[LLVMdev] Hoisting constant arguments from recursive functions

[Jon Harrop]

I am just considering various different designs for the code emitted by a HLVM 
and one alluring approach is to make all local variables immutable and 
replace loops with (tail) recursive functions. This makes it much easier to 
reason about injected code such as GC checks. However, I am concerned about 
the performance of the generated code.

Specifically, this is likely to result in small recursive functions that 
accept a large number of arguments, many of which have values that remain the 
same from one application/iteration to the next.

For example, a function to fill an array might become:

  let rec fill a i x =
    if i < length a then
      a.(i) <- x
      fill a (i + 1) x

where "a" and "x" have the same value from one call to the next and only the 
loop variable "i" changes.

My question is: will LLVM optimize this back into a function containing a loop 
that runs inside a single stack frame that contains the constants "a" and "x" 
or should my compiler perform this rewrite itself?

Also, would any simple rearrangements help LLVM, such as putting the constant 
function arguments to the back of the argument list?

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e