On Fri, 27 Nov 2020, Anders Roxell wrote:
> When building mips tinyconfig with clang the following warning show up:
>
> /tmp/arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
> register long sp __asm__("$sp");
> ^
> = 0
>
> Rework to make an explicit inline move.
It's not uninitialised, because by definition $sp has been initialised.
This code wouldn't have worked for the last 15 years otherwise.
It is however non-standard usage (which may not have been clarified at
the time this piece was made; I recall extensive discussions around this
feature over the years):
"6.47.5.2 Specifying Registers for Local Variables
.................................................
[...]
"The only supported use for this feature is to specify registers for
input and output operands when calling Extended 'asm' (*note Extended
Asm::). [...]"
so the change description should refer to that as the rationale rather
than an implementation-specific misleading warning (although that can be
quoted for a reference of course).
> diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
> index 09d5deea747f..21a4b94a0558 100644
> --- a/arch/mips/lib/uncached.c
> +++ b/arch/mips/lib/uncached.c
> @@ -37,10 +37,11 @@
> */
> unsigned long run_uncached(void *func)
> {
> - register long sp __asm__("$sp");
> register long ret __asm__("$2");
> long lfunc = (long)func, ufunc;
> long usp;
> + long sp;
> + asm ("move %0, $sp" : "=r" (sp));
Missing newline between the variable block and code here; you may well
swap it with the empty line below you have left. Also use `__asm__' for
consistency, and remove the extraneous space ahead of the parenthesis as
we don't use them in Linux. Using a tab to separate the mnemonic from
operands is also preferable, i.e. "move\t%0, $sp" in this case (using
actual tabs works better with multi-line assembly, but this is not one).
I think this should be OK once the issues I mentioned here have been
addressed.
Maciej
