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Strict aliasing problem in gcc 4.4.4


Hi all,

I have a question regarding strict aliasing and GCC 4.4.4. It has come to light in the recompilation of a third-party library that I'm using. I've created a small example of the type of code that can demonstrate the problem:

#include <stdio.h>

struct point
 {
 point(double x, double y)
   : x(x), y(y) { }

 double x, y;
 };

point function1(point* array, int len)
 {
 /* do some initialisation on array */
 for (int i = 0; i < len; i++)
   {
   array[i].x = i;
   array[i].y = i * 1.25;
   }

 return point(-1.0, -2.0);
 }

void function2(double* pts, int len)
 {
 for (int i = 0; i < len; i++)
   printf("%i %f\n", i, pts[i]);
 }

void do_ops(double* pts)
 {
 point* points = (point*)pts;
 points[0] = function1(&points[1], 3);
 }

int main(int, char**)
 {
 double pts[8];

#ifdef INLINE
 point* points = (point*)pts;
 points[0] = function1(points+1, 3);
#else
 do_ops(pts);
#endif

function2(pts, 8);

 return 0;
 }

To give some context, 'function1' and 'function2' are methods inside this 3rd party library (actually in the code above they are simplified approximations, but keeping the same function prototypes). A segment of code from elsewhere inside the library calls this function in a way similar to the following:

double pts[8];
point* points = (point*)pts;
points[0] = function1(points*1, 3);
function2(pts, 8);

The code above then puts all this together. If I compile this code in any of the following ways:

g++ test.cpp -Wall -Werror -o test
g++ test.cpp -Wall -Werror -o test -DINLINE
g++ test.cpp -Wall -Werror -o test -finline-functions
g++ test.cpp -Wall -Werror -o test -O2

then all well and good. The code compiles and runs without error.

However, if I compile in one of the following ways (which is my aim):

g++ test.cpp -Wall -Werror -o test -O2 -DINLINE
g++ test.cpp -Wall -Werror -o test -O2 -finline-functions
g++ test.cpp -Wall -Werror -o test -O3

Then I get this:

cc1plus: warnings being treated as errors
test.cpp: In function 'int main(int, char**)':
test.cpp:41: error: dereferencing pointer 'points' does break strict-aliasing rules
test.cpp:40: note: initialized from here


My question is, how can I fix the code to solve this error? The error is in the assignment to points[0], I believe.

I don't want to specify -fno-strict-aliasing nor remove the -Werror since this will have a impact on the whole library. I also cannot change how 'function1' and 'function2' operate, since they are used in multiple locations. I can however change the line that does the cast from double* to point* prior to the call to 'function1'.

But, I can't find the correct way to handle this cast. I've read about casting through a union (e.g. at http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html), but I can't seem to get it to work. This seems to me to be the best way to tackle it, unless someone can suggest another way. I expect it is something really simple that I've overlooked.

Please, could anyone help me with this?

Thanks

Andy



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