proposal: math/big: add Int.AddInt64, Int.CmpInt64

[TuomLarsen]

Please consider adding big.Int methods Inc and Dec which would increase or decrease the given Int by one. It would be similar to the following code, except for the allocation:

x.Add(x, big.NewInt(1))

The motivation is that it is quite common operation and the code using it would be simpler and saving one allocation.

Alternatively, please consider AddInt(64) and SubInt(64), which would still save one allocation when knowing the increment/decrement fits a (64-bit) machine word.

Cursory look at Go source shows that the first alternative could be useful here and here, and the second one here.

Yet another alternative would be to expose intOne from int.go.

[robpike]

You can declare some package variables called one, two, etc. and use them throughout.

x.Add(x, one)

This also lets you do things like

x.Cmp(one)

[ericlagergren]

Admittedly, being able to perform arithmetic with a big.Int would be beneficial: https://github.com/ericlagergren/decimal/blob/master/internal/arith/arith_amd64.go

https://github.com/ericlagergren/decimal/blob/master/internal/c/const.go

[TuomLarsen]

@robpike Yes, package variables are one way of how to approach (although each package would have to possibly allocate the same big.NewInt(1)) this but the proposal is more about avoid allocations and simplifying the notation for arguments which fit machine word. Kind of like GMP _si and ui methods, of which I find that +1/-1 are the most useful.

[rsc]

Int.AddInt64 seems strictly better than Inc/Dec and matches the Int64 and SetInt64 methods. (Probably don't need SubInt64 if we have a general AddInt64. Also probably don't need AddUint64, SubUint64.)

But what else would it require adding? And? AndNot? Cmp? Div? DivMod? Mod? Mul? Or? Quo? QuoRem? Rem? Sub after all? Xor?

Just declaring a few globals with the constants you need seems to be the right answer most of the time. It's not clear that doubling the API surface is a significant enough win.

[TuomLarsen]

Perhaps And, AndNot, Xor are not really necessary, Cmp can be worked around with Sign, IsInt64, Int64. So if I may summarize this gives 4 options:

• Add every arithmetic operator (Add, Sub, Mul, QuoRem, Quo, Rem, DivMod, Div, Mod) and maybe Exp, each for Int64 and Uint64. This would be quite complete (akin to GMP) but it seems too specialized for stdlib to compensate for so many new methods.

• Add only Add and Sub, for both Int64 and Uint64. I would argue Sub is needed even for Int64 case as the main motivation is to simplify code and to manually check whether an argument is negative or not to suitably change the sign so that it works for Add seems way too much work. And big.Int also has Sub even when it is not strictly necessary. Four methods, and really only two of which need a proper implementation as Int64 may be using Uin64 internally. The drawback is that it somehow "promises" there are more machine word methods like this, as shown by your question.

• And only Inc and Dec. No need to worry about Uint, Int64, it does not suggest other "missing" machine word methods. Albeit more limited that any option above, perhaps it's still better than nothing.

• Don't add anything. Clearly this is the easiest option but I believe that the above options have the potential to be generally useful.

[rsc]

Talked to @griesemer, who is inclined to start with just AddInt64 and CmpInt64 and wait for more compelling needs for any of the others. Those are clearly the most common.

[robpike]

I would start with none of them. If you add one, you'll end up adding them all, or at best dealing with a string of proposals to add them piecemeal. Please hold the line.

[TuomLarsen]

@rsc If I understood correctly, AddInt64 would need to distinguish positive and negative values as the internal implementation of big.Int uses big.nat, i.e. to add a negative number it subtracts its absolute value. If there would indeed be such mechanism, why not expose it, i.e. to add both AddUint64 and SubUint64? (With or without AddInt64?) It is similar to Int.SetInt64/Int.SetUint64, Rat.SetInt64/Rat.SetUint64, I think unsigned 64-bits are useful.

[bmkessler]

I don't understand the need for adding these methods to the standard library. As @robpike mentioned, package level variables already handle the proposed use case very well and the big.Int API is already very large.

[mazarin1]

Since AddInt64(x) would need to distinguish the sign and conver x to nat we can’t save much on allocations. And may be even more harmful for memory to do so in case of making increment functions. Only convenience I see is symantics. But I think std libs should follow: KISS.
Makes it easy to test, debug and understand

Solution like

  one := big.NewInt(1)
  counter := big.NewInt(0)
   ...
   counter.Add(counter,one)
   ...

are way better approach to issue.

[odeke-em]

I shall punt to Go1.14 as the jury is still hung on whether to implement it or not, but also there hasn't been movement.

[lmittmann]

The methods Inc and Dec seem like a nice pendant to ++ and -- for the integer primitive types. Also they would probably have some performance benefits opposed to x.Add(x, one) as less comparisons and allocations are necessary.

