Re: [PATCH v2] arm64: Add ASM modifier for xN register operands

Mark Rutland
2017-04-28T14:33:33Z

On Fri, Apr 28, 2017 at 08:18:52AM +0100, Ard Biesheuvel wrote:
> On 27 April 2017 at 23:52, Matthias Kaehlcke <mka@chromium.org> wrote:
> > El Thu, Apr 27, 2017 at 12:02:56PM +0100 Mark Rutland ha dit:
> >> On Wed, Apr 26, 2017 at 02:46:16PM -0700, Matthias Kaehlcke wrote:
> >> > Many inline assembly statements don't include the 'x' modifier when
> >> > using xN registers as operands. This is perfectly valid, however it
> >> > causes clang to raise warnings like this:
> >> >
> >> > warning: value size does not match register size specified by the
> >> > constraint and modifier [-Wasm-operand-widths]

[...]

> >> > - asm volatile("strb %w0, [%1]" : : "rZ" (val), "r" (addr));
> >> > + asm volatile("strb %w0, [%x1]" : : "rZ" (val), "r" (addr));
> >>
> >> In general, the '[%xN]' pattern looks *very* suspicious to me. Any
> >> address must be 64-bit, so this would mask a legitimate warning.
> >>
> >> Given the prototype of this function the code if fine either way, but
> >> were we to refactor things (e.g. making this a macro), that might not be
> >> true.
> >>
> >> ... so I'm not sure it make sense to alter instances used for addresses.
> >
> > Good point, I'll leave instances dealing with addresses untouched for now.
> >
>
> OK, I am confused now. We started this thread under the assumption
> that all unqualified placeholders are warned about by Clang. Given
> that this appears not to be the case, could we please first find out
> what causes the warnings? Is it necessary at all to add the x
> modifiers for 64-bit types?

FWIW, I grabbed a clang 4.0.0 binary and had a play.

It looks like clang only warns when an operand is less than 64 bits
wide, and there is no 'x' or 'w' modifier. Pointers a 64 bits wide, so
never produce a warning.

As far as I can tell, applying to both integers and pointers:

* GCC and clang always treat %N as meaning xN for an r constraint, and
you need to use %wN to get wN.

* If an operand type is 64 bits in size, clang will not produce a warning
regarding the operand size.

* If an x or w modifier is used, clang will not produce a warning
regarding the operand size, regardless of whether it matches the
register size. Clang is happy for %wN to be used on a pointer type.

* If an operand type is less than 64 bits in size, and neither an x or
w modifier is used, clang will produce a warning as above.

* If an operand type is greater than 64 bits in size, clang encounters
an internal error.

Given that, I think we *should not* use the x modifier to suppress this
warning, as I think for those cases we have a potential bug as outlined
in my prior reply.

Instead, we should use a temporary 64-bit variable (or cast input
operands to 64-bit), which avoids that and makes clang happy.

I've included my test below. Note that clang will produce other errors for
invalid asm (e.g. for mov w0, x0).

Thanks,
Mark.

---->8----
#define TEST(t, w1, w2) \
t foo_##t##w1##_##w2(t a, t b) \
{ \
asm ( \
"mov %" #w1 "0, %" #w2 "1" \
: "=r" (a) : "r" (b) \
); \
\
return a; \
}

#define TEST_TYPE(t) \
TEST(t, , ) \
TEST(t, w, ) \
TEST(t, w, w) \
TEST(t, w, x) \
TEST(t, x, ) \
TEST(t, x, w) \
TEST(t, x, x) \

TEST_TYPE(int)

TEST_TYPE(long)

typedef long * longp;
TEST_TYPE(longp)

TEST_TYPE(__int128)