Overview:
When using optimisation level -O3 the code generated from attached example is
wrong. It is accumalating the stream increment and thus the function readFloat
reads twice the same number and then it skips two in a row.
Problem seems to be in this line
const float val = *(const float *&)stream;
when I change it to
const float val = *(const float *)stream;
it starts working even on -O3.
Steps to reproduce:
Compile this code with -O3.
#include <stdio.h>
float readFloat(const char *&stream)
{
const float val = *(const float *&)stream;
stream += sizeof(float);
return val;
}
int main(int argc, char **argv)
{
float stream[] = { 2.0f,
1.0f, 2.0f,
3.0f, 4.0f,
5.0f, 6.0f };
const char *stream2 = (const char *) stream;
for (float i = 0, n = readFloat(stream2); i <= n; i += 1.0f) {
const float x = readFloat(stream2);
const float y = readFloat(stream2);
printf("%f,%f\n", x,y);
}
return 0;
}
Actual results:
Prints:
1.000000,1.000000
3.000000,3.000000
5.000000,5.000000
Expected results:
to print:
1.000000,2.000000
3.000000,4.000000
5.000000,6.000000
Build date & platform:
gcc (GCC) 4.1.2 20070626 (Red Hat 4.1.2-13)
Additional Builds and Platforms:
gcc (GCC) 4.0.2
gcc (GCC) 4.1.1
gcc (GCC) 4.2.0
This bug is not reproducible on gcc 3.3 series.
--
Summary: Wrong optimisation for -O3
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: honza at jikos dot cz
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32714
