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<body><span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - clang/LLVM 3.5.0 skips checking the for loop's condition when entering the loop"
href="http://llvm.org/bugs/show_bug.cgi?id=21308">bug 21308</a>
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<table border="1" cellspacing="0" cellpadding="8">
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<th>What</th>
<th>Removed</th>
<th>Added</th>
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<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
</td>
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<td style="text-align:right;">CC</td>
<td>
</td>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>INVALID
</td>
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - clang/LLVM 3.5.0 skips checking the for loop's condition when entering the loop"
href="http://llvm.org/bugs/show_bug.cgi?id=21308#c8">Comment # 8</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - clang/LLVM 3.5.0 skips checking the for loop's condition when entering the loop"
href="http://llvm.org/bugs/show_bug.cgi?id=21308">bug 21308</a>
from <span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span></b>
<pre>(In reply to <a href="show_bug.cgi?id=21308#c7">comment #7</a>)
<span class="quote">> So, your statement is that the compiler-generated code will refuse to verify
> a pointer against NULL even if the developer _explicitly_ checks for this in
> C code?</span >
Yes. This is exactly like checking for overflow *after* you perform signed
arithmetic that overflows. You can't put the undefined behavior genie back in
the bottle by checking for it after it happens.
<span class="quote">> Is this a new behavior since 3.5.0?</span >
Yes, and it was documented prominently in our release notes:
<a href="http://llvm.org/releases/3.5.0/tools/clang/docs/ReleaseNotes.html#c-language-changes-in-clang">http://llvm.org/releases/3.5.0/tools/clang/docs/ReleaseNotes.html#c-language-changes-in-clang</a>
You can build with -fsanitize=null to catch these bugs at runtime.</pre>
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