On Mon, Dec 17, 2007 at 11:25:35PM -0500, Ross Ridge wrote: > Ye, Joey writes: > >i. STACK_BOUNDARY in bits, which is enforced by hardware, 32 for i386 > >and 64 for x86_64. It is the minimum stack boundary. It is fixed. > > Strictly speaking by the above definition it would be 8 for i386. > The hardware doesn't force the stack to be 32-bit aligned, it just > performs poorly if it isn't.
We can change the wording. > > >v. INCOMING_STACK_BOUNDARY in bits, which is the stack boundary > >at function entry. If a function is marked with __attribute__ > >((force_align_arg_pointer)) or -mstackrealign option is provided, > >INCOMING = STACK_BOUNDARY. Otherwise, INCOMING == MIN(ABI_STACK_BOUNDARY, > >PREFERRED_STACK_BOUNDARY) because a function can be called via psABI > >externally or called locally with PREFERRED_STACK_BOUNDARY. > > This section doesn't make sense to me. The force_align_arg_pointer > attribute and -mstackrealign assume that the ABI is being > followed, while the -fpreferred-stack-boundary option effectively According to Apple engineer who implemented the -mstackrealign, on MacOS/ia32, psABI is 16byte, but -mstackrealign will assume 4byte, which is STACK_BOUNDARY. > changes the ABI. According your defintions, I would think > that INCOMING should be ABI_STACK_BOUNDARY in the first case, > and MAX(ABI_STACK_BOUNDARY, PREFERRED_STACK_BOUNDARY) in the second. That isn't true since some .o files may not be compiled with -fpreferred-stack-boundary or with a different value of -fpreferred-stack-boundary. > (Or just PREFERRED_STACK_BOUNDARY because a boundary less than the ABI's > should be rejected during command line processing.) On x86-64, ABI_STACK_BOUNDARY is 16byte, but the Linux kernel may want to use 8 byte for PREFERRED_STACK_BOUNDARY. > > >vi. REQUIRED_STACK_ALIGNMENT in bits, which is stack alignment required > >by local variables and calling other function. REQUIRED_STACK_ALIGNMENT > >== MAX(LOCAL_STACK_BOUNDARY,PREFERRED_STACK_BOUNDARY) in case of a > >non-leaf function. For a leaf function, REQUIRED_STACK_ALIGNMENT == > >LOCAL_STACK_BOUNDARY. > > Hmm... I think you should define STACK_BOUNDARY as the minimum > alignment that ABI requires the stack pointer to keep at all times. > ABI_STACK_BOUNDARY should be defined as the stack alignment the > ABI requires at function entry. In that case a leaf function's > REQUIRED_STACK_ALIGMENT should be MAX(LOCAL_STACK_BOUNDARY, > STACK_BOUNDARY). That is true since if the only local variable is char, LOCAL_STACK_BOUNDARY will be 1. But we want the stack to be aligned at STACK_BOUNDARY. We will update our proposal. > > >Because I386 PIC requires BX as GOT pointer and I386 may use AX, DX > >and CX as parameter passing registers, there are limited candidates for > >this proposal to choose. Current proposal suggests EDI, because it won't > >conflict with i386 PIC or regparm. > > Could you pick a call-clobbered register in cases where one is availale? Joey, Xuepeng, is that doable? > > >// Reserve two stack slots and save return address > >// and previous frame pointer into them. By > >// pointing new ebp to them, we build a pseudo > >// stack for unwinding > > Hmmm... I don't know much about the DWARF unwind information, but > couldn't it handle this case without creating the "pseudo frame"? > Or at least be extended so it could? Joey, Xuepeng, what do you think? H.J.
