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POWhat's the difference between -O3 and (-O2 + flags that man gcc says -O3 adds to -O2)?
 

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  1. POWhat's the difference between -O3 and (-O2 + flags that man gcc says -O3 adds to -O2)?
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    <p>I recently received a bug report about a program which fails to compile with the -O3 switch (see <a href="https://github.com/cschwan/sage-on-gentoo/issues/66" rel="nofollow noreferrer">https://github.com/cschwan/sage-on-gentoo/issues/66</a>). 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</p> <pre><code>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 </code></pre> <p>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 <code>x</code>):</p> <pre><code>-finline-functions -funswitch-loops -fpredictive-commoning -fgcse-after-reload -ftree-vectorize -fipa-cp-clone </code></pre> <p>I verified that by comparing the output of gcc when invoked with <code>-Q -O2 --help=optimizers</code> and <code>-Q -O3 --help=optimizers</code>. 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</p> <pre><code>-O3 != -O2 x </code></pre> <p><b>Now my question</b>: 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 ?</p>
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    1. USCiro Santilli 新疆改造中心 六四事件 法轮功
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    1. COI think, you should add to the problem -O3 run such options "-ftree-dump-all-all -frtl-dump-all-all" and to find last stage which have a hang in it. Also, chech the stack of gcc with starting it under `gdb --args` and stopping with Ctrl-C.
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