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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - O3 icmp combiner bug: (sgt icmt x+1,0) == (seq icmp x, 0) fail in case x==0x7f ff ff ff"
href="http://llvm.org/bugs/show_bug.cgi?id=20155">20155</a>
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<tr>
<th>Summary</th>
<td>O3 icmp combiner bug: (sgt icmt x+1,0) == (seq icmp x, 0) fail in case x==0x7f ff ff ff
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>troshkovdanil@mail.ru
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Hi!
C Test:
//------------ cmpXinc0.c:
#include <stdio.h>
#include <stdlib.h>
int cmpXinc0(int x) {
printf("x: 0x%x, %d\n", x, x);
++x;
printf("++x: 0x%x, %d\n", x, x);
return x > 0;
}
int main(int argc, char **argv) {
printf("~0U>>1: 0x%x, %d\n", ~0U>>1, ~0U>>1);
printf("arg: %s, 0x%x\n", argv[1], atoi(argv[1]));
if (cmpXinc0(atoi(argv[1])))
printf(">0: OK\n");
else
printf("<=0...\n");
return 0;
}
//-------------
Clang:
/home/dtroshkov/clang/build/Debug+Asserts/bin/clang cmpXinc0.c -o cmp_clang
Run:
./cmp_clang 2147483647
~0U>>1: 0x7fffffff, 2147483647
arg: 2147483647, 0x7fffffff
x: 0x7fffffff, 2147483647
++x: 0x80000000, -2147483648
<=0...
It is ok. Use O3:
Clang:
/home/dtroshkov/clang/build/Debug+Asserts/bin/clang -O3 cmpXinc0.c -o cmp_clang
Run
./cmp_clang 2147483647
~0U>>1: 0x7fffffff, 2147483647
arg: 2147483647, 0x7fffffff
x: 0x7fffffff, 2147483647
++x: 0x80000000, -2147483648
<span class="quote">>0: OK</span >
It is not ok...
The reason of this behavior is following code:
lib/Transforms/InstCombine/InstCombineCompares.cpp:
// icmp sgt (X + 1), Y -> icmp sge X, Y
if (A && NoOp0WrapProblem && Pred == CmpInst::ICMP_SGT &&
match(B, m_One()))
return new ICmpInst(CmpInst::ICMP_SGE, A, Op1);
Does anybody well know this area?
Please fix it because there is many similar cases in this area, and there is
similar code in lib/Analysis/InstructionSimplify.cpp:
// Special logic for binary operators.
BinaryOperator *LBO = dyn_cast<BinaryOperator>(LHS);
BinaryOperator *RBO = dyn_cast<BinaryOperator>(RHS);
if (MaxRecurse && (LBO || RBO)) {
// Analyze the case when either LHS or RHS is an add instruction.
Value *A = 0, *B = 0, *C = 0, *D = 0;
// LHS = A + B (or A and B are null); RHS = C + D (or C and D are null).
I think there are some more incorrect code in this area:
// Special logic for binary operators.
BinaryOperator *BO0 = dyn_cast<BinaryOperator>(Op0);
BinaryOperator *BO1 = dyn_cast<BinaryOperator>(Op1);
if (BO0 || BO1) {
CmpInst::Predicate Pred = I.getPredicate();
bool NoOp0WrapProblem = false, NoOp1WrapProblem = false;
if (BO0 && isa<OverflowingBinaryOperator>(BO0))
NoOp0WrapProblem = ICmpInst::isEquality(Pred) ||
(CmpInst::isUnsigned(Pred) && BO0->hasNoUnsignedWrap()) ||
(CmpInst::isSigned(Pred) && BO0->hasNoSignedWrap());
if (BO1 && isa<OverflowingBinaryOperator>(BO1))
NoOp1WrapProblem = ICmpInst::isEquality(Pred) ||
(CmpInst::isUnsigned(Pred) && BO1->hasNoUnsignedWrap()) ||
(CmpInst::isSigned(Pred) && BO1->hasNoSignedWrap());</pre>
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