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runtime: improve scan inner loop 

On every arch except amd64, it is faster to do x&(x-1) than x^(1<<n).

Most archs need 3 instructions for the latter: MOV $1, R; SLL n, R;
ANDN R, x. Maybe 4 if there's no ANDN.

Most archs need only 2 instructions to do x&(x-1). It takes 3 on
x86/amd64 because NEG only works in place.

Only amd64 can do x^(1<<n) in a single instruction.
(We could on 386 also, but that's currently not implemented.)

Change-Id: I3b74b7a466ab972b20a25dbb21b572baf95c3467
Reviewed-on: https://go-review.googlesource.com/c/go/+/672956
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Keith Randall 2025-04-24 11:10:05 -07:00
parent c31a5c571f
commit b30fa1bcc4
1 changed files with 7 additions and 2 deletions
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5
src/runtime/mbitmap.go
View File

@ -219,8 +219,13 @@ func (tp typePointers) nextFast() (typePointers, uintptr) {
} else {
i = sys.TrailingZeros32(uint32(tp.mask))
}
if GOARCH == "amd64" {
// BTCQ
tp.mask ^= uintptr(1) << (i & (ptrBits - 1))
} else {
// SUB, AND
tp.mask &= tp.mask - 1
}
// LEAQ (XX)(XX*8)
return tp, tp.addr + uintptr(i)*goarch.PtrSize
}