I found differences in code generation for simple constant expressions: 1<<24-1 generates a constant, 2^24-1 generates a quite expensive C function call. Code example below. I did expect Julia to replace both expressions by the constant 0xFFFFFF during optimization/code generation. Do we have an ol…

Interesting. I get same under 1.3.0-rc4.1 under Windows 10: julia> @code_native (() -> 2^24-1)() .text ; ┌ @ none:1 within `#19' pushq %rbp movq %rsp, %rbp ; │┌ @ none within `literal_pow' ; ││┌ @ none within `macro expansion' ; │││┌ @ intfuncs.jl:221 within `^' …

Simpler example. 2^4 is okay. 2^5 is right out.: julia> @code_native (() -> 2^4)() .text ; ┌ @ none:1 within `#37' pushq %rbp movq %rsp, %rbp movl $16, %eax popq %rbp retq nopl (%rax,%rax) ; └ julia> @code_native (() -> 2^5)() …

Maybe the complexity of power_by_squaring somehow makes it hard for LLVM to do the elimination? Although it manages to for 2 raised to the powers 4, 7,8, 15,16, 31,32, 63,64. Maybe add this to the Performance tips doc?

It would probably make sense for ^ to detect common bases (like 2, 2.0, ℯ, and 10.0) and jump out to specialized routines in those cases. In general, integer exponentiation is just doing a “smart” number of squarings, so that’s why small/easy exponents are more likely to constant fold than bigger o…

A simple-minded approach to compute 2^24-1 at compile time is to use Base.literal_pow: julia> @inline Base.literal_pow(::typeof(^), x::Base.HWNumber, ::Val{p}) where p = if p < 0 Base.literal_pow(^, inv(x), Val(p)) else Base.literal_pow(^, x, Val(…

The compiler should also definitely be able to constant fold 2^24 no matter how it is defined. The fact that it doesn’t is a bit of a compiler bug, though of course it may not be entirely trivial to fix.

No constant expression elimination for e.g. '2^24-1'

General Usage Performance

oxinabox November 12, 2019, 11:26am 8

Sorry, I did the bad thing and knew about a bug,
but never openned an issue for it

https://github.com/JuliaLang/julia/issues/33822

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No constant expression elimination for e.g. '2^24-1'

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