Convert fcmp with 0.0 from casted integers to icmp

This is already handled in general when it is known the
conversion can't lose bits with smaller integer types
casted into wider floating point types.

This pattern happens somewhat often in GPU programs that cast
workitem intrinsics to float, which are often compared with 0.

Specifically handle the special case of compares with zero which
should also be known to not lose information. I had a more general
version of this which allows equality compares if the casted float is
exactly representable in the integer, but I'm not 100% confident that
is always correct.

Also fold cases that aren't integers to true / false.

llvm-svn: 225265

1 files changed, 34 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 01beb53104b..37524b93a3c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "InstCombine.h"
+#include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/InstructionSimplify.h"
@@ -3624,18 +3625,49 @@ Instruction *InstCombiner::FoldFCmp_IntToFP_Cst(FCmpInst &I,
   int MantissaWidth = LHSI->getType()->getFPMantissaWidth();
   if (MantissaWidth == -1) return nullptr;  // Unknown.
 
+  IntegerType *IntTy = cast<IntegerType>(LHSI->getOperand(0)->getType());
+
   // Check to see that the input is converted from an integer type that is small
   // enough that preserves all bits.  TODO: check here for "known" sign bits.
   // This would allow us to handle (fptosi (x >>s 62) to float) if x is i64 f.e.
-  unsigned InputSize = LHSI->getOperand(0)->getType()->getScalarSizeInBits();
+  unsigned InputSize = IntTy->getScalarSizeInBits();
 
   // If this is a uitofp instruction, we need an extra bit to hold the sign.
   bool LHSUnsigned = isa<UIToFPInst>(LHSI);
   if (LHSUnsigned)
     ++InputSize;
 
+  if (I.isEquality()) {
+    FCmpInst::Predicate P = I.getPredicate();
+    bool IsExact = false;
+    APSInt RHSCvt(IntTy->getBitWidth(), LHSUnsigned);
+    RHS.convertToInteger(RHSCvt, APFloat::rmNearestTiesToEven, &IsExact);
+
+    // If the floating point constant isn't an integer value, we know if we will
+    // ever compare equal / not equal to it.
+    if (!IsExact) {
+      // TODO: Can never be -0.0 and other non-representable values
+      APFloat RHSRoundInt(RHS);
+      RHSRoundInt.roundToIntegral(APFloat::rmNearestTiesToEven);
+      if (RHS.compare(RHSRoundInt) != APFloat::cmpEqual) {
+        if (P == FCmpInst::FCMP_OEQ || P == FCmpInst::FCMP_UEQ)
+          return ReplaceInstUsesWith(I, Builder->getFalse());
+
+        assert(P == FCmpInst::FCMP_ONE || P == FCmpInst::FCMP_UNE);
+        return ReplaceInstUsesWith(I, Builder->getTrue());
+      }
+    }
+
+    // TODO: If the constant is exactly representable, is it always OK to do
+    // equality compares as integer?
+  }
+
+  // Comparisons with zero are a special case where we know we won't lose
+  // information.
+  bool IsCmpZero = RHS.isPosZero();
+
   // If the conversion would lose info, don't hack on this.
-  if ((int)InputSize > MantissaWidth)
+  if ((int)InputSize > MantissaWidth && !IsCmpZero)
     return nullptr;
 
   // Otherwise, we can potentially simplify the comparison.  We know that it
@@ -3676,8 +3708,6 @@ Instruction *InstCombiner::FoldFCmp_IntToFP_Cst(FCmpInst &I,
     return ReplaceInstUsesWith(I, Builder->getFalse());
   }
 
-  IntegerType *IntTy = cast<IntegerType>(LHSI->getOperand(0)->getType());
-
   // Now we know that the APFloat is a normal number, zero or inf.
 
   // See if the FP constant is too large for the integer.  For example,