GCC multiple optimization flags

Question

I have some legacy code that compiles with both -02 and -03 set. From the GCC man file I get the guarantee that:

-O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize options.

So, at first glance it would seem likely that turning both of these flags on would be the same as just -O3. However, that got me thinking is that the right thing to do in that case as -O2 is probably the "safer" option. Obviously, it is a simple matter compile some code with all of the permutations and see what happens in each case, but I was wondering if anyone knows if there is a specific policy that GCC has in regard to specifying multiple optimizations levels and if so what is the reasoning behind it?

Answer 1

From the man page:

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

Answer 2

For over-concerned users like my self, here is a code begging for optimization:

$ cat dominant_flag.c #include <stdio.h> int foo(int i) {     return 3*i+122; } int main(int argc, char **argv) {     return foo(0xface); // meant to be optimized out } 

And Here are four compilation scenarios:

$ gcc -g -O0     dominant_flag.c -o flag0 $ gcc -g -O3     dominant_flag.c -o flag3 $ gcc -g -O0 -O3 dominant_flag.c -o flag03 $ gcc -g -O3 -O0 dominant_flag.c -o flag30 

Once I look for the constant 0xface, I see it exists in the non optimized versions:

$ objdump -S -D flag0  | grep -w "\$0xface" # 61e: bf ce fa 00 00 mov $0xface,%edi $ objdump -S -D flag30 | grep -w "\$0xface" # 61e: bf ce fa 00 00 mov $0xface,%edi 

and optimized out in the optimized versions:

$ objdump -S -D flag3  | grep -w "\$0xface" $ objdump -S -D flag03 | grep -w "\$0xface" 

The whole foo call is gone:

$ objdump -S -D flag03 | sed -n "297,298p;299q"  4f0:   b8 e4 f0 02 00          mov    $0x2f0e4,%eax # <--- hex(3*0xface+122)=0x2f0e4  4f5:   c3                      retq