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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang seems to generate C code wrongly"
href="https://bugs.llvm.org/show_bug.cgi?id=35085">35085</a>
</td>
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<tr>
<th>Summary</th>
<td>Clang seems to generate C code wrongly
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.8
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>FreeBSD
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>voidbrainvoid@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=19344" name="attach_19344" title="Artifacts to reproduce the bug and some infos from my environment">attachment 19344</a> <a href="attachment.cgi?id=19344&action=edit" title="Artifacts to reproduce the bug and some infos from my environment">[details]</a></span>
Artifacts to reproduce the bug and some infos from my environment
Hi there,
I am was facing some issues with Clang when trying to compile an ARC4
implementation that was working pretty well in GCC.
Inspecting my code a little more I discovered that maybe Clang is generating
wrong code for the following code construction:
#include <stdio.h>
struct wtf_ctx {
unsigned char S[256];
int i;
};
int main(int argc, char **argv) {
struct wtf_ctx wc;
for (wc.i = 0; wc.i < 256; wc.S[wc.i] = wc.i++) // <-- This exact one.
;
for (wc.i = 0; wc.i < 256; wc.i++) {
printf("S[%.2X] = %.2X\n", wc.i, wc.S[wc.i]);
}
return 0;
}
What I have noticed is that GCC code sets the array positions with the expected
values: [0] = 0, [1] = 1 .. [255] = 255.
Unlike, Clang code seems to use undefined values.
I abstracted all the ARC4 algorithm complications to isolate what could be a
"bug". The code above is able to reproduce those
differences between GCC and Clang.
The compilation command lines are simply: "gcc weird.c -oweird" and "clang
weird.c -oweird".
Originally I have ran this code in FreeBSD but I also have the same issue in a
Linux 32-bit SMP box (distro Slackware 14.00)
with the same Clang version and the "bug" is recurrent there too.
I am also attaching some information that maybe could be relevant and useful to
the development team.
Thanks in advance!</pre>
</div>
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