On 8/6/19 5:51 AM, Richard Earnshaw (lists) wrote:
On 18/07/2019 18:18, James Greenhalgh wrote:
On Mon, Jun 10, 2019 at 06:21:05PM +0100, Sylvia Taylor wrote:
Greetings,
This patch adds the intrinsic functions for:
- vld1_<mode>_x4
- vst1_<mode>_x4
- vld1q_<mode>_x4
- vst1q_<mode>_x4
Bootstrapped and tested on aarch64-none-linux-gnu.
Ok for trunk? If yes, I don't have any commit rights, so can someone
please commit it on my behalf.
Hi,
I'm concerned by this strategy for implementing the arm_neon.h builtins:
+__extension__ extern __inline int8x8x4_t
+__attribute__ ((__always_inline__, __gnu_inline__, __artificial__))
+vld1_s8_x4 (const int8_t *__a)
+{
+ union { int8x8x4_t __i; __builtin_aarch64_simd_xi __o; } __au;
+ __au.__o
+ = __builtin_aarch64_ld1x4v8qi ((const __builtin_aarch64_simd_qi
*) __a);
+ return __au.__i;
+}
As far as I know this is undefined behaviour in C++11. This was the best
resource I could find pointing to the relevant standards paragraphs.
https://stackoverflow.com/questions/11373203/accessing-inactive-union-member-and-undefined-behavior
That said, GCC explicitly allows it, so maybe this is fine?
https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Optimize-Options.html#Type-punning
Can anyone from the languages side chime in on whether we're exposing
undefined behaviour (in either C or C++) here?
Yes, this is a GNU extension. My only question is whether or not this
can be disabled within GCC if you're trying to check for strict
standards conformance of your code?
It's undefined behavior: doing something reasonable is a conformant
interpretation of undefined behavior.
I don't imagine that ubsan checks for this case, but it's possible.
And if so, is there a way of making sure that this header still works in that
case?
The well-defined solution is memcpy. Or, in C++20, bit_cast (not
implemented yet).
Jason