[PATCH] D28927: [ValueTracking] Add comment that CannotBeOrderedLessThanZero does the wrong thing for powi.

[Justin Lebar via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rL293243: [ValueTracking] Add comment that CannotBeOrderedLessThanZero does the wrong… (authored by jlebar).

Changed prior to commit:
  https://reviews.llvm.org/D28927?vs=85098&id=85991#toc

Repository:
  rL LLVM

https://reviews.llvm.org/D28927

Files:
  llvm/trunk/lib/Analysis/ValueTracking.cpp


Index: llvm/trunk/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp
@@ -2602,6 +2602,11 @@
                                             const TargetLibraryInfo *TLI,
                                             bool SignBitOnly,
                                             unsigned Depth) {
+  // TODO: This function does not do the right thing when SignBitOnly is true
+  // and we're lowering to a hypothetical IEEE 754-compliant-but-evil platform
+  // which flips the sign bits of NaNs.  See
+  // https://llvm.org/bugs/show_bug.cgi?id=31702.
+
   if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V)) {
     return !CFP->getValueAPF().isNegative() ||
            (!SignBitOnly && CFP->getValueAPF().isZero());
@@ -2678,8 +2683,22 @@
         if (Exponent->getBitWidth() <= 64 && Exponent->getSExtValue() % 2u == 0)
           return true;
       }
+      // TODO: This is not correct.  Given that exp is an integer, here are the
+      // ways that pow can return a negative value:
+      //
+      //   pow(x, exp)    --> negative if exp is odd and x is negative.
+      //   pow(-0, exp)   --> -inf if exp is negative odd.
+      //   pow(-0, exp)   --> -0 if exp is positive odd.
+      //   pow(-inf, exp) --> -0 if exp is negative odd.
+      //   pow(-inf, exp) --> -inf if exp is positive odd.
+      //
+      // Therefore, if !SignBitOnly, we can return true if x >= +0 or x is NaN,
+      // but we must return false if x == -0.  Unfortunately we do not currently
+      // have a way of expressing this constraint.  See details in
+      // https://llvm.org/bugs/show_bug.cgi?id=31702.
       return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
                                              Depth + 1);
+
     case Intrinsic::fma:
     case Intrinsic::fmuladd:
       // x*x+y is non-negative if y is non-negative.


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D28927.85991.patch
Type: text/x-patch
Size: 1995 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170127/9023acbc/attachment.bin>