https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106101
--- Comment #22 from Andreas Krebbel <krebbel at gcc dot gnu.org> --- The longer a have been looking at these STRICT_LOW_PART issue the more I think that STRICT_LOW_PART is an awful way to express what we need: - the information needed to understand what it is doing is distributed across 3 RTXs (strict_low_part (subreg:mode1 (reg:mode2 xx) OFS)) - the big problems arise since the involved RTXs are separately optimized and we might end up with partial information without a clear definition of how to deal with that - actually it is really hard to handle the RTXs as one unit. Recursively walking RTXs needs to record whether we are in a STRICT_LOW_PART or not. I think it might make sense to explore other ways to express this: 1. SUBREG flag - Looks easy, but it would be hard to catch all places which should care about that flag. 2. Introduce a new RTX code which has a mode and an offset attached but does not require an additional SUBREG anymore. 3. Since a STRICT_LOW_PART is essentially a bit insertion operation we could express it always with a ZERO_EXTRACT target operand and get rid of STRICT_LOW_PART entirely. A ZERO_EXTRACT would be somewhat more cumbersome to deal with, since it would always require to check the bit width and offset for all the cases which just use mode boundaries. But at least most passes know how to deal with them already.
