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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Simple no-op loops with a termination flag are not optimized out"
href="https://bugs.llvm.org/show_bug.cgi?id=34171">34171</a>
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<th>Summary</th>
<td>Simple no-op loops with a termination flag are not optimized out
</td>
</tr>
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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>PC
</td>
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<th>OS</th>
<td>Linux
</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>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>geoffrey@allott.email
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>The following C code results in a loop with many iterations even with -O3; I
would expect it to be optimized out:
------------------------
void do_nothing() {
int i=200, q=0;
while(!q) if (!--i) i=q=1;
}
------------------------
; Function Attrs: norecurse nounwind readnone sspstrong uwtable
define void @do_nothing() local_unnamed_addr #0 {
br label %1
; <label>:1: ; preds = %1, %0
%2 = phi i32 [ 200, %0 ], [ %3, %1 ]
%3 = add nsw i32 %2, -25
%4 = icmp eq i32 %3, 0
br i1 %4, label %5, label %1
; <label>:5: ; preds = %1
ret void
}
------------------------
movl $-200, %eax
.LBB0_1:
addl $25, %eax
jne .LBB0_1
retq
------------------------
(Any number low enough seems to optimize out - I assume because of some
hardcoded limits that simply evaluate some number of loops.)
It seems as though the problem is the fact that `i` is set to 1 on exit of the
loop - if this is removed then the loop is optimized away no matter the inital
value of i.
This situation can come up very often whenever a flag is used as the condition
to exit a loop. For instance, no-op loops in rust are not optimized out by llvm
because they terminate in such a way.</pre>
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