a8e0641d5b
Our prior CL 649078 teaches the compiler to use a pointer to
runtime.zeroVal as the data pointer for an interface in cases it where
it can see that a zero value struct or array is being used in
an interface conversion.
This applies to some uses with reflect, such as:
s := S{}
v := reflect.ValueOf(s)
This CL builds on that to do a cheap pointer check in reflect.IsZero
to see if the Value points to runtime.zeroVal, which means it is a zero
value.
An alternative might be to do an initial pointer check in the typ.Equal
function for types where it makes sense to do but doesn't already.
This CL gives a performance boost of -51.71% geomean for
BenchmarkZero/IsZero, with most of the impact there on
arrays of structs. (The left column is CL 649078 and the right column
is this CL).
goos: linux
goarch: amd64
pkg: reflect
cpu: Intel(R) Xeon(R) CPU @ 2.80GHz
│ find-zeroVal │ check-zeroVal │
│ sec/op │ sec/op vs base │
Zero/IsZero/ByteArray/size=16-4 4.171n ± 0% 3.123n ± 0% -25.13% (p=0.000 n=20)
Zero/IsZero/ByteArray/size=64-4 3.864n ± 0% 3.129n ± 0% -19.02% (p=0.000 n=20)
Zero/IsZero/ByteArray/size=1024-4 3.878n ± 0% 3.126n ± 0% -19.39% (p=0.000 n=20)
Zero/IsZero/BigStruct/size=1024-4 5.061n ± 0% 3.273n ± 0% -35.34% (p=0.000 n=20)
Zero/IsZero/SmallStruct/size=16-4 4.191n ± 0% 3.275n ± 0% -21.87% (p=0.000 n=20)
Zero/IsZero/SmallStructArray/size=64-4 8.636n ± 0% 3.127n ± 0% -63.79% (p=0.000 n=20)
Zero/IsZero/SmallStructArray/size=1024-4 80.055n ± 0% 3.126n ± 0% -96.10% (p=0.000 n=20)
Zero/IsZero/Time/size=24-4 3.865n ± 0% 3.274n ± 0% -15.29% (p=0.000 n=20)
geomean 6.587n 3.181n -51.71%
Note these are of course micro benchmarks with easily predicted
branches. The extra branch we introduce in the CL might hurt if there
was for example a tight loop where 50% of the values used the
global zeroVal and 50% didn't in a way that is not well predicted,
although if the typ.Equal for many types already does an initial
pointer check, it might not matter much.
For the older BenchmarkIsZero in reflect, this change does not help.
(The compiler does not use the global zeroVal as the data word for the
interfaces in this benchmark because values are part of a larger value
that is too big to be used in the global zeroVal, and also a piece of
the larger value is mutated and is not zero).
│ find-zeroVal │ check-zeroVal │
│ sec/op │ sec/op vs base │
IsZero/ArrayComparable-4 14.58n ± 0% 14.59n ± 0% ~ (p=0.177 n=20)
IsZero/ArrayIncomparable-4 163.8n ± 0% 167.5n ± 0% +2.26% (p=0.000 n=20)
IsZero/StructComparable-4 6.847n ± 0% 6.847n ± 0% ~ (p=0.703 n=20)
IsZero/StructIncomparable-4 35.41n ± 0% 35.10n ± 0% -0.86% (p=0.000 n=20)
IsZero/ArrayInt_4-4 8.631n ± 0% 8.363n ± 0% -3.10% (p=0.000 n=20)
IsZero/ArrayInt_1024-4 265.5n ± 0% 265.4n ± 0% ~ (p=0.288 n=20)
IsZero/ArrayInt_1024_NoZero-4 135.8n ± 0% 136.2n ± 0% +0.33% (p=0.000 n=20)
IsZero/Struct4Int-4 8.451n ± 0% 8.386n ± 0% -0.77% (p=0.000 n=20)
IsZero/ArrayStruct4Int_1024-4 265.2n ± 0% 266.0n ± 0% +0.30% (p=0.000 n=20)
IsZero/ArrayChanInt_1024-4 265.5n ± 0% 265.4n ± 0% ~ (p=0.605 n=20)
IsZero/StructInt_512-4 135.8n ± 0% 135.8n ± 0% ~ (p=0.396 n=20)
geomean 55.22n 55.12n -0.18%
Updates #71323
Change-Id: Ie083853a5bff03856277a293d94532a681f4a8d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/654135
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
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| lib | ||
| misc | ||
| src | ||
| test | ||
| .gitattributes | ||
| .gitignore | ||
| codereview.cfg | ||
| CONTRIBUTING.md | ||
| go.env | ||
| LICENSE | ||
| PATENTS | ||
| README.md | ||
| SECURITY.md | ||
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