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RFC: lower x^literal as x^Val{literal} for integer literals #20530

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Feb 16, 2017
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RFC: lower x^literal as x^Val{literal} for integer literals #20530

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b453ba6
lower x^literal as x^Val{literal} for small integer literals (closes …
stevengj Feb 8, 2017
ce13a79
simplify smallnum check since all Ints get inlined as fixnums into Sc…
stevengj Feb 8, 2017
7eeefee
do x^Val{p} transformation for all literal p, not just small ones
stevengj Feb 8, 2017
5411736
fix comment
stevengj Feb 8, 2017
a7f1f45
docs for x^Val{p}
stevengj Feb 9, 2017
f23f250
documentation tweak
stevengj Feb 9, 2017
3a0d354
added test for x^literal
stevengj Feb 9, 2017
d1dab59
note experimental feature [ci skip]
stevengj Feb 16, 2017
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4 changes: 4 additions & 0 deletions NEWS.md
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Expand Up @@ -73,6 +73,10 @@ Language changes
* The `typealias` keyword is deprecated, and should be replaced with
`Vector{T} = Array{T,1}` or a `const` assignment.

* Experimental feature: `x^n` for integer literals `n` (e.g. `x^3`
or `x^-3`) is now lowered to `x^Val{n}`, to enable compile-time
specialization for literal integer exponents ([#20530]).

Breaking changes
----------------

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5 changes: 5 additions & 0 deletions base/intfuncs.jl
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Expand Up @@ -195,6 +195,11 @@ end
^(x::Number, p::Integer) = power_by_squaring(x,p)
^(x, p::Integer) = power_by_squaring(x,p)

# x^p for any literal integer p is lowered to x^Val{p},
# to enable compile-time optimizations specialized to p.
# However, we still need a fallback that calls the general ^:
^{p}(x, ::Type{Val{p}}) = x^p
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How about we rewrite this as ^(x, p) = internal_pow(x, p) and then dispatch on Val in internal_pow? I ask because this is going to be a magnet for ambiguities since packages are likely to specialize on the first argument. E.g., PainterQubits/Unitful.jl#54

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@stevengj stevengj Feb 17, 2017 •
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Unitful will maybe want to write its own ^(x, Val{p}) methods that are type-stable anyway, however, so in this case the ambiguity warning is helpful.

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@stevengj stevengj Feb 17, 2017 •
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But I guess it doesn't hurt to do this, and it may be helpful in other cases?

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@stevengj stevengj Feb 17, 2017 •
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I dunno, the more I think about it the more I think that the ambiguity warning is actually helpful here. Particularly if we start inlining x^Val{p}, anyone specializing purely on the first argument (and not restricting to p::Number, as most types would) will want to explicitly decide what to do for the Val{p} case.

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these won't be warnings any more, they'll be errors the first time someone happens to call the ambiguous signature, unless you're going out of your way to check for ambiguities

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Unitful wasn't even getting through precompilation, because some of these run at build time and the ambiguity error kicks in. These are fixable errors, as witnessed by that PR, but I don't personally see much downside to engineering the dispatch chain to avoid breakage.

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The downside is that packages that really should be thinking about doing something different for Val{p} will get no warning of the change.

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But I don't feel strongly about this; I've updated #20648 to use internal_pow as you suggest.


# b^p mod m

"""
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5 changes: 5 additions & 0 deletions base/promotion.jl
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Expand Up @@ -253,6 +253,11 @@ end

Exponentiation operator. If `x` is a matrix, computes matrix exponentiation.

If `y` is an `Int` literal (e.g. `2` in `x^2` or `-3` in `x^-3`), the Julia code
`x^y` is transformed by the compiler to `x^Val{y}`, to enable compile-time
specialization on the value of the exponent. (As a default fallback,
however, `x^Val{y}` simply calls the `^(x,y)` function.)

```jldoctest
julia> 3^5
243
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4 changes: 4 additions & 0 deletions src/julia-syntax.scm
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Expand Up @@ -2057,6 +2057,10 @@
(expand-forms
`(call (core _apply) ,f ,@(tuple-wrap argl '())))))

((and (eq? f '^) (length= e 4) (integer? (cadddr e)))
(expand-forms
`(call ^ ,(caddr e) (call (core apply_type) (top Val) ,(cadddr e)))))

((and (eq? f '*) (length= e 4))
(expand-transposed-op
e
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12 changes: 12 additions & 0 deletions test/numbers.jl
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Expand Up @@ -2902,6 +2902,18 @@ end
@test rem2pi(T(-4), RoundUp) == -4
end

import Base.^
immutable PR20530; end
^(::PR20530, p::Int) = 1
^{p}(::PR20530, ::Type{Val{p}}) = 2
@testset "literal powers" begin
x = PR20530()
p = 2
@test x^p == 1
@test x^2 == 2
@test [x,x,x].^2 == [2,2,2]
end

@testset "iszero" begin
# Numeric scalars
for T in [Float16, Float32, Float64, BigFloat,
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