Re: [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Mon, May 04, 2020 at 06:14:43PM -0700, Jesse Brandeburg wrote:
> On Mon, 4 May 2020 12:51:12 -0700
> Nick Desaulniers <ndesaulniers@google.com> wrote:
> 
> > Sorry for the very late report.  It turns out that if your config
> > tickles __builtin_constant_p just right, this now produces invalid
> > assembly:
> > 
> > $ cat foo.c
> > long a(long b, long c) {
> >   asm("orb\t%1, %0" : "+q"(c): "r"(b));
> >   return c;
> > }
> > $ gcc foo.c
> > foo.c: Assembler messages:
> > foo.c:2: Error: `%rax' not allowed with `orb'
> > 
> > The "q" constraint only has meanting on -m32 otherwise is treated as
> > "r".
> > 
> > Since we have the mask (& 0xff), can we drop the `b` suffix from the
> > instruction? Or is a revert more appropriate? Or maybe another way to
> > skin this cat?
> 
> Figures that such a small change can create problems :-( Sorry for the
> thrash!
> 
> The patches in the link below basically add back the cast, but I'm
> interested to see if any others can come up with a better fix that
> a) passes the above code generation test
> b) still keeps sparse happy
> c) passes the test module and the code inspection
> 
> If need be I'm OK with a revert of the original patch to fix the issue
> in the short term, but it seems to me there must be a way to satisfy
> both tools.  We went through several iterations on the way to the final
> patch that we might be able to pluck something useful from.

For me the below seems to work:


diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index b392571c1f1d1..139122e5b25b1 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "orb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" (CONST_MASK(nr) & 0xff)
+			: "iq" ((u8)(CONST_MASK(nr) & 0xff))
 			: "memory");
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
@@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr)
 	if (__builtin_constant_p(nr)) {
 		asm volatile(LOCK_PREFIX "andb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" (CONST_MASK(nr) ^ 0xff));
+			: "iq" ((u8)(CONST_MASK(nr) ^ 0xff)));
 	} else {
 		asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
 			: : RLONG_ADDR(addr), "Ir" (nr) : "memory");


-- 
With Best Regards,
Andy Shevchenko