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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization opportunity when returning function pointers based on run-time boolean"
href="https://bugs.llvm.org/show_bug.cgi?id=41980">41980</a>
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<th>Summary</th>
<td>Missed optimization opportunity when returning function pointers based on run-time boolean
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>vittorio.romeo@outlook.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
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<p>
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<pre>Given the following two functions:
int f() { return 0; }
int g() { return 1; }
And the following code to invoke one of them depending on a boolean `b`:
int t0(bool b) { return (b ? &f : &g)(); }
int t1(bool b) { return b ? f() : g(); }
int t2(bool b) { return b ? t0(true) : t0(false); }
Both `g++ (trunk)` and `clang++ (trunk)` with `-std=c++2a -Ofast -march=native`
fail to optimize the following code:
int main(int ac, char**) { return t0(ac & 1); }
Producing the following assembly:
<span class="quote">> main:
> and edi, 1
> mov eax, OFFSET FLAT:f()
> mov edx, OFFSET FLAT:g()
> cmove rax, rdx
> jmp rax
> </span >
Invoking `t1` or `t2` (instead of `t0`) produces the following optimized
assembly:
<span class="quote">> main:
> mov eax, edi
> not eax
> and eax, 1
> ret</span >
Everything can be reproduced live on **gcc.godbolt.org**:
<a href="https://godbolt.org/z/gh7270">https://godbolt.org/z/gh7270</a></pre>
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