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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - !range metadata pessimizes generated assembly"
href="https://llvm.org/bugs/show_bug.cgi?id=30505">30505</a>
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<th>Summary</th>
<td>!range metadata pessimizes generated assembly
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>3.9
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Transformation Utilities
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>simonas+llvm.org@kazlauskas.me
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17340" name="attach_17340" title="Case where pessimisation happens">attachment 17340</a> <a href="attachment.cgi?id=17340&action=edit" title="Case where pessimisation happens">[details]</a></span>
Case where pessimisation happens
In the attached test case the instruction
%1 = load i8, i8* %0, !range !1
has range metadata attached to it. In LLVM 3.9 Combine Redundant instructions
pass replaces this load with
%1 = load i8, i8* %0, align 1, !range !1
%trunc = trunc i8 %1 to i2
The truncation ends up never getting removed and results in extra and
unnecessary `andb $3, %dil` instruction in the optimised assembly. This extra
instruction is causing some noticeable overhead in a tight loop.
---
LLVM 3.8 does not apply the transformation detailed above. Therefore it does
not emit the superfluous `andb` instruction either.
As far as fixes are concerned, perhaps it would make sense to only `trunc` to
the bitwidths supported by the target machine?</pre>
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