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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - `i = i % constant` for static local `i` is not optimized"
href="https://bugs.llvm.org/show_bug.cgi?id=39509">39509</a>
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<th>Summary</th>
<td>`i = i % constant` for static local `i` is not optimized
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<th>Product</th>
<td>clang
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<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
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>antoshkka@gmail.com
</td>
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Consider the following example:
unsigned next_trivial() {
static int i = 0;
auto ret = i;
++i;
i = i % 10;
return ret;
}
For that example a very suboptimal assembly with multiplication, many registers
usage and multiple instructions is generated.
However, the above example could be rewritten in the following way:
unsigned next_trivial_optim() {
static int i = 0;
auto ret = i;
++i;
if (i == 10) { i = 0; }
return ret;
}
For the above code snippet a very short and clear assembly is produced, without
any multiplications and unnecessary instructions:
mov eax, dword ptr [rip + next_trivial_optim()::i]
lea ecx, [rax + 1]
xor edx, edx
cmp ecx, 10
cmovne edx, ecx
mov dword ptr [rip + next_trivial_optim()::i], edx
ret
Please, add an optimization to do that transformation.</pre>
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