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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine"
href="https://llvm.org/bugs/show_bug.cgi?id=27153">27153</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>InstCombine
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>davemm@cs.rutgers.edu
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, santosh.nagarakatte@gmail.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>LLVM optimizes this code:
define float @foo(i32 %x) #0 {
entry:
%y = and i32 %x, 268435455
%a = sitofp i32 %y to float
%b = fadd float %a, -8388608.0
ret float %b
}
to:
define float @foo(i32 %x) #0 {
entry:
%y = and i32 %x, 268435455
%addconv = add nsw i32 %y, -8388608
%b = sitofp i32 %addconv to float
ret float %b
}
This program calls foo and prints the result:
#include "stdio.h"
float foo(int x);
int main(int args, char** argv) {
float y = foo(33554434);
printf("%f\n",y);
return 0;
}
Without optimizations, this yields 25165824.000000
With optimizations, this yields 25165826.000000
This arises because 33554434 cannot be precisely represented as a float, so the
sitofp is constrained to return 33554432 or 33554436. This yields 25165824 or
25165828 when added to -8388608.
In the optimized code, the addition is performed in i32, yielding the exact
answer 25165826, which can be represented exactly as a float. Note that this is
different from both of the possible return values given above.
Here is the relevant code from InstCombineAddSub.cpp:
// Check for (fadd double (sitofp x), y), see if we can merge this into an
// integer add followed by a promotion.
if (SIToFPInst *LHSConv = dyn_cast<SIToFPInst>(LHS)) {
// (fadd double (sitofp x), fpcst) --> (sitofp (add int x, intcst))
// ... if the constant fits in the integer value. This is useful for
things
// like (double)(x & 1234) + 4.0 -> (double)((X & 1234)+4) which no longer
// requires a constant pool load, and generally allows the add to be better
// instcombined.
if (ConstantFP *CFP = dyn_cast<ConstantFP>(RHS)) {
Constant *CI =
ConstantExpr::getFPToSI(CFP, LHSConv->getOperand(0)->getType());
if (LHSConv->hasOneUse() &&
ConstantExpr::getSIToFP(CI, I.getType()) == CFP &&
WillNotOverflowSignedAdd(LHSConv->getOperand(0), CI, I)) {
// Insert the new integer add.
Value *NewAdd = Builder->CreateNSWAdd(LHSConv->getOperand(0),
CI, "addconv");
return new SIToFPInst(NewAdd, I.getType());
}
}</pre>
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