Module Sarek_codegen.Sarek_ir_for_bound

Evaluate a for upper bound once, before the first iteration.

Sarek for i = a to b has OCaml semantics: b is evaluated once. The C-family, GLSL and WGSL loops are spelled for (i = a; i <= b; i++), whose condition re-reads b every iteration, so a bound that reads something the body changes (a mut variable, an array element) iterates a different number of times than the interpreter, Native and PTX.

hoist rewrites every SFor (v, lo, hi, dir, body) whose hi is not stable into SLet (sarek_for_hi_N, hi, SFor (v, lo, sarek_for_hi_N, dir, body)) (the sarek_ prefix is reserved, so the name cannot collide with user code). A bound is stable, and left untouched, when nothing in the loop can change it: literals, variables the loop body never assigns, thread and block indices, array lengths, and pure arithmetic or casts of those. Common loops such as for i = 0 to n - 1l therefore keep their generated source.

Limit. A bound of Int32.max_int (Upto) or Int32.min_int (Downto) is not supported on any backend: the loop variable would have to step past the bound, which overflows int32 (undefined behaviour in the C dialects, wrap-around and an endless loop in PTX and in the interpreter, whose Int32.add wraps). The loop start is hoisted too, ahead of the bound, when it is not pure, so the order start-then-bound is kept.