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rather than:
double xd;
__fp16 xs = xd;
In some older implementations, __fp16 cannot be used as an argument or result type, though it can be used as a
field in a structure passed as an argument or result, or passed via a pointer. The predefined macro
__ARM_FP16_ARGS should be defined if __fp16 can be used as an argument and result. C++ name mangling is
“Dh” as defined in [cxxabi], and is the same for both the IEEE and alternative formats.
In this example, the floating-point addition is done in single (32-bit) precision:
void add(__fp16 *z, __fp16 const *x, __fp16 const *y, int n) {
int i;
for (i = 0; i < n; ++i) z[i] = x[i] + y[i];
}
4.2 Predefined macros
Several predefined macros are defined. Generally these define features of the ARM architecture being targeted,
or how the C/C++ implementation uses the architecture. These macros are detailed in section 6. All ACLE
predefined macros start with the prefix __ARM.
4.3 Intrinsics
ACLE standardizes intrinsics to access the NEON (Advanced SIMD) extension. These intrinsics are intended to
be compatible with existing implementations. Before using the NEON intrinsics or data types, the <arm_neon.h>
header must be included. The NEON intrinsics are defined in section 12. Note that the NEON intrinsics and data
types are in the user namespace.
ACLE also standardizes other intrinsics to access ARM instructions which do not map directly to C operators –
generally either for optimal implementation of algorithms, or for accessing specialist system-level features.
Intrinsics are defined further in various following sections.
Before using the non-NEON intrinsics, the <arm_acle.h> header should be included.
Whether intrinsics are macros, functions or built-in operators is unspecified. For example:
it is unspecified whether applying #undef to an intrinsic removes the name from visibility
it is unspecified whether it is possible to take the address of an intrinsic
However, each argument must be evaluated at most once. So this definition is acceptable:
#define __rev(x) __builtin_bswap32(x)
but this is not:
#define __rev(x) ((((x) & 0xff) << 24) | (((x) & 0xff00) << 8) | \
(((x) & 0xff0000) >> 8) | ((x) >> 24))
4.3.1 Constant arguments to intrinsics
Some intrinsics may require arguments that are constant at compile-time, to supply data that is encoded into the
immediate fields of an instruction. Typically, these intrinsics require an integral-constant-expression in a specified
range, or sometimes a string literal. An implementation should produce a diagnostic if the argument does not meet
the requirements.
4.4 Header files
<arm_acle.h> is provided to make the non-NEON intrinsics available. These intrinsics are in the C
implementation namespace and begin with double underscores. It is unspecified whether they are available
without the header being included. The __ARM_ACLE macro should be tested before including the header:
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