[Bug middle-end/66565] Problems and limitation GCC cost metrics and ways to improve the situation

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66565

--- Comment #2 from David Edelsohn <dje at gcc dot gnu.org> ---
The high-level consensus at the GNU Cauldron 2015 GCC Cost Model BoF is:

1) Create gimple_costs() instead of creating potentially invalid fragments to
query rtx_costs().

2) Cost models can utilize GCC Pass phase to set expectations for optimization
changes by later passes.

3) Cost models should be queried for relative comparisons of the potential
instruction sequences, not for comparisons against magic constants.