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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Suboptimal machine code for atomic bool load + test"
href="https://bugs.llvm.org/show_bug.cgi?id=38405">38405</a>
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<th>Summary</th>
<td>Suboptimal machine code for atomic bool load + test
</td>
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<th>Product</th>
<td>new-bugs
</td>
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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>All
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<th>Status</th>
<td>NEW
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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>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>gonzalobg88@gmail.com
</td>
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<th>CC</th>
<td>chandlerc@gmail.com, efriedma@codeaurora.org, hfinkel@anl.gov, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>The following C++ code compiled with clang -O3 -std=c++17
(<a href="https://godbolt.org/g/CAMK9k">https://godbolt.org/g/CAMK9k</a>):
#include <atomic>
std::atomic<bool> flag_atomic{false};
bool flag_nonatomic{false};
extern void f1();
extern void f2();
void branchAtomic() {
if (flag_atomic.load(std::memory_order_relaxed)) {
f1();
} else {
f2();
}
}
void branchNonatomic() {
if (flag_nonatomic) {
f1();
} else {
f2();
}
}
produces different code for the atomic and non-atomic functions, but in this
particular case, it should probably be the same and emit a cmpb instruction on
both (right? I am not 100% sure):
branchAtomic(): # @branchAtomic()
movb flag_atomic(%rip), %al
testb $1, %al
jne .LBB0_1
jmp _Z2f2v # TAILCALL
.LBB0_1:
jmp _Z2f1v # TAILCALL
branchNonatomic(): # @branchNonatomic()
cmpb $0, flag_nonatomic(%rip)
je .LBB1_2
jmp _Z2f1v # TAILCALL
.LBB1_2:
jmp _Z2f2v # TAILCALL
flag_atomic:
.zero 1
flag_nonatomic:
.byte 0 # 0x0</pre>
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