Index: llvm/include/llvm/Analysis/IVDescriptors.h
===================================================================
--- llvm/include/llvm/Analysis/IVDescriptors.h
+++ llvm/include/llvm/Analysis/IVDescriptors.h
@@ -136,7 +136,8 @@
   static InstDesc isConditionalRdxPattern(RecurKind Kind, Instruction *I);
 
   /// Returns identity corresponding to the RecurrenceKind.
-  static Constant *getRecurrenceIdentity(RecurKind K, Type *Tp);
+  static Constant *getRecurrenceIdentity(RecurKind K, Type *Tp,
+                                         FastMathFlags FMF);
 
   /// Returns the opcode corresponding to the RecurrenceKind.
   static unsigned getOpcode(RecurKind Kind);
Index: llvm/lib/Analysis/IVDescriptors.cpp
===================================================================
--- llvm/lib/Analysis/IVDescriptors.cpp
+++ llvm/lib/Analysis/IVDescriptors.cpp
@@ -761,7 +761,8 @@
 
 /// This function returns the identity element (or neutral element) for
 /// the operation K.
-Constant *RecurrenceDescriptor::getRecurrenceIdentity(RecurKind K, Type *Tp) {
+Constant *RecurrenceDescriptor::getRecurrenceIdentity(RecurKind K, Type *Tp,
+                                                      FastMathFlags FMF) {
   switch (K) {
   case RecurKind::Xor:
   case RecurKind::Add:
@@ -779,7 +780,14 @@
     return ConstantFP::get(Tp, 1.0L);
   case RecurKind::FAdd:
     // Adding zero to a number does not change it.
-    return ConstantFP::get(Tp, 0.0L);
+    // FIXME: Ideally we should not need to check FMF for FAdd and should always
+    // use -0.0. However, this will currently result in mixed vectors of 0.0/-0.0.
+    // Instead, we should ensure that 1) the FMF from FAdd are propagated to the PHI
+    // nodes where possible, and 2) PHIs with the nsz flag + -0.0 use 0.0. This would
+    // mean we can then remove the check for noSignedZeros() below (see D98963).
+    if (FMF.noSignedZeros())
+      return ConstantFP::get(Tp, 0.0L);
+    return ConstantFP::get(Tp, -0.0L);
   case RecurKind::UMin:
     return ConstantInt::get(Tp, -1);
   case RecurKind::UMax:
Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -4661,7 +4661,7 @@
         }
       } else {
         Constant *IdenC = RecurrenceDescriptor::getRecurrenceIdentity(
-            RK, VecTy->getScalarType());
+            RK, VecTy->getScalarType(), RdxDesc->getFastMathFlags());
         Iden = IdenC;
 
         if (!ScalarPHI) {
@@ -9123,7 +9123,7 @@
       Value *NewCond = State.get(Cond, Part);
       VectorType *VecTy = cast<VectorType>(NewVecOp->getType());
       Constant *Iden = RecurrenceDescriptor::getRecurrenceIdentity(
-          Kind, VecTy->getElementType());
+          Kind, VecTy->getElementType(), RdxDesc->getFastMathFlags());
       Constant *IdenVec =
           ConstantVector::getSplat(VecTy->getElementCount(), Iden);
       Value *Select = State.Builder.CreateSelect(NewCond, NewVecOp, IdenVec);
Index: llvm/test/Transforms/LoopVectorize/X86/reduction-fastmath.ll
===================================================================
--- llvm/test/Transforms/LoopVectorize/X86/reduction-fastmath.ll
+++ llvm/test/Transforms/LoopVectorize/X86/reduction-fastmath.ll
@@ -129,8 +129,8 @@
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x float> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP8:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[VEC_PHI1:%.*]] = phi <4 x float> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP9:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x float> [ <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, [[VECTOR_PH]] ], [ [[TMP8:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI1:%.*]] = phi <4 x float> [ <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, [[VECTOR_PH]] ], [ [[TMP9:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[INDEX]], 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr float, float* [[ARRAY:%.*]], i32 [[TMP0]]
@@ -153,7 +153,7 @@
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[LOOP_EXIT_LOOPEXIT:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i32 [ 4096, [[MIDDLE_BLOCK]] ], [ 0, [[LOOP_PREHEADER]] ]
-; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi float [ 0.000000e+00, [[LOOP_PREHEADER]] ], [ [[TMP11]], [[MIDDLE_BLOCK]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi float [ -0.000000e+00, [[LOOP_PREHEADER]] ], [ [[TMP11]], [[MIDDLE_BLOCK]] ]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IDX:%.*]] = phi i32 [ [[IDX_INC:%.*]], [[LOOP]] ], [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ]
@@ -168,7 +168,7 @@
 ; CHECK-NEXT:    [[SUM_INC_LCSSA:%.*]] = phi float [ [[SUM_INC]], [[LOOP]] ], [ [[TMP11]], [[MIDDLE_BLOCK]] ]
 ; CHECK-NEXT:    br label [[LOOP_EXIT]]
 ; CHECK:       loop.exit:
-; CHECK-NEXT:    [[SUM_LCSSA:%.*]] = phi float [ 0.000000e+00, [[ENTRY:%.*]] ], [ [[SUM_INC_LCSSA]], [[LOOP_EXIT_LOOPEXIT]] ]
+; CHECK-NEXT:    [[SUM_LCSSA:%.*]] = phi float [ -0.000000e+00, [[ENTRY:%.*]] ], [ [[SUM_INC_LCSSA]], [[LOOP_EXIT_LOOPEXIT]] ]
 ; CHECK-NEXT:    ret float [[SUM_LCSSA]]
 ;
 entry:
@@ -177,7 +177,7 @@
 
