Messages in this thread | | | Date | Fri, 28 Apr 2017 15:33:33 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2] arm64: Add ASM modifier for xN register operands |
| |
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)
| |