Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'
On Mon, 14 Dec 2020 at 21:05, Thomas Bogendoerfer
wrote:
>
> On Fri, Dec 11, 2020 at 11:24:37AM +0100, Anders Roxell wrote:
> > When building mips tinyconfig with clang the following warning show up:
> >
> > arch/mips/lib/uncached.c:45:6: warning: variable 'sp' is uninitialized when
> > used here [-Wuninitialized]
> > if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
> > ^~
> > arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to
> > silence this warning
> > register long sp __asm__("$sp");
> > ^
> > = 0
> > 1 warning generated.
> > [1] https://docs.w3cub.com/gcc~10/local-register-variables
> > Signed-off-by: Anders Roxell
> > ---
> > arch/mips/lib/uncached.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
>
> applied to mips-next.
We have noticed these build failures on stable tree also.
May I request you to initiate the process to get into the stable tree
and for the following branches.
- linux-5.10.y
- linux-5.9.y
- linux-5.4.y
- linux-4.19.y
- Naresh
Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'
On Fri, Dec 11, 2020 at 11:24:37AM +0100, Anders Roxell wrote:
> When building mips tinyconfig with clang the following warning show up:
>
> arch/mips/lib/uncached.c:45:6: warning: variable 'sp' is uninitialized when
> used here [-Wuninitialized]
> if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
> ^~
> arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence
> this warning
> register long sp __asm__("$sp");
> ^
> = 0
> 1 warning generated.
>
> Rework to make an explicit inline move, instead of the non-standard use
> of specifying registers for local variables. This is what's written
> from the gcc-10 manual [1] about specifying registers for local
> variables:
>
> "6.47.5.2 Specifying Registers for Local Variables
> .
> [...]
>
> "The only supported use for this feature is to specify registers for
> input and output operands when calling Extended 'asm' (*note Extended
> Asm::). [...]".
>
> [1] https://docs.w3cub.com/gcc~10/local-register-variables
> Signed-off-by: Anders Roxell
> ---
> arch/mips/lib/uncached.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
applied to mips-next.
Thomas.
--
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.[ RFC1925, 2.3 ]
Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'
On Fri, Dec 11, 2020 at 07:54:07PM +, Maciej W. Rozycki wrote:
> On Fri, 11 Dec 2020, Anders Roxell wrote:
>
> > diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> > index 09d5deea747f..f80a67c092b6 100644
> > --- a/arch/mips/lib/uncached.c
> > +++ b/arch/mips/lib/uncached.c
> > @@ -37,10 +37,12 @@
> > */
> > unsigned long run_uncached(void *func)
> > {
> > - register long sp __asm__("$sp");
> > register long ret __asm__("$2");
> > long lfunc = (long)func, ufunc;
> > long usp;
> > + long sp;
> > +
> > + __asm__("move %0, $sp" : "=r" (sp));
>
> I thought it might be better to make `sp' global instead, so that it's
> the compiler that chooses how to schedule accesses. Have you tried that?
>
> Maciej
This will not work, as the LLVM Mips backend does not support using $sp
as a global register variable:
https://github.com/llvm/llvm-project/commit/1440bb2a26ff13df1b29762658ee122cc0bc47ae
$ make -skj"$(nproc)" ARCH=mips CROSS_COMPILE=mipsel-linux-gnu- LLVM=1 O=out \
distclean malta_kvm_guest_defconfig vmlinux
fatal error: error in backend: Invalid register name global variable
...
Cheers,
Nathan
Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'
On Fri, 11 Dec 2020, Anders Roxell wrote:
> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index 09d5deea747f..f80a67c092b6 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -37,10 +37,12 @@
> */
> unsigned long run_uncached(void *func)
> {
> - register long sp __asm__("$sp");
> register long ret __asm__("$2");
> long lfunc = (long)func, ufunc;
> long usp;
> + long sp;
> +
> + __asm__("move %0, $sp" : "=r" (sp));
I thought it might be better to make `sp' global instead, so that it's
the compiler that chooses how to schedule accesses. Have you tried that?
Maciej
Re: [PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'
On Fri, Dec 11, 2020 at 2:24 AM Anders Roxell wrote:
>
> When building mips tinyconfig with clang the following warning show up:
>
> arch/mips/lib/uncached.c:45:6: warning: variable 'sp' is uninitialized when
> used here [-Wuninitialized]
> if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
> ^~
> arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence
> this warning
> register long sp __asm__("$sp");
> ^
> = 0
> 1 warning generated.
>
> Rework to make an explicit inline move, instead of the non-standard use
> of specifying registers for local variables. This is what's written
> from the gcc-10 manual [1] about specifying registers for local
> variables:
>
> "6.47.5.2 Specifying Registers for Local Variables
> .
> [...]
>
> "The only supported use for this feature is to specify registers for
> input and output operands when calling Extended 'asm' (*note Extended
> Asm::). [...]".
>
> [1] https://docs.w3cub.com/gcc~10/local-register-variables
> Signed-off-by: Anders Roxell
Link: https://github.com/ClangBuiltLinux/linux/issues/606
Reported-by: Nathan Chancellor
Reported-by: Naresh Kamboju
Reviewed-by: Nick Desaulniers
> ---
> arch/mips/lib/uncached.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index 09d5deea747f..f80a67c092b6 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -37,10 +37,12 @@
> */
> unsigned long run_uncached(void *func)
> {
> - register long sp __asm__("$sp");
> register long ret __asm__("$2");
> long lfunc = (long)func, ufunc;
> long usp;
> + long sp;
> +
> + __asm__("move %0, $sp" : "=r" (sp));
>
> if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
> usp = CKSEG1ADDR(sp);
> --
> 2.29.2
>
--
Thanks,
~Nick Desaulniers
