[LoopVectorizer] Require no-signed-zeros-fp-math=true for fmin/fmax

[kmclaughlin]

Summary

Currently, setting the no-nans-fp-math attribute to true will allow
loops with fmin/fmax to vectorize, though we should be requiring that
no-signed-zeros-fp-math is also set.

This patch adds the check for no-signed-zeros at the function level and includes
tests to make sure we don't vectorize functions with only one of the attributes
associated.

[spatel]

llvm/lib/Analysis/IVDescriptors.cpp

597–599

That logic is hard to follow. How about inverting the condition, so we are positively identifying a min/max candidate?

I'm thinking:

if (isIntMinMaxRecurrenceKind(Kind) ||
    (FMF.noNans() && FMF.noSignedZeros() && 
     isFPMinMaxRecurrenceKind(Kind)))
  return isMinMaxSelectCmpPattern(I, Prev);

llvm/test/Transforms/LoopVectorize/float-minmax-instruction-flag.ll

151

I know we have an FMF mess currently, but can you add a test comment here to explain that this should not vectorize even after we switch to IR-level FMF? Or if there are other tests already checking for that scenario, point to those.

[kmclaughlin]

• Reordered the conditions in isRecurrenceInstr
• Added a comment to explain the @minloopmissingnsz test

llvm/lib/Analysis/IVDescriptors.cpp

597–599

Thanks for the suggestion, this is much easier to follow

[spatel]

LGTM

llvm/include/llvm/Analysis/IVDescriptors.h

public:
   };
 
   /// Returns a struct describing if the instruction 'I' can be a recurrence
   /// variable of type 'Kind'. If the recurrence is a min/max pattern of
   /// select(icmp()) this function advances the instruction pointer 'I' from the
   /// compare instruction to the select instruction and stores this pointer in
   /// 'PatternLastInst' member of the returned struct.
   static InstDesc isRecurrenceInstr(Instruction *I, RecurKind Kind,
-                                    InstDesc &Prev, bool HasFunNoNaNAttr);
+                                    InstDesc &Prev, FastMathFlags FMF);
 
   /// Returns true if instruction I has multiple uses in Insts
   static bool hasMultipleUsesOf(Instruction *I,
                                 SmallPtrSetImpl<Instruction *> &Insts,
                                 unsigned MaxNumUses);
 
   /// Returns true if all uses of the instruction I is within the Set.
   static bool areAllUsesIn(Instruction *I, SmallPtrSetImpl<Instruction *> &Set);
public:
   /// Returns the opcode corresponding to the RecurrenceKind.
   static unsigned getOpcode(RecurKind Kind);
 
   /// Returns true if Phi is a reduction of type Kind and adds it to the
   /// RecurrenceDescriptor. If either \p DB is non-null or \p AC and \p DT are
   /// non-null, the minimal bit width needed to compute the reduction will be
   /// computed.
   static bool AddReductionVar(PHINode *Phi, RecurKind Kind, Loop *TheLoop,
-                              bool HasFunNoNaNAttr,
+                              FastMathFlags FMF,
                               RecurrenceDescriptor &RedDes,
                               DemandedBits *DB = nullptr,
                               AssumptionCache *AC = nullptr,
                               DominatorTree *DT = nullptr);
 
   /// Returns true if Phi is a reduction in TheLoop. The RecurrenceDescriptor
   /// is returned in RedDes. If either \p DB is non-null or \p AC and \p DT are
   /// non-null, the minimal bit width needed to compute the reduction will be

llvm/lib/Analysis/IVDescriptors.cpp

while (!Worklist.empty()) {
     for (Value *O : cast<User>(Val)->operands())
       if (auto *I = dyn_cast<Instruction>(O))
         if (TheLoop->contains(I) && !Visited.count(I))
           Worklist.push_back(I);
   }
 }
 
 bool RecurrenceDescriptor::AddReductionVar(PHINode *Phi, RecurKind Kind,
-                                           Loop *TheLoop, bool HasFunNoNaNAttr,
+                                           Loop *TheLoop, FastMathFlags FuncFMF,
                                            RecurrenceDescriptor &RedDes,
                                            DemandedBits *DB,
                                            AssumptionCache *AC,
                                            DominatorTree *DT) {
   if (Phi->getNumIncomingValues() != 2)
     return false;
 
