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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 - IsPowerOfTwo not treating INT_MIN as a negative number"
href="https://bugs.llvm.org/show_bug.cgi?id=37569">37569</a>
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<th>Summary</th>
<td>IsPowerOfTwo not treating INT_MIN as a negative number
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>pankajgode@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
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<pre>Overview:
Transformation of sdiv to 'shift, add, shift', gives incorrect result for
division by 0x80000000 i.e. INT_MIN (-2147483648).
Specifically for the test code below:
#include <stdio.h>
#include <stdlib.h>
volatile int myInt;
int number[6];
int gen_int (int i) {
return i + myInt;
}
void init() {
number[gen_int(4)] = gen_int (0x80000000);
}
int main() {
register int *p;
p = number;
init();
*(p + 5) = *(p + 4) / (int) 0x80000000;
if ( *(p+5)==1)
printf(" \n Answer=%d Expected.\n", *(p+5));
else
printf(" \n Answer=%d . Not Expected! \n", *(p+5));
return 0;
}
Steps to Reproduce:
Compilation command: clang -O0 t_unit1.c -mllvm -fast-isel=false
(Disabling -fast-isel to enable converting it to shift and add)
LLVM was compiled for X86 target.
Test was executed on X86_64 Linux machine.
Actual Result: Answer=-1 . Not Expected!
i.e. Division operation shows -1 as answer, which is incorrect.
Expected Result: Answer=1 Expected.
i.e. As it is division by same number so answer should be 1.
Observations:
The division below is essentially 0x80000000/0x80000000
*(p + 24) = *(p + 4) / (int) 0x80000000;
which gets converted to:
sarl $31, %edx
shrl %edx
addl %edx, %ecx
sarl $31, %ecx
The last 'sarl' was introduced with the below commit.
<a href="https://github.com/llvm-mirror/llvm/commit/7ac44f2885a25fad268acae3d3503eb0114d4c83#diff-d818c34776d200e6983e01b0819bb974">https://github.com/llvm-mirror/llvm/commit/7ac44f2885a25fad268acae3d3503eb0114d4c83#diff-d818c34776d200e6983e01b0819bb974</a>
This appears to be relevant code in lib/CodeGen/SelectionDAG/DAGCombiner.cpp:
SDValue Srl = DAG.getNode(ISD::SRL, DL, VT, Sign, Inexact);
SDValue Add = DAG.getNode(ISD::ADD, DL, VT, N0, Srl);
AddToWorklist(Srl.getNode());
AddToWorklist(Add.getNode()); // Divide by pow2
SDValue Sra = DAG.getNode(ISD::SRA, DL, VT, Add, C1);
// If dividing by a positive value, we're done. Otherwise, the result must
// be negated.
if (KnownNegatives.none())
return Sra;</pre>
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