> Am 14.06.2023 um 17:41 schrieb Segher Boessenkool > <seg...@kernel.crashing.org>: > > Hi! > >> On Wed, Jun 14, 2023 at 07:59:04AM +0000, Richard Biener wrote: >>> On Wed, 14 Jun 2023, Jiufu Guo wrote: >>> 3. "set (mem/c:DI (reg/f:DI 1 1) unspec:DI (const_int 0 [0]) >>> UNSPEC_TIE". >>> This avoids using BLK on unspec, but using DI. >> >> That gives the MEM a size which means we can interpret the (set ..) >> as killing a specific area of memory, enabling DSE of earlier >> stores. > > Or DSE can delete this tie even, if it can see some later store to the > same location without anything in between that can read what the tie > stores. > > BLKmode avoids all of this. You can call that elegant, you can call it > cheating, you can call it many things -- but it *works*. > >> AFAIU this special instruction is only supposed to prevent >> code motion (of stack memory accesses?) across this instruction? > > Form rs6000.md: > ; This is to explain that changes to the stack pointer should > ; not be moved over loads from or stores to stack memory. > (define_insn "stack_tie" That suggests it’s the hard register value that‘s protected, not the memory pointed to. I suppose that means an unspec volatile with the reg as input would serve the same? Or maybe that’s not the whole story. > and from rs6000-logue.cc: > /* This ties together stack memory (MEM with an alias set of frame_alias_set) > and the change to the stack pointer. */ > static void > rs6000_emit_stack_tie (rtx fp, bool hard_frame_needed) I cannot make sense of that comment, but not sure if I really want to know … > A big reason this is needed is because of all the hard frame pointer > stuff, which the generic parts of GCC require, but there is no register > for that in the Power architecture. Nothing is an issue here in most > cases, but sometimes we need to do unusual things to the stack, say for > alloca. > >> I'd say a >> >> (may_clobber (mem:BLK (reg:DI 1 1))) > > "clobber" always means "may clobber". (clobber X) means X is written > with some unspecified value, which may well be whatever value it > currently holds. Via some magical means or whatever, there is no > mechanism specified, just the effects :-) > >> might be more to the point? I've used "may_clobber" which doesn't >> exist since I'm not sure whether a clobber is considered a kill. >> The docs say "Represents the storing or possible storing of an >> unpredictable..." - what is it? Storing or possible storing? > > It is the same thing. "clobber" means the same thing as "set", except > the value that is written is not specified. > >> I suppose stack_tie should be less strict than the documented >> (clobber (mem:BLK (const_int 0))) (clobber all memory). > > "clobber" is nicer than the set to (const_int 0). Does it work though? > All this code is always fragile :-/ I'm all for this change, don't get > me wrong, but preferably things stay in working order. > > We use "stack_tie" as a last resort heavy hammer anyway, in all normal > cases we explain the actual data flow explicitly and correctly, also > between the various registers used in the *logues. > > > Segher
Re: [PATCH] rs6000: replace '(const_int 0)' to 'unspec:BLK [(const_int 0)]' for stack_tie
Richard Biener via Gcc-patches Wed, 14 Jun 2023 09:25:32 -0700
- Re: [PATCH] rs6000: replace... Richard Sandiford via Gcc-patches
- Re: [PATCH] rs6000: replace... Richard Biener via Gcc-patches
- Re: [PATCH] rs6000: replace... Richard Sandiford via Gcc-patches
- Re: [PATCH] rs6000: replace... Segher Boessenkool
- Re: [PATCH] rs6000: replace... Segher Boessenkool
- Re: [PATCH] rs6000: replace '(c... Jiufu Guo via Gcc-patches
- Re: [PATCH] rs6000: replace '(c... Segher Boessenkool
- Re: [PATCH] rs6000: replace '(const... Jiufu Guo via Gcc-patches
- Re: [PATCH] rs6000: replace '(c... Segher Boessenkool
- Re: [PATCH] rs6000: replace '(const... Segher Boessenkool
- Re: [PATCH] rs6000: replace '(c... Richard Biener via Gcc-patches
- Re: [PATCH] rs6000: replace... Segher Boessenkool
- Re: [PATCH] rs6000: replace '(const_int ... Segher Boessenkool
- Re: [PATCH] rs6000: replace '(const... Jiufu Guo via Gcc-patches
- Re: [PATCH] rs6000: replace '(c... Segher Boessenkool
- Re: [PATCH] rs6000: replace... Jiufu Guo via Gcc-patches