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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization: clang does not infer from assumptions"
href="https://bugs.llvm.org/show_bug.cgi?id=44577">44577</a>
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<th>Summary</th>
<td>Missed optimization: clang does not infer from assumptions
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>9.0
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<th>Hardware</th>
<td>PC
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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>mafagafogigante@gmail.com
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>As of Clang 9.0.0, no inference is done based on __builtin_assume. I have
tested using "-std=c++17 -O2", but -O3 seems to do the same.
Example 1
bool lessThan(int a, int b) {
__builtin_assume(a < b);
return a < b;
}
compiles into
lessThan(int, int):
mov al, 1
ret
as one would expect. If the assumption is negated (!(a < b)), the compiler will
optimize it to return 0. However, if the assumption is any of (a == b), (a >=
b), (a > b), or (a == b || a > b) a comparison is emitted.
Example 2
bool lessThan(int a, int b, int c) {
__builtin_assume(a < b);
__builtin_assume(b < c);
return a < c;
}
also requires a comparison, even though a < c can be inferred from the
assumptions.</pre>
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