Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ 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"
@@ -3606,18 +3607,49 @@
   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
@@ -3658,8 +3690,6 @@
     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,
Index: test/Transforms/InstCombine/cast-int-fcmp-eq-0.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/cast-int-fcmp-eq-0.ll
@@ -0,0 +1,454 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_0_uitofp(
+; CHECK-NEXT: icmp eq i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_n0_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+define i1 @i32_cast_cmp_oeq_int_n0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_0_sitofp(
+; CHECK-NEXT: icmp eq i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_n0_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+define i1 @i32_cast_cmp_oeq_int_n0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_one_int_0_uitofp(
+; CHECK-NEXT: icmp ne i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_one_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp one float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_one_int_n0_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp one
+define i1 @i32_cast_cmp_one_int_n0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp one float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_one_int_0_sitofp(
+; CHECK-NEXT: icmp ne i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_one_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp one float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_one_int_n0_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp one
+define i1 @i32_cast_cmp_one_int_n0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp one float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ueq_int_0_uitofp(
+; CHECK-NEXT: icmp eq i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_ueq_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp ueq float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ueq_int_n0_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp ueq
+define i1 @i32_cast_cmp_ueq_int_n0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp ueq float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ueq_int_0_sitofp(
+; CHECK-NEXT: icmp eq i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_ueq_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp ueq float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ueq_int_n0_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp ueq
+define i1 @i32_cast_cmp_ueq_int_n0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp ueq float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_une_int_0_uitofp(
+; CHECK-NEXT: icmp ne i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_une_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp une float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_une_int_n0_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp une
+define i1 @i32_cast_cmp_une_int_n0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp une float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_une_int_0_sitofp(
+; CHECK-NEXT: icmp ne i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_une_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp une float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_une_int_n0_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp une
+define i1 @i32_cast_cmp_une_int_n0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp une float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ogt_int_0_uitofp(
+; CHECK: icmp ne i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_ogt_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp ogt float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ogt_int_n0_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp ogt
+define i1 @i32_cast_cmp_ogt_int_n0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp ogt float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ogt_int_0_sitofp(
+; CHECK: icmp sgt i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_ogt_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp ogt float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ogt_int_n0_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp ogt
+define i1 @i32_cast_cmp_ogt_int_n0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp ogt float %f, -0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ole_int_0_uitofp(
+; CHECK: icmp eq i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_ole_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp ole float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ole_int_0_sitofp(
+; CHECK: icmp slt i32 %i, 1
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_ole_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp ole float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_olt_int_0_uitofp(
+; CHECK: ret i1 false
+define i1 @i32_cast_cmp_olt_int_0_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp olt float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_olt_int_0_sitofp(
+; CHECK: icmp slt i32 %i, 0
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_olt_int_0_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp olt float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i64_cast_cmp_oeq_int_0_uitofp(
+; CHECK-NEXT: icmp eq i64 %i, 0
+; CHECK-NEXT: ret
+define i1 @i64_cast_cmp_oeq_int_0_uitofp(i64 %i) {
+  %f = uitofp i64 %i to float
+  %cmp = fcmp oeq float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i64_cast_cmp_oeq_int_0_sitofp(
+; CHECK-NEXT: icmp eq i64 %i, 0
+; CHECK-NEXT: ret
+define i1 @i64_cast_cmp_oeq_int_0_sitofp(i64 %i) {
+  %f = sitofp i64 %i to float
+  %cmp = fcmp oeq float %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i64_cast_cmp_oeq_int_0_uitofp_half(
+; CHECK-NEXT: icmp eq i64 %i, 0
+; CHECK-NEXT: ret
+define i1 @i64_cast_cmp_oeq_int_0_uitofp_half(i64 %i) {
+  %f = uitofp i64 %i to half
+  %cmp = fcmp oeq half %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i64_cast_cmp_oeq_int_0_sitofp_half(
+; CHECK-NEXT: icmp eq i64 %i, 0
+; CHECK-NEXT: ret
+define i1 @i64_cast_cmp_oeq_int_0_sitofp_half(i64 %i) {
+  %f = sitofp i64 %i to half
+  %cmp = fcmp oeq half %f, 0.0
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_0_uitofp_ppcf128(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_0_uitofp_ppcf128(i32 %i) {
+  %f = uitofp i32 %i to ppc_fp128
+  %cmp = fcmp oeq ppc_fp128 %f, 0xM00000000000000000000000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i24max_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+
+; XCHECK: icmp eq i32 %i, 16777215
+; XCHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i24max_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x416FFFFFE0000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i24max_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+
+; XCHECK: icmp eq i32 %i, 16777215
+; XCHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i24max_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x416FFFFFE0000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i24maxp1_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+
+; XCHECK: icmp eq i32 %i, 16777216
+; XCHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i24maxp1_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x4170000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i24maxp1_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+
+; XCHECK: icmp eq i32 %i, 16777216
+; XCHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i24maxp1_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x4170000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i32umax_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i32umax_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x41F0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i32umax_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i32umax_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x41F0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i32imin_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i32imin_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0xC1E0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i32imin_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i32imin_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0xC1E0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i32imax_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i32imax_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x41E0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_i32imax_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_i32imax_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0x41E0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_negi32umax_uitofp(
+; CHECK: uitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_negi32umax_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0xC1F0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_int_negi32umax_sitofp(
+; CHECK: sitofp
+; CHECK: fcmp oeq
+; CHECK-NEXT: ret
+define i1 @i32_cast_cmp_oeq_int_negi32umax_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0xC1F0000000000000
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_half_uitofp(
+; CHECK: ret i1 false
+define i1 @i32_cast_cmp_oeq_half_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_oeq_half_sitofp(
+; CHECK: ret i1 false
+define i1 @i32_cast_cmp_oeq_half_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp oeq float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_one_half_uitofp(
+; CHECK: ret i1 true
+define i1 @i32_cast_cmp_one_half_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp one float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_one_half_sitofp(
+; CHECK: ret i1 true
+define i1 @i32_cast_cmp_one_half_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp one float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ueq_half_uitofp(
+; CHECK: ret i1 false
+define i1 @i32_cast_cmp_ueq_half_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp ueq float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_ueq_half_sitofp(
+; CHECK: ret i1 false
+define i1 @i32_cast_cmp_ueq_half_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp ueq float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_une_half_uitofp(
+; CHECK: ret i1 true
+define i1 @i32_cast_cmp_une_half_uitofp(i32 %i) {
+  %f = uitofp i32 %i to float
+  %cmp = fcmp une float %f, 0.5
+  ret i1 %cmp
+}
+
+; CHECK-LABEL: @i32_cast_cmp_une_half_sitofp(
+; CHECK: ret i1 true
+define i1 @i32_cast_cmp_une_half_sitofp(i32 %i) {
+  %f = sitofp i32 %i to float
+  %cmp = fcmp une float %f, 0.5
+  ret i1 %cmp
+}