ValueTracking: Document some difficult isKnownNeverInfinity cases

Add a comment and some negative tests. I'd like to have test coverage
and explicit handling of all the math operations for clarity.

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index c97092b..dbf2ef5 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3833,6 +3833,24 @@ bool llvm::isKnownNeverInfinity(const Value *V, const TargetLibraryInfo *TLI,
       case Intrinsic::maximum:
         return isKnownNeverInfinity(Inst->getOperand(0), TLI, Depth + 1) &&
                isKnownNeverInfinity(Inst->getOperand(1), TLI, Depth + 1);
+      case Intrinsic::log:
+      case Intrinsic::log10:
+      case Intrinsic::log2:
+        // log(+inf) -> +inf
+        // log([+-]0.0) -> -inf
+        // log(-inf) -> nan
+        // log(-x) -> nan
+        // TODO: We lack API to check the == 0 case.
+        return false;
+      case Intrinsic::exp:
+      case Intrinsic::exp2:
+      case Intrinsic::pow:
+      case Intrinsic::powi:
+      case Intrinsic::fma:
+      case Intrinsic::fmuladd:
+        // These can return infinities on overflow cases, so it's hard to prove
+        // anything about it.
+        return false;
       default:
         break;
       }