Hi Mark, Thanks for that explanation, it makes sense now to me.
Cheers! On Mon, 2018-05-14 at 22:55 -0400, Mark H Weaver wrote: > Hi Brian, > > Brian <gomesbas...@gmail.com> writes: > > > Today I found that top level defines have a significant performance > > impact on Guile (2.2.3). The following program takes about 108 > > seconds > > to complete on my ThinkPad (an i5-5200U with Arch Linux): > > [...] > > By simply wrapping that code in a lambda the program finished in > > about > > 47 seconds. Using lets instead of defines is equally effective. > > > > I was quite surprised because I initially thought some optimization > > would just substitute those useless nodes symbols away, but it > > seems > > like that's not the case... > > Right. The problem is that toplevel variables can be mutated by > arbitrary code from other modules, e.g. by 'module-set!', so the > compiler cannot make any assumptions about what values those > variables > will contain at runtime. > > For non-toplevel variables, the situation is quite different. In > Scheme, non-toplevel variables can be accessed only from within their > lexical scope, so if such a variable is not 'set!' from within its > scope, the compiler knows that it can never be mutated. In that > case, > it can assume that the variable will always contain its initial > value, > which enables a great many optimizations including partial > evaluation. > > Mark
