runtime: restore old mp.fastrand initialization

CL 337350 changed mp.fastrand from a [2]uint32 to a uint64 and changed
the initialization to a single call of int64Hash. However, int64Hash
returns uintptr, so 32-bit systems this always left the most
significant 32 bits of mp.fastrand initialized to 0. The new code also
did not protect against initializing mp.fastrand to 0, which on a
system that does not implement math.Mul64 (most 32-bit systems) would
lead fastrand to always return 0.

This CL restores the mp.fastrand initialization to what it was before
CL 337350, adjusted for the change from [2]uint32 to uint64.

Change-Id: I663b415d9424d967e8e665ce2d017604dcd5b204
Reviewed-on: https://go-review.googlesource.com/c/go/+/383916
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/runtime/proc.go

@@ -802,8 +802,18 @@ func mcommoninit(mp *m, id int64) {
  mp.id = mReserveID()
  }
 
- // cputicks is not very random in startup virtual machine
- mp.fastrand = uint64(int64Hash(uint64(mp.id), fastrandseed^uintptr(cputicks())))
+ lo := uint32(int64Hash(uint64(mp.id), fastrandseed))
+ hi := uint32(int64Hash(uint64(cputicks()), ^fastrandseed))
+ if lo|hi == 0 {
+ hi = 1
+ }
+ // Same behavior as for 1.17.
+ // TODO: Simplify ths.
+ if goarch.BigEndian {
+ mp.fastrand = uint64(lo)<<32 | uint64(hi)
+ } else {
+ mp.fastrand = uint64(hi)<<32 | uint64(lo)
+ }
 
  mpreinit(mp)
  if mp.gsignal != nil {