bc2cpp-range-map-inline.md

bc2cpp: (a..b).map { } as an inlined loop

RANGE_MAP_INLINE (recognize_range_map_regions in tools/bc2cpp/codegen_loop_regions.rb, emit_range_map_inline in tools/bc2cpp/codegen_loop_inline.rb) compiles (a..b).map { |i| BODY } and (a...b).map { |i| BODY } to a counted loop that pushes the block's value for each i. Without it the site calls the compiled Enumerable#collect, which runs Range#each and, for every element, collects |*val| into an Array and dispatches block.call(*val): an allocation and a funcall per element.

When a site is inlined

• The receiver is a Range literal of two Integer literals. The register's dominating writer is a RANGE_INC/RANGE_EXC whose operands are Integer literals smaller than 2**30 in magnitude, so the loop's Integer header never refuses and the counter stays a Fixnum on every target, the 32-bit ones included. A computed or Float bound keeps the call.
• map on a Range is mruby's own. BLOCK_CORE_DIRECT's proof (block_core_target) must find that map on a Range resolves to core Enumerable#collect and each to core Range#each: no program definition on the chain, no native registration, prepend, outside definer or dynamic installer. Those are the bodies the loop follows (3rd/mruby/mrblib/enum.rb, range.rb's Integer path): ary.push(block.call(i)) for each i.
• The block takes one plain parameter.

next v gives the element, break v the call's value, and a return leaves the method.

Inside a BLOCK_FALLBACK body

A block nested in an inlined loop is compiled as a standalone cfunc (BLOCK_FALLBACK). A Range map in such a body is inlined there too, before the body's own nested pass would make it one more cfunc called per element. That is PPU::TILE_LUT's shape: the outer Array map is inlined into the method, the (0..7).map body becomes a cfunc, and its (0...0x10000).map now runs as a loop inside that cfunc.

The inlined block reads the host function's own registers at level 0, as in a method, and the frames the host captured at level 1 and further out, through the host's captured-upvar parameters (inline_host_upvar_name; a write roots the value as the host's own SETUPVAR does). A block that nests another block or lambda, or that returns from the method, stays a nested cfunc.

Switch and check

BC2CPP_RANGE_MAP_INLINE=0 keeps every site as a call. scripts/bc2cpp/range_map_inline_check.rb asserts the inlined and kept shapes and compares interpreted and compiled runs for inclusive, exclusive, empty and negative ranges, next, break, return, a captured local, an exception in the block, GC in the block, the nested BLOCK_FALLBACK shape (reads and writes of outer locals), GC.stress, and a Fiber.