👀 Views: 1 💬 Answers: 1 📅 Created: 2025-06-07

gcc optimization structs C

I can't seem to get I'm stuck on something that should probably be simple. I'm experiencing unexpected behavior when compiling my C program with GCC 12.2.0 using optimization flags. The program involves a complex struct that contains nested structs, and when I compile it with `-O2`, the output doesn't match my expectations. Specifically, the calculations based on the struct values seem to be incorrect, but they work fine with no optimization. I have a struct defined as follows: ```c typedef struct { int a; float b; } Inner; typedef struct { Inner inner; int c; } Outer; Outer create_outer(int a, float b, int c) { Outer o; o.inner.a = a; o.inner.b = b; o.c = c; return o; } float compute_value(Outer o) { return o.inner.a * o.inner.b + o.c; } int main() { Outer o = create_outer(5, 2.0, 3); float result = compute_value(o); printf("Result: %f\n", result); return 0; } ``` When I compile this code with `gcc -O2 myfile.c -o myprogram`, I get a result of `15.000000`, which is incorrect; it should be `13.000000`. However, if I compile it with `gcc -O0 myfile.c -o myprogram`, it returns the expected value. I tried simplifying the structs and breaking down the computation into more steps, but the scenario continues with optimization enabled. I also tried using different optimization levels like `-O1` and `-Os`, but the incorrect output remains consistently with `-O2`. Could this be a compiler bug, or is there something wrong with how I'm defining or using the structs? Any suggestions for debugging this would be greatly appreciated. My development environment is Windows. Any ideas what could be causing this? I'm using C 3.11 in this project. Any advice would be much appreciated. This is for a application running on Windows 10.