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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 - Mips2 and Mips3 produce inefficient lowerings for partial shifts"
href="https://bugs.llvm.org/show_bug.cgi?id=32293">32293</a>
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<th>Summary</th>
<td>Mips2 and Mips3 produce inefficient lowerings for partial shifts
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>enhancement
</td>
</tr>
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<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>kyle+llvm@iteratee.net
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Mips2 and Mips3 produce inefficient lowerings for partial shifts:
on Mips2 shl lowers to code that looks like:
if (!x)
... (Don't modify x)
if (!x)
...
specifically:
# BB#0: # %entry
sllv $1, $5, $7
andi $6, $7, 32
bnez $6, $BB4_2
move $2, $1
# BB#1: # %entry
sllv $2, $4, $7
not $3, $7
srl $4, $5, 1
srlv $3, $4, $3
or $2, $2, $3
$BB4_2: # %entry
bnez $6, $BB4_4
addiu $3, $zero, 0
# BB#3: # %entry
move $3, $1
$BB4_4: # %entry
jr $ra
nop
With more careful lowering, the double compare can be eliminated:
# BB#0: # %entry
andi $6, $7, 32
beqz $6, $BB4_2
sllv $1, $5, $7
# BB#1:
move $2, $1
jr $ra
addiu $3, $zero, 0
$BB4_2: # %entry
sllv $2, $4, $7
not $3, $7
srl $4, $5, 1
srlv $3, $4, $3
jr $ra
or $2, $2, $3
The lowering produces 2 selects which each create a branch instruction on the
same condition.
If we're worried about this, we could try to produce better code for these
targets in selection dag.</pre>
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