On Mon, Aug 6, 2012 at 5:20 AM, Parang Saraf <parang.sa...@gmail.com> wrote: > Hi, > > I am facing some weird issues with the optimization flags. I tried > posting this in the gcc-help but got no response. Here is my query: > > I can see the flags enabled for each optimization level using "gcc -Q > -On -help=optimizers" (flags enabled) > Also, when I compile a program for a particular optimization level, I > can see the flags used by gcc using, "gcc -Q -v -On program.c" (flags > used). > I am working with gcc 4.4.5. I found a few anomalies, which are mentioned > below: > > 1. There are a few flags which are documented in O1 but are enabled > for O0. Example: -funit-at-a-time. This flag is both enabled and used > at O0 > 2. There are a few flags, which are documented in O1 and are not > enabled for O0 (which is correct), but are still used for O0. Example: > -fauto-inc-dec flag is documented in O1 and is not enabled for O0 but > is still used by O0. > 3. Flags which are enabled but are not used. Example: flags -fdce and > -fdse are enabled for O0, O1 but is never used by the compiler. > 4. Here is the complete list of flags distribution: > O0: documented 0; enabled 35; used 43 > O1: documented 26; enabled 56; used 66 > O2: documented 55; enabled 80; used 91 > O3: documented 61; enabled 86; used 97 > > I understand that there are a few flags which are used internally by > the gcc. But my question is, if some of the flags are shown as enabled > for a particular optimization level, why are they are not used during > compilation? > > Also, ideally if I am using -O0, i should not be able to add/enable > optimization flags. But when i enable some of the O1 optimization > flags at O0, I could see gcc using them. For example: "gcc -Q -v -O0 > -ftree-fre program.c". Here -ftree-fre is an optimization flag from > O1, but still it gets enabled at -O0. > > Most importantly: How can I make my gcc 4.5.4 adhere strictly with the > 4.5.4 documentation? If I have to recompile gcc and make some changes > to the config files, then please let me know. > > I will greatly appreciate your response in this regard.
It is a common misunderstanding that -On is just an alias for a set of -fX options. It does not work this way. In case of -funit-at-a-time this is a bug in the documentation. Note that GCC 4.5.x is no longer maintained. Richard. > Regards > Parang
