Question

I recently received a bug report about a program which fails to compile with the -O3 switch (see https://github.com/cschwan/sage-on-gentoo/issues/66). In particular, the problem is that compilation hangs at a certain point. The issue is solved by compiling with -O2 (I am well aware of the fact that programs compiled with -O3 may be broken, but I did not know that -O3 may hangup the compiler). If you want to reproduce the issue run

wget http://perso.ens-lyon.fr/xavier.pujol/fplll/libfplll-3.0.12.tar.gz
tar -xf libfplll-3.0.12.tar.gz
cd libfplll-3.0.12
./configure CXXFLAGS="-O3"
make

I wondered why -O3 hangs up the compiler and so I tried to track down the issue. First, I tried to find out the the difference between -O2 between -O3. Gcc’s man page states that -O3 enables the switches of -O2 and the following ones (lets call them x):

-finline-functions -funswitch-loops -fpredictive-commoning -fgcse-after-reload
-ftree-vectorize -fipa-cp-clone

I verified that by comparing the output of gcc when invoked with -Q -O2 --help=optimizers and -Q -O3 --help=optimizers. I then planned to selectively remove the switches in order to find the one which causes the problem. However, compilation works fine with -O2 and the additional switches above, so I conclude

-O3 != -O2 x

Now my question: Does anybody know if there is a further difference between -O2 and -O3 (undocumented?), has anyone experienced a similar behavior ? Is this maybe a compiler bug ?

Answer 1

Man pages can be outdated, but you can find actual lists for O2 and O3.

To get full list (almost, check “update”) of -f optimization options actually used, I suggest you use the -fverbose-asm -save-temps (or -fverbose-asm -S) – there is a full list at a top of asm file (*.s).

For gcc-4.6.0 I got x (the difference between O2 and O3) to be:

 -fgcse-after-reload
 -finline-functions
 -fipa-cp-clone
 -fpredictive-commoning
 -ftree-loop-distribute-patterns
 -ftree-vectorize
 -funswitch-loops

Another source of information for your question is the sources of GCC (file gcc/opts.c and possibly gcc/common.opt) as gcc-4.6.0:

/* -O3 optimizations.  */
{ OPT_LEVELS_3_PLUS, OPT_ftree_loop_distribute_patterns, NULL, 1 },
{ OPT_LEVELS_3_PLUS, OPT_fpredictive_commoning, NULL, 1 },
/* Inlining of functions reducing size is a good idea with -Os
   regardless of them being declared inline.  */
{ OPT_LEVELS_3_PLUS_AND_SIZE, OPT_finline_functions, NULL, 1 },
{ OPT_LEVELS_3_PLUS, OPT_funswitch_loops, NULL, 1 },
{ OPT_LEVELS_3_PLUS, OPT_fgcse_after_reload, NULL, 1 },
{ OPT_LEVELS_3_PLUS, OPT_ftree_vectorize, NULL, 1 },
{ OPT_LEVELS_3_PLUS, OPT_fipa_cp_clone, NULL, 1 },

I have also checked, does gcc check -On in other files (cscope symbol search for x_optimize).

The only additional usage of n from option -On is saving it value into macro __OPTIMIZE__. So some headers can behave differently for value of this macro equal 2 or 3.

UPDATE: There are questions about it in GCC WIKI:

• “Is -O1 (-O2,-O3 or -Os) equivalent to individual -foptimization options?”

No. First, individual optimization options (-f*) do not enable optimization, an option -Os or -Ox with x > 0 is required. Second, the -Ox flags enable many optimizations that are not controlled by any individual -f* option. There are no plans to add individual options for controlling all these optimizations.

• “What specific flags are enabled by -O1 (-O2, -O3 or -Os)?“

Varies by platform and GCC version. You can get GCC to tell you what flags it enables by doing this:

touch empty.c
gcc -O1 -S -fverbose-asm empty.c
cat empty.s