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<body><span class="vcard"><a class="email" href="mailto:lebedev.ri@gmail.com" title="Roman Lebedev <lebedev.ri@gmail.com>"> <span class="fn">Roman Lebedev</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - Simple C program produces different output if compiled with -O1 instead of -O0"
href="https://bugs.llvm.org/show_bug.cgi?id=44424">bug 44424</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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<tr>
<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>INVALID
</td>
</tr>
<tr>
<td style="text-align:right;">CC</td>
<td>
</td>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - Simple C program produces different output if compiled with -O1 instead of -O0"
href="https://bugs.llvm.org/show_bug.cgi?id=44424#c1">Comment # 1</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - Simple C program produces different output if compiled with -O1 instead of -O0"
href="https://bugs.llvm.org/show_bug.cgi?id=44424">bug 44424</a>
from <span class="vcard"><a class="email" href="mailto:lebedev.ri@gmail.com" title="Roman Lebedev <lebedev.ri@gmail.com>"> <span class="fn">Roman Lebedev</span></a>
</span></b>
<pre>(In reply to Dirk Hoffmann from <a href="show_bug.cgi?id=44424#c0">comment #0</a>)
<span class="quote">> Test program: main.cpp
>
>
> #include <stdint.h>
> #include <stdio.h>
>
> bool foo(uint32_t x, uint32_t y)
> {
> uint32_t z = (uint16_t)x * (uint16_t)y;</span >
This line contains UB - the multiplication does not happen in uint16_t,
but the uint16_t values are promoted to int, so this is signed overflow.
<a href="https://godbolt.org/z/wPp7Xq">https://godbolt.org/z/wPp7Xq</a>
<span class="quote">> return z & 0x80000000;
> }
>
> int main(int argc, char *argv[])
> {
> printf("%d\n", foo(0x7FFFF, 0x7FFFF));
> }
>
>
>
> Compiler:
>
> Apple clang version 11.0.0 (clang-1100.0.33.8)
> Target: x86_64-apple-darwin18.7.0
> Thread model: posix
>
> Output:
>
> > clang -O0 main.cpp; ./a.out
> 1
>
> > clang -O1 main.cpp; ./a.out
> 0</span ></pre>
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