remove -O3 from hands-on

lectures/parallelism/slides.qmd

@@ -126,12 +126,12 @@ module load gcc
 ```
 2. Compile and run the serial example
 ```bash
-gcc -O3 main.c -o serial.x -lm
+gcc main.c -o serial.x -lm
 time ./serial.x   # use `time` to check the runtime
 ```
 3. Compile and run the example using OpenMP
 ```bash
-gcc -O3 -fopenmp main.c -o parallel.x -lm
+gcc -fopenmp main.c -o parallel.x -lm
 export OMP_NUM_THREADS=2
 time ./parallel.x
 ```