4.5. Assertions

The _nassert intrinsic generates no code and so is not a typical compiler intrinsic. Instead, it tells the compiler that the expression declared with the assert function is true. It can be used to assert that certain conditions are true, which in turn can be used by the compiler during its optimizations.

Warning

Code can fail at runtime if the condition specified in the _nassert is not true.

Listing 4.12 illustrates an example of using _nassert. In this case, the programmer is guaranteeing that the loop executes at least once and count is a multiple of 4. This enables the compiler to avoid generating code to check for count == 0 and peeled iterations during unrolling.

Listing 4.12 Using nassert to avoid generating unnecessary checks

#include <stdint.h>

int16_t sum(int16_t* input, int16_t count)
{
    int16_t sum = 0;
    int16_t i   = 0;

    _nassert(count > 0 && count % 4 == 0);

    for (i = 0; i < count; i++)
        sum += input[i];

    return sum;
}

Table 4.6 shows the assembly generated without and with the _nassert.

Table 4.6 Assembly comparison

Asm generated with _nassert (loop is unrolled 4x)

Asm generated without _nassert (loop is unrolled 2x)

||sum||: MOV AH,AL MOVB AL,#0 ASR AH,2 ADDB AH,#-1 MOVZ AR6,AH ||$C$L1||: ADD AL,*XAR4++ ADD AL,*XAR4++ ADD AL,*XAR4++ ADD AL,*XAR4++ BANZ ||$C$L1||,AR6-- LRETR

||sum||: MOV AH,AL MOVB XAR7,#0 BF ||$C$L4||,LEQ CMPB AH,#2 BF ||$C$L1||,GEQ MOV PL,#0 BF ||$C$L3||,UNC ||$C$L1||: AND AH,AH,#0xfffe MOV PL,AH MOVL XAR5,XAR4 MOV AH,AL ASR AH,1 ADDB AH,#-1 MOVZ AR6,AH ||$C$L2||: MOV AH,AR7 ADD AH,*XAR5++ ADD AH,*XAR5++ MOVZ AR7,AH BANZ ||$C$L2||,AR6-- ||$C$L3||: TBIT AL,#0 BF ||$C$L4||,NTC MOVL ACC,XAR4 SETC SXM ADD ACC,PL MOVL XAR4,ACC MOV AL,AR7 ADD AL,*+XAR4[0] MOVZ AR7,AL ||$C$L4||: MOV AL,AR7 LRETR