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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Honor __builtin_expect through inlining"
href="http://llvm.org/bugs/show_bug.cgi?id=19230">19230</a>
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<th>Summary</th>
<td>Honor __builtin_expect through inlining
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Transformation Utilities
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>wielkiegie@gmail.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12269" name="attach_12269" title="Test case">attachment 12269</a> <a href="attachment.cgi?id=12269&action=edit" title="Test case">[details]</a></span>
Test case
A common usage model of __builtin_expect is to create helper macros called
'likely' and 'unlikely':
#if __has_builtin(__builtin_expect)
# define likely(x) __builtin_expect(!!(x), 1)
# define unlikely(x) __builtin_expect(!!(x), 0)
#else
# define likely(x) (x)
# define unlikely(x) (x)
#endif
However using macros (especially in C++) has a major drawback: short names
pollute namespace and long ones are harder to use.
I wanted to fix this by using inline functions (which can reside in a
namespace) instead of macros:
#if __has_builtin(__builtin_expect)
inline bool likely (const bool x) { return __builtin_expect(x, 1); }
inline bool unlikely(const bool x) { return __builtin_expect(x, 0); }
#else
inline bool likely (const bool x) { return x; }
inline bool unlikely(const bool x) { return x; }
#endif
These inline functions above work as expected in GCC but unfortunately not in
LLVM/Clang (tested both with -O3). Apparently, the branch weight metadata from
llvm.expect intrinsic are dropped before the inlining occurs. Doing the
inlining explicitly before any other optimizations fixes this problem.
$ clang++ llvm-expect.cpp -o - -emit-llvm -c -O3 -DUSE_MACROS | llvm-dis | grep
branch_weights
!1 = metadata !{metadata !"branch_weights", i32 64, i32 4}
$ clang++ llvm-expect.cpp -o - -emit-llvm -c -O3 | llvm-dis | grep
branch_weights
$ clang++ llvm-expect.cpp -o - -emit-llvm -c | opt -inline | opt -O3 | llvm-dis
| grep branch_weights
!1 = metadata !{metadata !"branch_weights", i32 64, i32 4}</pre>
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