runtime: short path for equal pointers in arm64 memequal

If memequal is invoked with the same pointers as arguments it ends up
comparing the whole memory contents, instead of just comparing the pointers.

This effectively makes an operation that could be O(1) into O(n). All the
other architectures already have this optimization in place. For
instance, arm64 also have it, in memequal_varlen.

Such optimization is very specific, one case that it will probably benefit is
programs that rely heavily on interning of strings.

goos: darwin
goarch: arm64
pkg: bytes
                 │      old.txt       │               new.txt                │
                 │       sec/op       │    sec/op     vs base                │
Equal/same/1-8           2.678n ± ∞ ¹   2.400n ± ∞ ¹   -10.38% (p=0.008 n=5)
Equal/same/6-8           3.267n ± ∞ ¹   2.431n ± ∞ ¹   -25.59% (p=0.008 n=5)
Equal/same/9-8           2.981n ± ∞ ¹   2.385n ± ∞ ¹   -19.99% (p=0.008 n=5)
Equal/same/15-8          2.974n ± ∞ ¹   2.390n ± ∞ ¹   -19.64% (p=0.008 n=5)
Equal/same/16-8          2.983n ± ∞ ¹   2.380n ± ∞ ¹   -20.21% (p=0.008 n=5)
Equal/same/20-8          3.567n ± ∞ ¹   2.384n ± ∞ ¹   -33.17% (p=0.008 n=5)
Equal/same/32-8          3.568n ± ∞ ¹   2.385n ± ∞ ¹   -33.16% (p=0.008 n=5)
Equal/same/4K-8         78.040n ± ∞ ¹   2.378n ± ∞ ¹   -96.95% (p=0.008 n=5)
Equal/same/4M-8      78713.000n ± ∞ ¹   2.385n ± ∞ ¹  -100.00% (p=0.008 n=5)
Equal/same/64M-8   1348095.000n ± ∞ ¹   2.381n ± ∞ ¹  -100.00% (p=0.008 n=5)
geomean                  43.52n         2.390n         -94.51%
¹ need >= 6 samples for confidence interval at level 0.95

                 │    old.txt    │                     new.txt                      │
                 │      B/s      │         B/s          vs base                     │
Equal/same/1-8     356.1Mi ± ∞ ¹         397.3Mi ± ∞ ¹        +11.57% (p=0.008 n=5)
Equal/same/6-8     1.711Gi ± ∞ ¹         2.298Gi ± ∞ ¹        +34.35% (p=0.008 n=5)
Equal/same/9-8     2.812Gi ± ∞ ¹         3.515Gi ± ∞ ¹        +24.99% (p=0.008 n=5)
Equal/same/15-8    4.698Gi ± ∞ ¹         5.844Gi ± ∞ ¹        +24.41% (p=0.008 n=5)
Equal/same/16-8    4.995Gi ± ∞ ¹         6.260Gi ± ∞ ¹        +25.34% (p=0.008 n=5)
Equal/same/20-8    5.222Gi ± ∞ ¹         7.814Gi ± ∞ ¹        +49.63% (p=0.008 n=5)
Equal/same/32-8    8.353Gi ± ∞ ¹        12.496Gi ± ∞ ¹        +49.59% (p=0.008 n=5)
Equal/same/4K-8    48.88Gi ± ∞ ¹       1603.96Gi ± ∞ ¹      +3181.17% (p=0.008 n=5)
Equal/same/4M-8    49.63Gi ± ∞ ¹    1637911.85Gi ± ∞ ¹   +3300381.91% (p=0.008 n=5)
Equal/same/64M-8   46.36Gi ± ∞ ¹   26253069.97Gi ± ∞ ¹  +56626517.99% (p=0.008 n=5)
geomean            6.737Gi               122.7Gi            +1721.01%
¹ need >= 6 samples for confidence interval at level 0.95

Fixes #64381

Change-Id: I7d423930a688edd88c4ba60d45e097296d9be852
GitHub-Last-Rev: ae8189fafb1cba87b5394f09f971746ae9299273
GitHub-Pull-Request: golang/go#64419
Reviewed-on: https://go-review.googlesource.com/c/go/+/545416
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>

diff --git a/src/bytes/bytes_test.go b/src/bytes/bytes_test.go
index f0733edd3f447eb6d78ae1274f20f72191d2053c..5e8cf85fd90ab9cf3b1ce90066f49984e3aa8c26 100644 (file)
--- a/src/bytes/bytes_test.go
+++ b/src/bytes/bytes_test.go
@@ -629,6 +629,11 @@ func BenchmarkEqual(b *testing.B) {
        })
 
        sizes := []int{1, 6, 9, 15, 16, 20, 32, 4 << 10, 4 << 20, 64 << 20}
+
+       b.Run("same", func(b *testing.B) {
+               benchBytes(b, sizes, bmEqual(func(a, b []byte) bool { return Equal(a, a) }))
+       })
+
        benchBytes(b, sizes, bmEqual(Equal))
 }
 

diff --git a/src/internal/bytealg/equal_arm64.s b/src/internal/bytealg/equal_arm64.s
index d3aabba5871eedf52d848decc3bf3838a10a7ae4..4db951547443d57ae736980eaf3d3729fe072df5 100644 (file)
--- a/src/internal/bytealg/equal_arm64.s
+++ b/src/internal/bytealg/equal_arm64.s
@@ -9,6 +9,9 @@
 TEXT runtime·memequal<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-25
        // short path to handle 0-byte case
        CBZ     R2, equal
+       // short path to handle equal pointers
+       CMP     R0, R1
+       BEQ     equal
        B       memeqbody<>(SB)
 equal:
        MOVD    $1, R0