   // Reduction variables are only found in the loop header block.
if (!Cur->isCommutative() && !IsAPhi && !isa<SelectInst>(Cur) &&
         !isa<ICmpInst>(Cur) && !isa<FCmpInst>(Cur) &&
         !VisitedInsts.count(dyn_cast<Instruction>(Cur->getOperand(0))))
       return false;
 
     // Any reduction instruction must be of one of the allowed kinds. We ignore
     // the starting value (the Phi or an AND instruction if the Phi has been
     // type-promoted).
     if (Cur != Start) {
-      ReduxDesc = isRecurrenceInstr(Cur, Kind, ReduxDesc, HasFunNoNaNAttr);
+      ReduxDesc = isRecurrenceInstr(Cur, Kind, ReduxDesc, FuncFMF);
       if (!ReduxDesc.isRecurrence())
         return false;
       // FIXME: FMF is allowed on phi, but propagation is not handled correctly.
       if (isa<FPMathOperator>(ReduxDesc.getPatternInst()) && !IsAPhi) {
         FastMathFlags CurFMF = ReduxDesc.getPatternInst()->getFastMathFlags();
         if (auto *Sel = dyn_cast<SelectInst>(ReduxDesc.getPatternInst())) {
           // Accept FMF on either fcmp or select of a min/max idiom.
           // TODO: This is a hack to work-around the fact that FMF may not be
RecurrenceDescriptor::isConditionalRdxPattern(RecurKind Kind, Instruction *I) {
   if (m_FMul(m_Value(Op1), m_Value(Op2)).match(I1) && (I1->isFast()))
     return InstDesc(Kind == RecurKind::FMul, SI);
 
   return InstDesc(false, I);
 }
 
 RecurrenceDescriptor::InstDesc
 RecurrenceDescriptor::isRecurrenceInstr(Instruction *I, RecurKind Kind,
-                                        InstDesc &Prev, bool HasFunNoNaNAttr) {
+                                        InstDesc &Prev, FastMathFlags FMF) {
   Instruction *UAI = Prev.getUnsafeAlgebraInst();
   if (!UAI && isa<FPMathOperator>(I) && !I->hasAllowReassoc())
     UAI = I; // Found an unsafe (unvectorizable) algebra instruction.
 
   switch (I->getOpcode()) {
   default:
     return InstDesc(false, I);
   case Instruction::PHI:
RecurrenceDescriptor::isRecurrenceInstr(Instruction *I, RecurKind Kind,
   case Instruction::FAdd:
     return InstDesc(Kind == RecurKind::FAdd, I, UAI);
   case Instruction::Select:
     if (Kind == RecurKind::FAdd || Kind == RecurKind::FMul)
       return isConditionalRdxPattern(Kind, I);
     LLVM_FALLTHROUGH;
   case Instruction::FCmp:
   case Instruction::ICmp:
     if (!isIntMinMaxRecurrenceKind(Kind) &&
-        (!HasFunNoNaNAttr || !isFPMinMaxRecurrenceKind(Kind)))
+        (!(FMF.noNaNs() && FMF.noSignedZeros()) ||
+         !isFPMinMaxRecurrenceKind(Kind)))
       return InstDesc(false, I);
     return isMinMaxSelectCmpPattern(I, Prev);
   }
 }
 
 bool RecurrenceDescriptor::hasMultipleUsesOf(
     Instruction *I, SmallPtrSetImpl<Instruction *> &Insts,
     unsigned MaxNumUses) {

 }
 bool RecurrenceDescriptor::isReductionPHI(PHINode *Phi, Loop *TheLoop,
                                           RecurrenceDescriptor &RedDes,
                                           DemandedBits *DB, AssumptionCache *AC,
                                           DominatorTree *DT) {
 
   BasicBlock *Header = TheLoop->getHeader();
   Function &F = *Header->getParent();
-  bool HasFunNoNaNAttr =
-      F.getFnAttribute("no-nans-fp-math").getValueAsString() == "true";
+  FastMathFlags FMF;
+  FMF.setNoNaNs(
+    F.getFnAttribute("no-nans-fp-math").getValueAsString() == "true");
+  FMF.setNoSignedZeros(
+    F.getFnAttribute("no-signed-zeros-fp-math").getValueAsString() == "true");
 
