On Mon, May 30, 2016 at 7:54 PM, Erik Schnetter <schnet...@gmail.com> wrote: > Yichao > > What is the known bug here: That Julia doesn't handle this more efficiently,
The bug is that the type instability here is spurious. > or that the variable `x` can change its type from Int to Float64? > > In this example, are there two variables `x` (the argument and the > explicitly declared local variable), or should the declaration be an error? > > -erik > > On Mon, May 30, 2016 at 7:47 PM, Yichao Yu <yyc1...@gmail.com> wrote: >> >> On Mon, May 30, 2016 at 7:23 PM, David P. Sanders <dpsand...@gmail.com> >> wrote: >> > >> > >> > El lunes, 30 de mayo de 2016, 19:11:47 (UTC-4), FANG Colin escribió: >> >> >> >> function t1(n::Int, x::Int, a::Float64) >> >> x::Float64 = x >> >> for i in 1:n >> >> x += a >> >> end >> >> x >> >> end >> >> @time t1(10^6, 1, 1.0) >> >> >> >> 0.005445 seconds (1.00 M allocations: 15.259 MB) >> > >> > >> > In t1, x changes type during the function, from Int to Float64, so the >> > function is type *un*stable, as shown by @code_warntype, >> > and as suggested by the huge number of allocations. >> > >> > In t2, x is always a Float64, and the function is type stable. >> > >> >> >> >> >> >> >> >> >> >> >> >> function t2(n::Int, y::Int, a::Float64) >> >> x::Float64 = y >> >> for i in 1:n >> >> x += a >> >> end >> >> x >> >> end >> >> @time t2(10^6, 1, 1.0) >> >> >> >> 0.001044 seconds (6 allocations: 192 bytes) >> >> >> >> >> >> >> >> >> >> The @code_warntype of the 2 functions are very similar. However, the >> >> llvm >> >> code generated from t2 is a lot simpler. >> > >> > >> > The @code_warntype of the two functions is very *different*. (This is >> > easier >> > to see in the REPL than in the notebook, if >> > that is the problem.) >> > >> >> >> >> >> >> Does it suggest that if we want to change the type of an argument, we'd >> >> better create a new variable? >> >> This is a known bug. Fortunately it's easy to catch with code_warntype. > > > > > -- > Erik Schnetter <schnet...@gmail.com> > http://www.perimeterinstitute.ca/personal/eschnetter/
