Hi, > Michael, Jan, > > When aligning stack for those functions who have dynamic stack > allocation, we must use an available callee-saved register in prologue. > We named this hard register DRAP. It is worthwhile to emphasize that > *free* here means "free in prologue". After prologue, a virtual register > will be used instead.
I am bit confused here. If I wanted a free register in prologue only, I would probably look at the caller saved ones. But I gues it is just typo. I don't see much value in making the register callee-saved especially if you say that virtual reg (pseudo?) is used afterward. > > Given the definition of free, we can fix the DRAP register to simplify > the implementation. Original GCC only have limited cases that use > callee-saved register in prologue, such as setting GOT pointer as far as > I know. So choosing the DRAP register is easy: just avoid GOT pointer > register, which is EBX in i386 and RBX in x86_64. As HJ said, R12 is a > good candiate. When you just need a temporary in prologue, I think you can go with RAX in most cases getting shortest code. It is used by x86-64 stdargs prologue and by i386 regparm. You can improve bit broken ix86_eax_live_at_start_p to test this. Using alternative choice if RAX is taken. > > It will be more complicated if GOT pointer register is not fixed. In > this case, the DRAP candidate must be avoid using GOT register, or vice > versa. When will current GCC decide the register to use as GOT pointer? See ix86_select_alt_pic_regnum it is done in prologue too. There are few cases 64bit prologues needs temporary already. (when stack frame is gigantic). I never attempted to optimize those at all, but if you will have more cureful way of chosing temporary around, you might consider looking for cases R11_REG is used and update them. Honza > > Thanks - Joey
