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SubjectRe: mainline build failure of powerpc allmodconfig for prom_init_check
On Sun, Jul 17, 2022 at 07:44:22AM -0700, Linus Torvalds wrote:
> On Sun, Jul 17, 2022 at 2:13 AM Sudip Mukherjee
> <> wrote:
> > I was trying to check it. With gcc-11 the assembly code generated is
> > not using memset, but using __memset.
> > But with gcc-12, I can see the assembly code is using memset. One
> > example from the assembly:
> You could try making the 'args' array in 'struct prom_args' be marked
> 'volatile'.
> Ie something like this:
> --- a/arch/powerpc/kernel/prom_init.c
> +++ b/arch/powerpc/kernel/prom_init.c
> @@ -115,6 +115,6 @@ struct prom_args {
> __be32 service;
> __be32 nargs;
> __be32 nret;
> - __be32 args[10];
> + volatile __be32 args[10];
> };
> because I think it's just the compilers turning the small loop over
> those fields into a "memset()".

Yes. See <>
near the end:
Most of the compiler support routines used by GCC are present in
libgcc, but there are a few exceptions. GCC requires the freestanding
environment provide memcpy, memmove, memset and memcmp. Finally, if
__builtin_trap is used, and the target does not implement the trap
pattern, then GCC emits a call to abort.

Can't we simply have a small simple implementation of these functions in
arch/powerpc/boot/? This stuff is not performance-critical, and this is
not the first time we hit these problems.


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