On 11/11/20 3:55 AM, Jakub Jelinek via Gcc-patches wrote: > On Wed, Nov 11, 2020 at 11:43:34AM +0100, Philipp Tomsich wrote: >> The patch addresses this by disallowing that rule, if an exact power-of-2 is >> seen as C1. The reason why I would prefer to have this canonicalised the >> same way the (X & C1) * C2 is canonicalised, is that cleaning this up during >> combine is more difficult on some architectures that require multiple insns >> to represent the shifted constant (i.e. C1 << C2). > It is bad to have many exceptions for the canonicalization > and it is unclear why exactly these were chosen, and it doesn't really deal > with say: > (x & 0xabcdef12ULL) << 13 > being less expensive on some targets than > (x << 13) & (0xabcdef12ULL << 13). > (x & 0x7ffff) << 3 vs. (x << 3) & 0x3ffff8 on the other side is a wash on > many targets. > As I said, it is better to decide which one is better before or during > expansion based on target costs, sure, combine can't catch everything.
I think Jakub is hitting a key point here. Gimple should canonicalize on what is simpler from a gimple standpoint, not what is better for some set of targets. Target dependencies like this shouldn't be introduced until expansion time. Jeff
