Ian Lance Taylor writes: > truncate has a machine independent meaning. Yes, I guess with your definition below it does. It's interesting though that Jim had said the opposite in the excerpts posted by Jeff:
And a later message from Jim: Truncate converts a value from a larger to a smaller mode in a machine dependent way. A subreg just chops off the high bits. A truncate does not. The name might be a little confusing, but the whole point of truncate is to have something that operates differently than a subreg. Combine is clearly making an invalid transformation. > Yes. The bits in Nmode's mask are determined by the truncate. The > other bits are don't-care. If the result of the truncate happens to > wind up in a register, then in some cases PROMOTE_MODE will apply. And IIUC this don't-care nature of the other bits that allows backends to define the upper bits. For example to have sign-bit copies there in registers to enforce the MIPS64 SI mode representation. And treating the don't care bits outside SI mode in this way is true for any other SI-mode operations performed on registers not just truncate, right? Hmm, nice. Adam
