runtime: improve runtime.overLoadFactor calculation

[josharian]

runtime.overLoadFactor could use some improvement.

First, a bug: uintptr is 32 bits wide on a 32 bit system, so calculating uintptr(1<<B) can overflow when B >= 32. That's Go 1.9 material, and I'll send a CL shortly. Maybe even 1.8.2, given the devastating result (infinite loop) and simplicity of fix. Marking 1.8.2 for discussion.

Second, an improvement: We should rewrite this to use integer arithmetic and pay more careful attention to potential overflow. That should probably wait for Go 1.10.

Exposed by CL 40854 when working with a hint of 1 << 62. CL 40854 will also help with the bug, once it is correctly implemented.

[gopherbot]

CL https://golang.org/cl/42185 mentions this issue.

[josharian]

Keith points out that the bug can't occur in 1.8, and the initial fix is in, so setting to 1.10 to use int calculations.

[ianlancetaylor]

This was fixed by https://golang.org/cl/54655.