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

Ard Biesheuvel
2017-04-28T14:43:56Z

On 28 April 2017 at 15:33, Mark Rutland <mark.rutland@arm.com> wrote:
> 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.
>

OK, good. That is a departure from previous behavior of Clang, which
was causing build errors before due to the fact that msr/mrs
instructions involving 32-bit values must still use x registers.

> * 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.
>

Yes, I think that makes sense.

> 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)