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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [MIPS] Missing ANDI optimization"
href="https://bugs.llvm.org/show_bug.cgi?id=43481">43481</a>
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<th>Summary</th>
<td>[MIPS] Missing ANDI optimization
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: MIPS
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>arichardson.kde@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>We noticed that clang generates unnecessary SLL instructions in one of our
benchmark hot paths (and apparently GCC does not): <a href="https://godbolt.org/z/xKcCqT">https://godbolt.org/z/xKcCqT</a>
Consider
```
#include <stdint.h>
uint64_t foo(uint64_t a)
{
uint64_t b = a / 16;
uint64_t c = b & 0x7UL;
return ((uint64_t)1UL << c);
}
```
At present, the MIPS backend produces:
```
foo(unsigned long): # @foo(unsigned long)
sll $1, $4, 0
srl $1, $1, 4
andi $1, $1, 7
daddiu $2, $zero, 1
jr $ra
dsllv $2, $2, $1
```
The `sll` is introduced because the `srl` requires that its input be zero
extended (which, well, seems silly, but so it goes). In any case, because
`andi` has a 16-bit immediate, some arithmetic and lookahead could find that
`0x7 << 4` fits and so make this be
```
andi $1, $4, 0x70
srl $1, $1, 4
daddiu $2, $zero, 1
jr $ra
dsllv $2, $2, $1
```
Alternatively, whatever's concluding that it can use 32-bit values internally
could stop doing that and this could instead just be
```
dsrl $1, $4, 4
andi $1, $1, 7
daddiu $2, $zero, 1
jr $ra
dsllv $2, $2, $1
```
This problem was found by Nathaniel Wesley Filardo and reported as
<a href="https://github.com/CTSRD-CHERI/llvm-project/issues/343">https://github.com/CTSRD-CHERI/llvm-project/issues/343</a></pre>
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