Re: [PATCH] riscv: pass machine size to sparse

[Palmer Dabbelt]

On Mon, 28 May 2018 23:14:20 PDT (-0700), yamada.masahiro@xxxxxxxxxxxxx wrote:
> 2018-05-29 15:11 GMT+09:00 Christoph Hellwig <hch@xxxxxxxxxxxxx>:
> > On Mon, May 28, 2018 at 06:35:05PM +0200, Luc Van Oostenryck wrote:
> > > By default, sparse assumes a 64bit machine when compiled on x86-64
> > > and 32bit when compiled on anything else.
> > >
> > > This can of course create all sort of problems when this doesn't
> > > correspond to the target's machine size, like issuing false
> > > warnings like: 'shift too big (32) for type unsigned long' or
> > > is 64bit while sparse was compiled on a 32bit machine, or worse,
> > > to not emit legitimate warnings.
> > >
> > > Fix this by passing the appropriate -m32/-m64 flag to sparse.
> >
> > Can we please move this to the common Kbuild code using the
> > CONFIG_64BIT syombol?  This really should not need boiler plate in
> > every architecture.
>
>
> I agree.
>
> Luc did so for -mbig/little-endian:
> https://patchwork.kernel.org/patch/10433957/
>
> We should do likewise for -m32/64.

Sorry for being a bit slow here, but I like the idea of making the 32/64-bit 
issue generic as it seems like it'll be necessary for every port.  Looking 
through the patch for big/little-endian I did notice:

* RISC-V compilers set "__riscv_xlen" to the length of an X (integer) register 
 in bits.
* RISC-V compilers define "__riscv", and it doesn't appear we inform sparse 
 about that.

These two might not be that interesting, but we do already have some cases 
where we're checking for __riscv_xlen in Linux.  I've yet to successfully use 
sparse, but adding at least

   CHECKFLAGS += -D__riscv

seems reasonable, and possibly also some sort of

   ifeq ($(CONFIG_ARCH_RV64I),y)
           CHECKFLAGS += -D__riscv_xlen=64
   else
           CHECKFLAGS += -D__riscv_xlen=32
   fi

might be necessary.  We strive to follow the generic rules for ABI-related 
stuff like __LP64__ but I don't think there's any generic mapping for XLEN.  
Similarly there's "__riscv_flen" and "__riscv_float_abi_*", but those are less 
likely to be used by the kernel so they're probably not worth worrying about 
for now.

There's also a bunch of other RISC-V macros, the only one of which we're 
currently using is "__riscv_muldiv" (and that's in a manner that's unlikely to 
trigger any sort of static analysis).  Between a lack of Kconfig options and a 
glibc port we're essentially mandating IMA right now, so these probably don't 
matter.

Thanks!