Kenneth Zadeck wrote:
I have one question about EH_USES on the ia-64 (the only place it is used).
I know this is a bit late, but I didn't see any answer to this on this list, and figured you might still need one.
The IA-64 port uses EH_USES for registers that hold frame state. We can optionally store some frame state in local registers in the current register window. These are obviously live at the epilogue because the epilogue code uses them. These are also live at EH edges, but this isn't obvious because there are no explicit uses in the rtl. The uses are hidden inside the unwind library. So EH_USES is used to make sure these regs are live at entry to an EH edge. EH_USES serves the same purpose as EPILOGUE_USES, except one is for EH edges and one is for epilogues.
Why only IA-64 needs this, offhand, I don't know. Maybe it has to do with the IA-64 unwinder that only IA-64 uses. Or maybe it has something to do with how IA-64 register windows work. I don't care enough to take a closer look.
These registers do not exist before the prologue/epilogue code is emitted, and hence they do not exist before reload.
These are normal allocatable registers. If there are free registers, then we use some of them for storing frame state. If there aren't any free registers, then the frame state goes on the stack which is the normal place for it. So there is no latent register allocation problem here.
We only use 5 of 128 integer registers this way, so relatively speaking, this isn't a lot of registers. Since these are local register windowed regs, they are pretty cheap.
-- Jim Wilson, GNU Tools Support, http://www.specifix.com
