> Am 23.12.2022 um 15:48 schrieb Segher Boessenkool > <seg...@kernel.crashing.org>: > > Hi! > >> On Fri, Dec 23, 2022 at 08:36:36PM +0800, Jiufu Guo wrote: >> It seems some limitations there. e.g. 1. "subreg:DF on DI register" >> may not work well on pseudo, > > It is perfectly normal: > A hard register may be accessed in various modes throughout one > function, but each pseudo register is given a natural mode > and is accessed only in that mode. When it is necessary to describe > an access to a pseudo register using a nonnatural mode, a @code{subreg} > expression is used. > > and: > @code{subreg} expressions are used to refer to a register in a machine > mode other than its natural one, or to refer to one register of > a multi-part @code{reg} that actually refers to several registers. > > Each pseudo register has a natural mode. If it is necessary to > operate on it in a different mode, the register must be > enclosed in a @code{subreg}. > > and we even have: > @item hard registers > It is seldom necessary to wrap hard registers in @code{subreg}s; such > registers would normally reduce to a single @code{reg} rtx. This use of > @code{subreg}s is discouraged and may not be supported in the future. > >> and 2. to convert high-part:DI to SF, >> a "shift/rotate" is needed, and then we need to "emit shift insn" >> in cse. I may need to update this patch. > > Hrm. The machine insns to do this is just mtvsrd;xscvspdpn, but for > converting the lowpart it is mtvsrws;xscvspdpn (this needs p9 or > later). We should arrive at those patterns, and we should try to not > go via the more expensive formulations with shifts, which don't describe > the hardware well, and which overestimate the cost of it. > > None of this belongs in generic code at all imo. At expand time it > should be expanded to something that works and can be optimised well, > so not anything with :BLK (which has to be put in memory, something with > unbounded size cannot be put in registers), not anything specifically > tailored to any cpu, something nice and regular. Using a subreg (of a > pseudo!) is the standard way of writing a bitcast. > > So generic code would do a (subreg:SF (reg:SI) 0) to express a 32-bit > integer bitcast to an IEEE SP number, and our machine description should > make it work nicely. There’s also a byte offset in subreg, so (subreg:sf (reg:di) 4) is a Highpart bitcast. Note whether targets actually support subreg operations needs to be queried and I’m not sure how subreg with offset validation should work there. Richard > > > Segher
Re: [PATCH] loading float member of parameter stored via int registers
Richard Biener via Gcc-patches Fri, 23 Dec 2022 08:20:30 -0800
- [PATCH] loading float member of parameter s... Jiufu Guo via Gcc-patches
- Re: [PATCH] loading float member of pa... Richard Biener via Gcc-patches
- Re: [PATCH] loading float member o... guojiufu via Gcc-patches
- Re: [PATCH] loading float memb... Richard Biener via Gcc-patches
- Re: [PATCH] loading float ... Jiufu Guo via Gcc-patches
- Re: [PATCH] loading f... Richard Biener via Gcc-patches
- Re: [PATCH] loadi... Segher Boessenkool
- Re: [PATCH] l... Jiufu Guo via Gcc-patches
- Re: [PATCH] loading f... Jiufu Guo via Gcc-patches
- Re: [PATCH] loadi... Segher Boessenkool
- Re: [PATCH] l... Richard Biener via Gcc-patches
- Re: [PATCH] l... Segher Boessenkool
- Re: [PATCH] l... Richard Biener via Gcc-patches
- Re: [PATCH] l... Segher Boessenkool
- Re: [PATCH] l... Jiufu Guo via Gcc-patches
- Re: [PATCH] l... Jiufu Guo via Gcc-patches
- Re: [PATCH] l... Jiufu Guo via Gcc-patches
- Re: [PATCH] l... Segher Boessenkool
- Re: [PATCH] l... Andrew Pinski via Gcc-patches
- Re: [PATCH] l... Jiufu Guo via Gcc-patches
- Re: [PATCH] l... Segher Boessenkool