runtime: add physHugePageShift

This change adds physHugePageShift which is defined such that
1 << physHugePageShift == physHugePageSize. The purpose of this variable
is to avoid doing expensive divisions in key functions, such as
(*mspan).hugePages.

This change also does a sweep of any place we might do a division or mod
operation with physHugePageSize and turns it into bit shifts and other
bitwise operations.

Finally, this change adds a check to mallocinit which ensures that
physHugePageSize is always a power of two. osinit might choose to ignore
non-powers-of-two for the value and replace it with zero, but mallocinit
will fail if it's not a power of two (or zero). It also derives
physHugePageShift from physHugePageSize.

This change helps improve the performance of most applications because
of how often (*mspan).hugePages is called.

Updates #32828.

Change-Id: I1a6db113d52d563f59ae8fd4f0e130858859e68f
Reviewed-on: https://go-review.googlesource.com/c/go/+/186598
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/runtime/mem_linux.go b/src/runtime/mem_linux.go
index cda2c78eaf..524915fb31 100644
--- a/src/runtime/mem_linux.go
+++ b/src/runtime/mem_linux.go
@@ -68,11 +68,11 @@ func sysUnused(v unsafe.Pointer, n uintptr) {
 		// flag on the huge pages containing v and v+n-1, and
 		// only if those aren't aligned.
 		var head, tail uintptr
-		if uintptr(v)%physHugePageSize != 0 {
+		if uintptr(v)&(physHugePageSize-1) != 0 {
 			// Compute huge page containing v.
 			head = uintptr(v) &^ (physHugePageSize - 1)
 		}
-		if (uintptr(v)+n)%physHugePageSize != 0 {
+		if (uintptr(v)+n)&(physHugePageSize-1) != 0 {
 			// Compute huge page containing v+n-1.
 			tail = (uintptr(v) + n - 1) &^ (physHugePageSize - 1)
 		}