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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - MIPS back end performs inefficient subtraction of immediates"
href="https://llvm.org/bugs/show_bug.cgi?id=26291">26291</a>
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<th>Summary</th>
<td>MIPS back end performs inefficient subtraction of immediates
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: MIPS
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>csdavec@swan.ac.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Simple C test case:
int foo(char*,char*, char*);
int main(void)
{
char z[1048576];
foo(z,z,z);
}
The key part of the IR:
%z = alloca [1048576 x i8], align 1
This produces the following code for the stack pointer modification:
daddiu $5, $zero, 1
dsll $5, $5, 44
daddiu $5, $5, -1
dsll $5, $5, 20
daddiu $5, $5, -48
daddu $sp, $sp, $5
This would be much simpler as
lui $5, 16
dsubu $sp, $sp, $5
Even without rewriting the add as a subtraction, the constant could be
materialised as:
lui $5, 16
dsubu $5, $zero, $5</pre>
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