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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - clang's -O implies -O2 whereas gcc's -O implies -O1"
href="https://llvm.org/bugs/show_bug.cgi?id=23581">23581</a>
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<th>Summary</th>
<td>clang's -O implies -O2 whereas gcc's -O implies -O1
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.6
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>FreeBSD
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Frontend
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>yanegomi@gmail.com
</td>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<div>
<pre>I've been tracking down issues with debuggability on FreeBSD. It turns out that
the -O with clang means -O2, not -O1 like it does with gcc. From man gcc for
4.8.3:
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of
compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fcompare-elim -fcprop-registers -fdce -fdefer-pop
-fdelayed-branch -fdse -fguess-branch-probability -fif-conversion2
-fif-conversion -fipa-pure-const -fipa-profile -fipa-reference
-fmerge-constants -fsplit-wide-types -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copyrename
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-slsr -ftree-sra -ftree-pta
-ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so
does not interfere with debugging.
>From man clang for 3.6.0:
-O Equivalent to -O2.
-g Generate debug information. Note that Clang debug information
works best at -O0.
It seems counterintuitive for projects that have used gcc for over a decade to
change behavior in this way. Why was this done/shouldn't clang pattern itself
after gcc to some degree?</pre>
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