Joern Rennecke wrote:
Quoting "Paulo J. Matos" <pocma...@gmail.com>:
Hi all,
After reading the internal docs about rtx_costs I am left wondering
what they exactly are estimating.
- Are they estimating in the beginning of expand how many insns will be
generated from a particular insn until the assembler is generated?
- or Are they estimating how many assembler instructions will be
generated for a particular insn?
- or something else?
When optimizing for speed, they estimate execution cycles, when
optimizing for space, they estimate code size. In each case,
normalized so that COSTS_N_INSNS (1) is equivalent to the cost of
a simple instruction. Or at least that's the theoretical goal.
Individual ports might deviate from that for historical and/or
practical reasons.
Thanks for the reply. The name of the macro COSTS_N_INSNS was making me
think I was estimating numbers of insns instead of number of assembler
instructions even though when optimising for size the latter makes more
sense.