-  if (AddReductionVar(Phi, RecurKind::Add, TheLoop, HasFunNoNaNAttr, RedDes, DB,
-                      AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::Add, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found an ADD reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::Mul, TheLoop, HasFunNoNaNAttr, RedDes, DB,
-                      AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::Mul, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a MUL reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::Or, TheLoop, HasFunNoNaNAttr, RedDes, DB,
-                      AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::Or, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found an OR reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::And, TheLoop, HasFunNoNaNAttr, RedDes, DB,
-                      AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::And, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found an AND reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::Xor, TheLoop, HasFunNoNaNAttr, RedDes, DB,
-                      AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::Xor, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a XOR reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::SMax, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::SMax, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a SMAX reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::SMin, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::SMin, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a SMIN reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::UMax, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::UMax, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a UMAX reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::UMin, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::UMin, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a UMIN reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::FMul, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::FMul, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found an FMult reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::FAdd, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::FAdd, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found an FAdd reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::FMax, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::FMax, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a float MAX reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RecurKind::FMin, TheLoop, HasFunNoNaNAttr, RedDes,
-                      DB, AC, DT)) {
+  if (AddReductionVar(Phi, RecurKind::FMin, TheLoop, FMF, RedDes, DB, AC, DT)) {
     LLVM_DEBUG(dbgs() << "Found a float MIN reduction PHI." << *Phi << "\n");
     return true;
   }
   // Not a reduction of known type.
   return false;
 }
 
 bool RecurrenceDescriptor::isFirstOrderRecurrence(

llvm/test/Transforms/LoopVectorize/X86/reduction-fastmath.ll

 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds float, float* [[A:%.*]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP3:%.*]] = getelementptr inbounds float, float* [[A]], i64 [[TMP1]]
 ; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr inbounds float, float* [[TMP2]], i32 0
 ; CHECK-NEXT:    [[TMP5:%.*]] = bitcast float* [[TMP4]] to <4 x float>*
 ; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <4 x float>, <4 x float>* [[TMP5]], align 4
 ; CHECK-NEXT:    [[TMP6:%.*]] = getelementptr inbounds float, float* [[TMP2]], i32 4
 ; CHECK-NEXT:    [[TMP7:%.*]] = bitcast float* [[TMP6]] to <4 x float>*
 ; CHECK-NEXT:    [[WIDE_LOAD2:%.*]] = load <4 x float>, <4 x float>* [[TMP7]], align 4
-; CHECK-NEXT:    [[TMP8:%.*]] = fcmp nnan ninf oge <4 x float> [[WIDE_LOAD]], [[VEC_PHI]]
-; CHECK-NEXT:    [[TMP9:%.*]] = fcmp nnan ninf oge <4 x float> [[WIDE_LOAD2]], [[VEC_PHI1]]
+; CHECK-NEXT:    [[TMP8:%.*]] = fcmp nnan ninf nsz oge <4 x float> [[WIDE_LOAD]], [[VEC_PHI]]
+; CHECK-NEXT:    [[TMP9:%.*]] = fcmp nnan ninf nsz oge <4 x float> [[WIDE_LOAD2]], [[VEC_PHI1]]
 ; CHECK-NEXT:    [[TMP10]] = select <4 x i1> [[TMP8]], <4 x float> [[WIDE_LOAD]], <4 x float> [[VEC_PHI]]
 ; CHECK-NEXT:    [[TMP11]] = select <4 x i1> [[TMP9]], <4 x float> [[WIDE_LOAD2]], <4 x float> [[VEC_PHI1]]
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add i64 [[INDEX]], 8
 ; CHECK-NEXT:    [[TMP12:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[TMP12]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], [[LOOP8:!llvm.loop !.*]]
 ; CHECK:       middle.block:
-; CHECK-NEXT:    [[RDX_MINMAX_CMP:%.*]] = fcmp nnan ninf ogt <4 x float> [[TMP10]], [[TMP11]]
-; CHECK-NEXT:    [[RDX_MINMAX_SELECT:%.*]] = select nnan ninf <4 x i1> [[RDX_MINMAX_CMP]], <4 x float> [[TMP10]], <4 x float> [[TMP11]]
-; CHECK-NEXT:    [[TMP13:%.*]] = call nnan ninf float @llvm.vector.reduce.fmax.v4f32(<4 x float> [[RDX_MINMAX_SELECT]])
+; CHECK-NEXT:    [[RDX_MINMAX_CMP:%.*]] = fcmp nnan ninf nsz ogt <4 x float> [[TMP10]], [[TMP11]]
+; CHECK-NEXT:    [[RDX_MINMAX_SELECT:%.*]] = select nnan ninf nsz <4 x i1> [[RDX_MINMAX_CMP]], <4 x float> [[TMP10]], <4 x float> [[TMP11]]
+; CHECK-NEXT:    [[TMP13:%.*]] = call nnan ninf nsz float @llvm.vector.reduce.fmax.v4f32(<4 x float> [[RDX_MINMAX_SELECT]])
 ; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 [[WIDE_TRIP_COUNT]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[FOR_COND_CLEANUP_LOOPEXIT:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[FOR_BODY_LR_PH]] ]
 ; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi float [ -1.000000e+00, [[FOR_BODY_LR_PH]] ], [ [[TMP13]], [[MIDDLE_BLOCK]] ]
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.cond.cleanup.loopexit:
 ; CHECK-NEXT:    [[MAX_0__LCSSA:%.*]] = phi float [ [[MAX_0_:%.*]], [[FOR_BODY]] ], [ [[TMP13]], [[MIDDLE_BLOCK]] ]
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    [[MAX_0_LCSSA:%.*]] = phi float [ -1.000000e+00, [[ENTRY:%.*]] ], [ [[MAX_0__LCSSA]], [[FOR_COND_CLEANUP_LOOPEXIT]] ]
 ; CHECK-NEXT:    ret float [[MAX_0_LCSSA]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[MAX_013:%.*]] = phi float [ [[BC_MERGE_RDX]], [[SCALAR_PH]] ], [ [[MAX_0_]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds float, float* [[A]], i64 [[INDVARS_IV]]
 ; CHECK-NEXT:    [[TMP14:%.*]] = load float, float* [[ARRAYIDX]], align 4
-; CHECK-NEXT:    [[CMP1_INV:%.*]] = fcmp nnan ninf oge float [[TMP14]], [[MAX_013]]
+; CHECK-NEXT:    [[CMP1_INV:%.*]] = fcmp nnan ninf nsz oge float [[TMP14]], [[MAX_013]]
 ; CHECK-NEXT:    [[MAX_0_]] = select i1 [[CMP1_INV]], float [[TMP14]], float [[MAX_013]]
 ; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
 ; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT]], [[WIDE_TRIP_COUNT]]
 ; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_COND_CLEANUP_LOOPEXIT]], label [[FOR_BODY]], [[LOOP9:!llvm.loop !.*]]
 ;
 entry:
   %cmp12 = icmp sgt i32 %N, 0
   br i1 %cmp12, label %for.body.lr.ph, label %for.cond.cleanup
 
