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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __builtin_assume causes a call to a function marked as __attribute__((const)) or ((pure)) to be needlessly propagated"
href="https://bugs.llvm.org/show_bug.cgi?id=43010">43010</a>
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<th>Summary</th>
<td>__builtin_assume causes a call to a function marked as __attribute__((const)) or ((pure)) to be needlessly propagated
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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
</td>
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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
</td>
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>mike.k@digitalcarbide.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>In a situation when a call to an external __attribute__((const)) or
__attribute__((pure)) is encapsulated in a __builtin_assume, and then called,
the function is still called despite having no side-effects and the result
value being known.
extern int foo() __attribute__((const));
extern void oof();
void bar() {
__builtin_assume(foo() == 1);
if (foo() == 1) {
oof();
}
}
results in the following x86-64 assembly with -03:
bar(): # @bar()
push rax
call foo()
pop rax
jmp oof() # TAILCALL
The `call foo()`, as stated, is unnecessary.
If the code is refactored to use a temporary variable for the result, as such:
extern int foo() __attribute__((const));
extern void oof();
void bar() {
int foo_result = foo();
__builtin_assume(foo_result == 1);
if (foo_result == 1) {
oof();
}
}
It then produces the expected output:
bar(): # @bar()
jmp oof() # TAILCALL</pre>
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