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Microsoft-specific
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).
Example
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");
exit(1);
}
memptr = mempool;
a = init(M, N);
b = init(N, P);
c = init(M, P);
multiply(a, b, c);
}