Optimizer Incorrect Code Generation

#include <math.h>
#include <stdio.h>

typedef struct {
   int r, c; // number of rows and columns
   int capacity; // maximum number of elements that can be stored
 // in the memory block pointed at by 'el'
   double *el; // storage for elements (row-wise)
} CMAT;

int cmat_mm_mac(CMAT* C, const CMAT* A, const CMAT* B)
{
	int _r, _c, id;
	
	// Check inputs:
	if (A->c != B->r || C->r != A->r || C->c != B->c) {
		return 1;
	}
	// The output cannot be one of the inputs.
	if (A == C || B == C) {
		return 1;	
	}
	
	for (_r=0; _r < C->r; _r++)		//	Outer dims
		for (_c=0; _c < C->c; _c++)	//	 form new matrix
			for (id=0; id < A->c; id++)	// Inner dimension 
				C->el[_r*C->c+_c] += A->el[_r*A->c+id] * B->el[id*B->c+_c];	
	
	return 0;
}


// Here is a function to test the above: 
#include <assert.h>

#define TOL     1e-4
void testMMac(void)
{
   double A[] = { 
          0.2785, 0.9575, 0.1576,
          0.5469, 0.9649, 0.9706 
   }, B[] = { 
          0.9572, 0.1419, 0.4854,
          0.4218, 0.8003, 0.9157
   }, C[4] = { 0 }, Cexpected[] = {
          0.8575, 0.5877,
          1.7686, 1.3734
   };
   CMAT mA = { 2, 3, 6, NULL }, mB = { 3, 2, 6, NULL }, mC = { 2, 2, 4, NULL };
   int i;

   mA.el = A; mB.el = B; mC.el = C;

   cmat_mm_mac(&mC, &mA, &mB);
  
   for (i = 0; i < mC.r * mC.c; ++i) {
       printf("%.4f ", mC.el[i]);
       assert(fabs(mC.el[i] - Cexpected[i]) < TOL);
   }

   printf("\n\n");
}