12.57 __usax intrinsic

This intrinsic inserts a USAX instruction into the instruction stream generated by the compiler.

It enables you to exchange the halfwords of the second operand, subtract the high halfwords and add the low halfwords.
The GE bits in the APSR are set according to the results.

Syntax

unsigned int __usax(unsigned int val1, unsigned int val2)
Where:
val1
holds the first operand for the addition in the low halfword, and the first operand for the subtraction in the high halfword
val2
holds the second operand for the addition in the high halfword, and the second operand for the subtraction in the low halfword.

Return value

The __usax intrinsic returns:
  • The addition of the low halfword in the first operand and the high halfword in the second operand, in the low halfword of the return value.
  • The subtraction of the low halfword in the second operand from the high halfword in the first operand, in the high halfword of the return value.
Each bit in APSR.GE is set or cleared for each byte in the return value, depending on the results of the operation. If res is the return value, then:
  • If res[15:0] ≥ 0x10000 then APSR.GE[1:0] = 11 else 00.
  • If res[31:16] ≥ 0 then APSR.GE[3:2] = 11 else 00.

Example

unsigned int exchange_subtract_add(unsigned int val1, unsigned int val2)
{
    unsigned int res;
    res = __usax(val1,val2); /* res[15:0] = val1[15:0] + val2[31:16]
                                res[31:16] = val1[31:16] - val2[15:0]
                              */
    return res;
}
Related reference
9.144 ARMv6 SIMD intrinsics
Related information
USAX
ARM and Thumb instruction summary
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