkCompiler                     455k ± 0%         456k ± 0%  +0.08%  (p=0.000 n=10+9)
ExternalLinkCompiler             670k ± 0%         671k ± 0%  +0.06%  (p=0.000 n=10+10)
LinkWithoutDebugCompiler         113k ± 0%         113k ± 0%    ~     (p=0.149 n=10+10)

name                      old maxRSS/op     new maxRSS/op     delta
Template                        34.1M ± 1%        34.1M ± 1%    ~     (p=0.853 n=10+10)
Unicode                         35.1M ± 1%        34.6M ± 1%  -1.43%  (p=0.000 n=10+10)
GoTypes                         72.8M ± 3%        73.3M ± 2%    ~     (p=0.724 n=10+10)
Compiler                         288M ± 3%         295M ± 4%    ~     (p=0.393 n=10+10)
SSA                              630M ± 1%         622M ± 1%  -1.18%  (p=0.001 n=10+10)
Flate                           26.0M ± 1%        26.2M ± 2%    ~     (p=0.493 n=10+10)
GoParser                        28.6M ± 1%        28.5M ± 2%    ~     (p=0.256 n=10+10)
Reflect                         55.5M ± 2%        55.4M ± 1%    ~     (p=0.436 n=10+10)
Tar                             33.0M ± 1%        32.8M ± 2%    ~     (p=0.075 n=10+10)
XML                             38.7M ± 1%        39.0M ± 1%    ~     (p=0.053 n=9+10)
LinkCompiler                     164M ± 1%         164M ± 1%  -0.27%  (p=0.029 n=10+10)
ExternalLinkCompiler             174M ± 0%         173M ± 0%  -0.33%  (p=0.002 n=9+10)
LinkWithoutDebugCompiler         137M ± 0%         136M ± 2%    ~     (p=0.825 n=9+10)

Change-Id: I9ecf2a10024513abef8fbfbe519e44e0b29b6167
Reviewed-on: https://go-review.googlesource.com/c/go/+/242258
Trust: Martin Möhrmann <moehrmann@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/runtime/mstats.go b/src/runtime/mstats.go
index 6a8a34d1ed..b95b332134 100644
--- a/src/runtime/mstats.go
+++ b/src/runtime/mstats.go
@@ -78,6 +78,10 @@ type mstats struct {
 		nfree   uint64
 	}
 
+	// Add an uint32 for even number of size classes to align below fields
+	// to 64 bits for atomic operations on 32 bit platforms.
+	_ [1 - _NumSizeClasses%2]uint32
+
 	// Statistics below here are not exported to MemStats directly.
 
 	last_gc_nanotime uint64 // last gc (monotonic time)