noalias means that a function call doesn't modify or reference visible global state and only modifies the memory pointed to directly by pointer parameters (first-level indirections).

If a function is annotated as noalias, the optimizer can assume that only the parameters themselves, and only first-level indirections of pointer parameters, are referenced or modified inside the function.

The noalias annotation only applies within the body of the annotated function. Marking a function as __declspec(noalias) doesn't affect the aliasing of pointers returned by the function.

For another annotation that can impact aliasing, see __declspec(restrict).


The following sample demonstrates the use of __declspec(noalias).

When the function multiply that accesses memory is annotated __declspec(noalias), it tells the compiler that this function doesn't modify the global state except through the pointers in its parameter list.

// declspec_noalias.c
#include <stdio.h>
#include <stdlib.h>

#define M 800
#define N 600
#define P 700

float * mempool, * memptr;

float * ma(int size)
    float * retval;
    retval = memptr;
    memptr += size;
    return retval;

float * init(int m, int n)
    float * a;
    int i, j;
    int k=1;

    a = ma(m * n);
    if (!a) exit(1);
    for (i=0; i<m; i++)
        for (j=0; j<n; j++)
            a[i*n+j] = 0.1/k++;
    return a;

__declspec(noalias) void multiply(float * a, float * b, float * c)
    int i, j, k;

    for (j=0; j<P; j++)
        for (i=0; i<M; i++)
            for (k=0; k<N; k++)
                c[i * P + j] =
                          a[i * N + k] *
                          b[k * P + j];

int main()
    float * a, * b, * c;

    mempool = (float *) malloc(sizeof(float) * (M*N + N*P + M*P));

    if (!mempool)
        puts("ERROR: Malloc returned null");

    memptr = mempool;
    a = init(M, N);
    b = init(N, P);
    c = init(M, P);

    multiply(a, b, c);

See also