[griesemer]

Despite this proposal being accepted we have not moved ahead yet. @robpike is likely correct that this will simply invite piecemeal additions of more little helpers. There may be ways to improve performance of the existing code w/o changing the API.

[smasher164]

Since there doesn't appear to be a clear consensus on this addition to math/big, should this be moved back into active discussion, with the Accepted label removed?

[gopherbot]

Change https://golang.org/cl/334885 mentions this issue: math/big: add Int.AddInt64, Int.SubInt64 Int.CmpInt64

[vpereira01]

Since there doesn't appear to be a clear consensus on this addition to math/big, should this be moved back into active discussion, with the Accepted label removed?

Can someone clean the "Proposal-Accepted" label as it seems to not really being accepted?

---

Having crossed paths with z.Add(z, smallNumber) API and performance I have wondered if there was an improvement that could be made but that does not seem to be the case.

• As bmkessler stated, big.Int API is already big so there's should be a strong case to expand it further.
• As mazarin1 stated, allocations can not really be saved.
• A z.AddInt64(oneInt64) breaks the big.Int API pattern of always received two values x,y and storing the result in z.
• A z.AddInt64(xBigInt, oneInt64) expands/confuses the API and does not reduce allocations or seems to have a performance benefit.

The only improvement I see that could be done is advising in the docs not to do z.Add(x, big.NewInt(1)) but create a local variable of "oneBI" and do z.Add(x, oneBI).

---

If someone really needs to save allocations and/or improvement performance a fixed-precision package should be used and/or hacks which are just that, hacks. If you really need a hack check https://github.com/vpereira01/biigist/blob/master/bigintinc.go

[randall77]

Looks like that won't quite work, at least currently, because we haven't fixed #7921. Doing

type S struct {
	a []byte
	b [1]byte
}

func f(s *S) {
	s.a = s.b[:]
}

Causes s to escape in f. (Because some later code can read s.a and store the result in a global.)

[rsc]

If NewInt was as simple as:

func NewInt(x int64) *Int {
x := &Int{nat: []Word{Word(x)}}
return x
}

I think that would inline and not escape. So we just have to figure out how to preserve that with setting neg and negating x. Maybe sign shifting tricks are enough.

On 32-bit it might be harder because sometimes you need two words, but maybe we don't care as much if 32-bit can't be made non-allocating.

[randall77]

That doesn't work either, unfortunately. The x.nat field might escape even though x doesn't.

[vpereira01]

That doesn't work either, unfortunately. The x.nat field might escape even though x doesn't.

I'm probably missing something but can you double check that?

My simple test seems to indicate that z.Add(y, big.NewInt(5)) already does not escape.

user@host$ go version
go version go1.18.1 linux/amd64
user@host$ cat -n bigLeak.go 
     1  package main
     2
     3  import (
     4          "fmt"
     5          "math"
     6          "math/big"
     7  )
     8
     9  func main() {
    10          z, _ := big.NewInt(0).SetString("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF", 16)
    11          addI64nt(z, math.MaxInt32)
    12          fmt.Printf("result %v\n", z)
    13  }
    14
    15  func addI64nt(z *big.Int, y int64) {
    16          z.Add(z, big.NewInt(y))
    17  }
user@host$ go build -gcflags -m bigLeak.go
# command-line-arguments
./bigLeak.go:16:21: inlining call to big.NewInt
./bigLeak.go:10:20: inlining call to big.NewInt
./bigLeak.go:12:12: inlining call to fmt.Printf
./bigLeak.go:15:15: leaking param content: z
./bigLeak.go:16:21: new(big.Int) does not escape <--------------
./bigLeak.go:10:20: new(big.Int) escapes to heap
./bigLeak.go:12:12: ... argument does not escape

[randall77]

The big.Int struct itself is on the stack just fine. It's the [1]Word that is currently heap-allocated.

[rsc]

Is the [1]Word heap-allocated because of Add?
If we just did _ = big.NewInt(1) I would not expect that to heap allocate the words by itself.
The words must be escaping somewhere.

[randall77]

Ah yes, that's the problem. Add bottoms out to things like addVV which are in assembly. If I mark all that assembly as //go:noescape, then the [1]Word gets stack allocated.

[gopherbot]

Change https://go.dev/cl/411254 mentions this issue: math/big: make NewInt inlineable and zero allocation

[rsc]

OK, so it sounds like we don't need any new API here once this change lands.
There may be still be some optimization work to do, but that doesn't affect API and so wouldn't require proposals.
So I will move this along to likely decline.

[rsc]

Thanks @randall77!

[rsc]

Based on the discussion above, this proposal seems like a likely decline.
— rsc for the proposal review group

[rsc]

No change in consensus, so declined.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/422039 mentions this issue: math/big: disable TestNewIntAllocs on noopt builder