Recently I came across this TODO comment in runtime:

This sounds like a useful primitive to have when you need an atomic counter. This is similar to the accepted proposal for And/Or operators

Initially, as the comment suggests, we can use a similar stub implementation using a for loop and atomic Cas, but we should be able to write optimized assembly code per GOARCH.

We can check the machine code for such a routine in C and Go stub. For Risc-V we have AMOMAX and AMOMIN we can use, not sure about other arches.

There is a proposal to add atomic maximum/minimum to libstdc++. Quoting some of the motivations used there:

• optimal implementation of lock-free shared data structures - as in the motivating example later in this paper
• reductions in data-parallel applications
• recording the maximum so far reached in an optimization process, to allow unproductive threads to terminate
• collecting statistics, such as the largest item of input encountered by any worker thread.

For runtime/internal/atomic, we would need

Max
Max64
Maxuintptr
Min
Min64
Minuintptr

The proposed apis for sync/atomic would look something like this:

func MaxInt32(addr *int32, val int32) (old int32)
func MaxInt64(addr *int64, val int64) (old int64)
func MaxUint32(addr *uint32, val uint32) (old uint32)
func MaxUint64(addr *uint64, val uint64) (old uint64)
func MaxUintptr(addr *uintptr, val uintptr) (old uintptr)

func (*Int32) Max(val int32) (old int32)
func (*Int64) Max(val int64) (old int64)
func (*Uint32) Max(val uint32) (old uint32)
func (*Uint64) Max(val uint64) (old uint64)
func (*Uintptr) Max(val uintptr) (old uintptr)

func MinInt32(addr *int32, val int32) (old int32)
func MinInt64(addr *int64, val int64) (old int64)
func MinUint32(addr *uint32, val uint32) (old uint32)
func MinUint64(addr *uint64, val uint64) (old uint64)
func MinUintptr(addr *uintptr, val uintptr) (old uintptr)

func (*Int32) Min(val int32) (old int32)
func (*Int64) Min(val int64) (old int64)
func (*Uint32) Min(val uint32) (old uint32)
func (*Uint64) Min(val uint64) (old uint64)
func (*Uintptr) Min(val uintptr) (old uintptr)

Regarding the race detector, there are no fetch_max/fetch_min functions in tsan so that needs to be implemented as well, there shouldn't be a problem with this and I'm open to submit such changes to llvm upstream if that helps to move forward with this proposal.

Please feel free to share opinions and possible use cases for such a feature.