Subject: Re: [PATCH] powerpc/bug: Cast to unsigned long before passing to inline asm

From: Segher Boessenkool <[email-masked]>
Date: Tue, 31 Aug 2021 16:34:32 -0500

Hi!

On Tue, Aug 31, 2021 at 11:27:20PM +1000, Michael Ellerman wrote:
> Nathan filed an LLVM bug [2], in which Eli Friedman explained that "if
> you pass a value of a type that's narrower than a register to an inline
> asm, the high bits are undefined". In this case we are passing a bool
> to the inline asm, which is a single bit value, and so the compiler
> thinks it can leave the high bits of r30 unmasked, because only the
> value of bit 0 matters.
> 
> Because the inline asm compares the full width of the register (32 or
> 64-bit) we need to ensure the value passed to the inline asm has all
> bits defined. So fix it by casting to long.
> 
> We also already cast to long for the similar case in BUG_ENTRY(), which
> it turns out was added to fix a similar bug in 2005 in commit
> 32818c2eb6b8 ("[PATCH] ppc64: Fix issue with gcc 4.0 compiled kernels").

That points to <https://gcc.gnu.org/PR23422>, which shows the correct
explanation.

The code as it was did **not** pass a bool.  It perhaps passed an int
(so many macros in play, it is hard to tell for sure, but it is int or
long int, perhaps unsigned (which does not change anything here).  But
td wants a 64-bit register, not a 32-bit one (which are the only two
possibilities for the "r" constraint on PowerPC).  The cast to "long" is
fine for powerpc64, but it has nothing to do with "narrower than a
register".

If this is not the correct explanation for LLVM, it needs to solve some
other bug.

Segher