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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed Tail Call Optimization when specifying function return value with __builtin_unreachable or __builtin_assume"
href="https://bugs.llvm.org/show_bug.cgi?id=43007">43007</a>
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<th>Summary</th>
<td>Missed Tail Call Optimization when specifying function return value with __builtin_unreachable or __builtin_assume
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mike.k@digitalcarbide.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>When explicitly specifying that a function can only return a specific value
using either __builtin_unreachable or __builtin_assume, the optimizer fails to
perform tail call optimization.
This is observed for all architectures.
Example:
extern int function_returns_only_1_or_doesnt_return(int, int);
int foo1(int a, int b) {
const int result = function_returns_only_1_or_doesnt_return(a, b);
if (result == 1) {
return result;
}
else {
__builtin_unreachable();
}
}
int foo2(int a, int b) {
const int result = function_returns_only_1_or_doesnt_return(a, b);
__builtin_assume(result == 1);
return result;
}
int foo3(int a, int b) {
return function_returns_only_1_or_doesnt_return(a, b);
}
For the flags '-O3' for an x86-64 target, this emits the following assembly:
foo1(int, int): # @foo1(int, int)
push rax
call function_returns_only_1_or_doesnt_return(int, int)
mov eax, 1
pop rcx
ret
foo2(int, int): # @foo2(int, int)
push rax
call function_returns_only_1_or_doesnt_return(int, int)
mov eax, 1
pop rcx
ret
foo3(int, int): # @foo3(int, int)
jmp function_returns_only_1_or_doesnt_return(int, int) # TAILCALL
It would be expected that all three would perform tail call optimization.</pre>
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