If you change -0.0 into +0.0, what about <div>fsub -0.0, %x</div><div><br></div><div>I think we might get unoptimized code then.</div><div class="gmail_extra"><br><br><div class="gmail_quote">On Thu, Dec 6, 2012 at 9:53 AM, Duncan Sands <span dir="ltr"><<a href="mailto:baldrick@free.fr" target="_blank">baldrick@free.fr</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Michael,<div class="im"><br>
<br>
On 06/12/12 01:07, Michael Ilseman wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Author: milseman<br>
Date: Wed Dec  5 18:07:09 2012<br>
New Revision: 169452<br>
<br>
URL: <a href="http://llvm.org/viewvc/llvm-project?rev=169452&view=rev" target="_blank">http://llvm.org/viewvc/llvm-<u></u>project?rev=169452&view=rev</a><br>
Log:<br>
Have CannotBeNegativeZero() be aware of the nsz fast-math flag<br>
</blockquote>
<br></div>
does nsz mean that if there is a signed zero then the behaviour is undefined?<br>
Or does it mean negative zero can be treated like positive zero, and there's no<br>
need to get the sign of zero right?<br>
<br>
It seems to me that your patch pretty much assumes the first case.<br>
If it's instead the second then I think you would do better to have<br>
instcombine turn all instances of -0.0 into +0.0.<br>
<br>
Ciao, Duncan.<div class="HOEnZb"><div class="h5"><br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
Modified:<br>
     llvm/trunk/lib/Analysis/<u></u>ValueTracking.cpp<br>
<br>
Modified: llvm/trunk/lib/Analysis/<u></u>ValueTracking.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=169452&r1=169451&r2=169452&view=diff" target="_blank">http://llvm.org/viewvc/llvm-<u></u>project/llvm/trunk/lib/<u></u>Analysis/ValueTracking.cpp?<u></u>rev=169452&r1=169451&r2=<u></u>169452&view=diff</a><br>

==============================<u></u>==============================<u></u>==================<br>
--- llvm/trunk/lib/Analysis/<u></u>ValueTracking.cpp (original)<br>
+++ llvm/trunk/lib/Analysis/<u></u>ValueTracking.cpp Wed Dec  5 18:07:09 2012<br>
@@ -1312,7 +1312,12 @@<br>
<br>
    const Operator *I = dyn_cast<Operator>(V);<br>
    if (I == 0) return false;<br>
-<br>
+<br>
+  // Check if the nsz fast-math flag is set<br>
+  if (const FPMathOperator *FPO = dyn_cast<FPMathOperator>(I))<br>
+    if (FPO->hasNoSignedZeros())<br>
+      return true;<br>
+<br>
    // (add x, 0.0) is guaranteed to return +0.0, not -0.0.<br>
    if (I->getOpcode() == Instruction::FAdd &&<br>
        isa<ConstantFP>(I->getOperand(<u></u>1)) &&<br>
<br>
<br>
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</blockquote>
<br>
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