Verifying compiler optimizations in gcc/g++ by analyzing assembly listings

Question

I just asked a question related to how the compiler optimizes certain C++ code, and I was looking around SO for any questions about how to verify that the compiler has performed

Answer 1

In order to output the optimizations applied you can use:

-fopt-info-optimized

To see those that have not been applied

-fopt-info-missed

Beware that the output is sent to standard error stream so to see it you actually have to redirect that : ( hint 2>&1 )

Here is nice example of :

g++ -O3 -std=c++11 -march=native -mtune=native 
        -fopt-info-optimized h2d.cpp -o h2d 2>&1

h2d.cpp:225:3: note: loop vectorized
h2d.cpp:213:3: note: loop vectorized
h2d.cpp:198:3: note: loop vectorized
h2d.cpp:186:3: note: loop vectorized

You can check the interleaved output, when having applied -g with objdump -dS|c++filt , but that will not get you that far.Enjoy!