Jeff Law <l...@redhat.com> writes: > Ian Lance Taylor wrote: >> Jeff Law <l...@redhat.com> writes: >> >> >>>> No, that makes no sense. What I'm suggesting is that we fix the stack >>>> offsets of all local variables before register allocation, based on a >>>> conservative assessment of how many registers will be saved on the >>>> stack. Then we know during register allocation whether the memory >>>> reference will be in or out of the +- 16 byte range. >>> Just fixing the offset is not sufficient for the case I'm concerned >>> about because you don't know what offsets are valid until you know >>> precisely what registers are used in the insn. That's absolutely >>> critical. So you have to assume worst case which is you can only use >>> the +-16 byte insns which will pessimize just about every stack >>> reference. >>> >> >> You fix the offset of the value stored on the stack. Given that, you >> know which instruction you can use. >> > No, it's not that simple. If you spill, then you can change the > registers that are used and that in turn changes the set of valid > offsets in reg+d instructions.
We are just talking in circles. As I have been saying all along, I'm suggesting that gcc conservatively determine which registers will be used. Then spilling will not change the stack space allocated for saved registers. >>> Perhaps the confusion in this case is because we're not really >>> relaxing -- we're dealing with a case where the offset alone isn't >>> enough to determine if the addressing mode is valid. >>> >> >> The confusion may be that we may be talking about different things. >> I'm talking about removing this loop in the reload function: >> >> /* This loop scans the entire function each go-round >> and repeats until one repetition spills no additional hard regs. */ >> > Most of that loop's real work isn't in the address relaxation code, > but in digging through instructions to determine what needs to be > reloaded and how many registers are necessary to perform the requested > reloads. You are not using the word "relaxation" in the same sense that I meant it. I meant explicitly the process of finding the final set of elimination offsets. >> You know the size of the frame, by guessing conservatively as to how >> many registers will be required. >> > A conservative guess would be something like "every pseudo needs a > slot" -- anything less than that isn't conservative, it's optimistic. Not a conservative guess at pseudo registers, a conservative guess at hard registers. I'm sorry, but I think I'm done with this thread. I'm just repeating myself, and people are critiquing things which I am not saying. Ian