 for.body.lr.ph:
   %wide.trip.count = zext i32 %N to i64
   br label %for.body
 
 for.cond.cleanup:
   %max.0.lcssa = phi float [ -1.000000e+00, %entry ], [ %max.0., %for.body ]
   ret float %max.0.lcssa
 
 for.body:
   %indvars.iv = phi i64 [ 0, %for.body.lr.ph ], [ %indvars.iv.next, %for.body ]
   %max.013 = phi float [ -1.000000e+00, %for.body.lr.ph ], [ %max.0., %for.body ]
   %arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
   %0 = load float, float* %arrayidx, align 4
-  %cmp1.inv = fcmp nnan ninf oge float %0, %max.013
+  %cmp1.inv = fcmp nnan ninf nsz oge float %0, %max.013
   %max.0. = select i1 %cmp1.inv, float %0, float %max.013
   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
   %exitcond = icmp eq i64 %indvars.iv.next, %wide.trip.count
   br i1 %exitcond, label %for.cond.cleanup, label %for.body
 }
 
 ; Same as above, but this time the select already has matching FMF with its condition.
 
for.body:
   %0 = load float, float* %arrayidx, align 4
   %cmp1.inv = fcmp nnan ninf oge float %0, %max.013
   %max.0. = select nnan ninf i1 %cmp1.inv, float %0, float %max.013
   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
   %exitcond = icmp eq i64 %indvars.iv.next, %wide.trip.count
   br i1 %exitcond, label %for.cond.cleanup, label %for.body
 }
 
-attributes #0 = { "no-infs-fp-math"="true" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="false" "unsafe-fp-math"="false" }
+attributes #0 = { "no-infs-fp-math"="true" "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" "unsafe-fp-math"="false" }

llvm/test/Transforms/LoopVectorize/float-minmax-instruction-flag.ll

loop:                                             ; preds = %loop, %top
   %t7 = add i64 %t1, 1
   %t8 = icmp eq i64 %t7, 65537
   br i1 %t8, label %out, label %loop
 
 out:                                              ; preds = %loop
   ret float %t6
 }
 
