Hi.
I wonder how to calculate absolute value of a 32-bit signed integer in C as fast as possible. I saw that there is a FPU instruction VABS.F32, which do that in one cycle (above the floats). I thought, if it is possible to use it also with integers (sign bit is in both cases MSB bit). Maybe in some way with inline or embedded assembly?
Or do you have any other advice how to get absolute value of an integer in C in the fastest way.
Not to forget - I am speaking regarding Cortex-M4 processor.
Thanks
Hi,
I think you might have some misunderstandings.
Regarding the sign bit, the FPU data format is different from the format of the integer.
In the FPU case, data consists of a sign bit and an absolute value.
In the integer case, data has 2's complement format.
Therefore, the absolute value of FPU data can be gotten because the sign bit can be cleared.
To the contrary, to get the absolute value of the integer there would be 2 steps. The first is to compare with zero. The second is to negate the value if it is a negative value.
The instruction sequence will be the following and it will take at least 2 cycles.
cmp r0,#0 it lt neglt r0,r0
Best regards,
Yasuhiko Koumoto.
Hi matic and Yasuhiko,
If the original value needs to be preserved, below is Example 3.1 from section 3.3.7. Conditional execution of Cortex-M4 Devices Generic User Guide.
Example 3.1 shows the use of a conditional instruction to find the absolute value of a number. R0 = abs(R1).
Example 3.1. Absolute value
MOVS R0, R1 ; R0 = R1, setting flags
IT MI ; skipping next instruction if value 0 or positive
RSBMI R0, R0, #0 ; If negative, R0 = -R0
For both the negate and reverse subtract instructions, we have to handle the special case when the input is 0x80000000 (sign bit=1, all others are 0; -2147483648). We have to check the behavior of the flags particularly V (overflow).
Regards,
Goodwin