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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 - Failure to optimize add loop in -O3 as well as in -Oz"
href="https://bugs.llvm.org/show_bug.cgi?id=49024">49024</a>
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</tr>
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<th>Summary</th>
<td>Failure to optimize add loop in -O3 as well as in -Oz
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int f(uint8_t max_value) {
int sum = 0;
for (int i = 0; i < max_value; i++) {
sum += i;
}
return sum;
}
This can be optimized to `return (((unsigned _ExtInt(33))max_value) * (unsigned
_ExtInt(33))((uint32_t)max_value - 1)) >> 1;`. This transformation is done
under `-Oz`, but `-O3` seems to generate far inferior code, as the IR shows:
define dso_local i32 @_Z1fh(i8 zeroext %0) local_unnamed_addr #0 {
%2 = zext i8 %0 to i32
%3 = icmp eq i8 %0, 0
br i1 %3, label %14, label %4
4: ; preds = %1
%5 = add nsw i32 %2, -1
%6 = zext i32 %5 to i33
%7 = add nsw i32 %2, -2
%8 = zext i32 %7 to i33
%9 = mul i33 %6, %8
%10 = lshr i33 %9, 1
%11 = trunc i33 %10 to i32
%12 = add i32 %2, %11
%13 = add i32 %12, -1
br label %14
14: ; preds = %4, %1
%15 = phi i32 [ 0, %1 ], [ %13, %4 ]
ret i32 %15
}
Compared to the far simpler and faster code generated on -Oz:
define dso_local i32 @_Z1fh(i8 zeroext %0) local_unnamed_addr #0 {
%2 = zext i8 %0 to i32
%3 = zext i8 %0 to i33
%4 = add nsw i32 %2, -1
%5 = zext i32 %4 to i33
%6 = mul i33 %3, %5
%7 = lshr i33 %6, 1
%8 = trunc i33 %7 to i32
ret i32 %8
}
Godbolt comparison: <a href="https://godbolt.org/z/vW5jKz">https://godbolt.org/z/vW5jKz</a>
alive2 comparison: <a href="https://alive2.llvm.org/ce/z/NVhmdm">https://alive2.llvm.org/ce/z/NVhmdm</a></pre>
</div>
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