+define float @minloopmissingnsz(float* nocapture readonly %arg) #1 {
+; CHECK-LABEL: @minloopmissingnsz(
+; CHECK-NEXT:  top:
+; CHECK-NEXT:    [[T:%.*]] = load float, float* [[ARG:%.*]], align 4
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[T1:%.*]] = phi i64 [ [[T7:%.*]], [[LOOP]] ], [ 1, [[TOP:%.*]] ]
+; CHECK-NEXT:    [[T2:%.*]] = phi float [ [[T6:%.*]], [[LOOP]] ], [ [[T]], [[TOP]] ]
+; CHECK-NEXT:    [[T3:%.*]] = getelementptr float, float* [[ARG]], i64 [[T1]]
+; CHECK-NEXT:    [[T4:%.*]] = load float, float* [[T3]], align 4
+; CHECK-NEXT:    [[T5:%.*]] = fcmp olt float [[T2]], [[T4]]
+; CHECK-NEXT:    [[T6]] = select i1 [[T5]], float [[T2]], float [[T4]]
+; CHECK-NEXT:    [[T7]] = add i64 [[T1]], 1
+; CHECK-NEXT:    [[T8:%.*]] = icmp eq i64 [[T7]], 65537
+; CHECK-NEXT:    br i1 [[T8]], label [[OUT:%.*]], label [[LOOP]]
+; CHECK:       out:
+; CHECK-NEXT:    [[T6_LCSSA:%.*]] = phi float [ [[T6]], [[LOOP]] ]
+; CHECK-NEXT:    ret float [[T6_LCSSA]]
+;
+top:
+  %t = load float, float* %arg
+  br label %loop
+
+loop:                                             ; preds = %loop, %top
+  %t1 = phi i64 [ %t7, %loop ], [ 1, %top ]
+  %t2 = phi float [ %t6, %loop ], [ %t, %top ]
+  %t3 = getelementptr float, float* %arg, i64 %t1
+  %t4 = load float, float* %t3, align 4
+  %t5 = fcmp olt float %t2, %t4
+  %t6 = select i1 %t5, float %t2, float %t4
+  %t7 = add i64 %t1, 1
+  %t8 = icmp eq i64 %t7, 65537
+  br i1 %t8, label %out, label %loop
+
+out:                                              ; preds = %loop
+  ret float %t6
+}
+
 ; This would assert on FMF propagation.
 
 define void @not_a_min_max() {
 ; CHECK-LABEL: @not_a_min_max(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[F9_S0_V0:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[ADD:%.*]], [[LOOP]] ]
loop:
   %t15 = select reassoc nnan ninf nsz contract afn i1 %t14, float 0x36A0000000000000, float 0.0
   %add = add nuw nsw i32 %f9.s0.v0, 1
   br i1 false, label %end, label %loop
 
 end:
   ret void
 }
 
-attributes #0 = { "no-nans-fp-math"="true" }
+attributes #0 = { "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" }
+attributes #1 = { "no-nans-fp-math"="true" }

llvm/test/Transforms/LoopVectorize/minmax_reduction.ll

for.body:
   %indvars.iv.next = add i64 %indvars.iv, 1
   %exitcond = icmp eq i64 %indvars.iv.next, 1024
   br i1 %exitcond, label %for.end, label %for.body
 
 for.end:
   ret float %max.red.0
 }
 
+; As above, with the no-signed-zeros-fp-math attribute missing
+; CHECK-LABEL: @max_red_float_nsz(
+; CHECK-NOT: <2 x float>
+
+define float @max_red_float_nsz(float %max) #1 {
+entry:
+  br label %for.body
+
+for.body:
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %max.red.08 = phi float [ %max, %entry ], [ %max.red.0, %for.body ]
+  %arrayidx = getelementptr inbounds [1024 x float], [1024 x float]* @fA, i64 0, i64 %indvars.iv
+  %0 = load float, float* %arrayidx, align 4
+  %cmp3 = fcmp fast ogt float %0, %max.red.08
+  %max.red.0 = select i1 %cmp3, float %0, float %max.red.08
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 1024
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret float %max.red.0
+}
+
 ; Make sure any check-not directives are not triggered by function declarations.
 ; CHECK: declare
 
-attributes #0 = { "no-nans-fp-math"="true" }
+attributes #0 = { "no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" }
+attributes #1 = { "no-nans-fp-math"="true" }