 loop:
   %idx = phi i32 [ 0, %entry ], [ %idx.inc, %loop ]
-  %sum = phi float [ 0.000000e+00, %entry ], [ %sum.inc, %loop ]
+  %sum = phi float [ -0.000000e+00, %entry ], [ %sum.inc, %loop ]
   %address = getelementptr float, float* %array, i32 %idx
   %value = load float, float* %address
   %sum.inc = fadd reassoc float %sum, %value
@@ -186,7 +186,7 @@
   br i1 %be.cond, label %loop, label %loop.exit
 
 loop.exit:
-  %sum.lcssa = phi float [ %sum.inc, %loop ], [ 0.000000e+00, %entry ]
+  %sum.lcssa = phi float [ %sum.inc, %loop ], [ -0.000000e+00, %entry ]
   ret float %sum.lcssa
 }
 
@@ -201,8 +201,8 @@
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x float> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP8:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[VEC_PHI1:%.*]] = phi <4 x float> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP9:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x float> [ <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, [[VECTOR_PH]] ], [ [[TMP8:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI1:%.*]] = phi <4 x float> [ <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, [[VECTOR_PH]] ], [ [[TMP9:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[INDEX]], 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr float, float* [[ARRAY:%.*]], i32 [[TMP0]]
@@ -225,7 +225,7 @@
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[LOOP_EXIT_LOOPEXIT:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i32 [ 4096, [[MIDDLE_BLOCK]] ], [ 0, [[LOOP_PREHEADER]] ]
-; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi float [ 0.000000e+00, [[LOOP_PREHEADER]] ], [ [[TMP11]], [[MIDDLE_BLOCK]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi float [ -0.000000e+00, [[LOOP_PREHEADER]] ], [ [[TMP11]], [[MIDDLE_BLOCK]] ]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IDX:%.*]] = phi i32 [ [[IDX_INC:%.*]], [[LOOP]] ], [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ]
@@ -240,7 +240,7 @@
 ; CHECK-NEXT:    [[SUM_INC_LCSSA:%.*]] = phi float [ [[SUM_INC]], [[LOOP]] ], [ [[TMP11]], [[MIDDLE_BLOCK]] ]
 ; CHECK-NEXT:    br label [[LOOP_EXIT]]
 ; CHECK:       loop.exit:
-; CHECK-NEXT:    [[SUM_LCSSA:%.*]] = phi float [ 0.000000e+00, [[ENTRY:%.*]] ], [ [[SUM_INC_LCSSA]], [[LOOP_EXIT_LOOPEXIT]] ]
+; CHECK-NEXT:    [[SUM_LCSSA:%.*]] = phi float [ -0.000000e+00, [[ENTRY:%.*]] ], [ [[SUM_INC_LCSSA]], [[LOOP_EXIT_LOOPEXIT]] ]
 ; CHECK-NEXT:    ret float [[SUM_LCSSA]]
 ;
 entry:
@@ -249,7 +249,7 @@
 
 loop:
   %idx = phi i32 [ 0, %entry ], [ %idx.inc, %loop ]
-  %sum = phi float [ 0.000000e+00, %entry ], [ %sum.inc, %loop ]
+  %sum = phi float [ -0.000000e+00, %entry ], [ %sum.inc, %loop ]
   %address = getelementptr float, float* %array, i32 %idx
   %value = load float, float* %address
   %sum.inc = fadd reassoc contract float %sum, %value
@@ -258,7 +258,7 @@
   br i1 %be.cond, label %loop, label %loop.exit
 
 loop.exit:
-  %sum.lcssa = phi float [ %sum.inc, %loop ], [ 0.000000e+00, %entry ]
+  %sum.lcssa = phi float [ %sum.inc, %loop ], [ -0.000000e+00, %entry ]
   ret float %sum.lcssa
 }