Index: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -7361,6 +7361,7 @@
   bool Changed = false;
   SmallVector<Value *, 4> Incoming;
   SmallPtrSet<Value *, 16> VisitedInstrs;
+  unsigned MaxVecRegSize = R.getMaxVecRegSize();
 
   bool HaveVectorizedPhiNodes = true;
   while (HaveVectorizedPhiNodes) {
@@ -7387,8 +7388,18 @@
 
       // Look for the next elements with the same type.
       SmallVector<Value *, 4>::iterator SameTypeIt = IncIt;
+      Type *EltTy = (*IncIt)->getType();
+      unsigned EltSize = EltTy->isSized() ? DL->getTypeSizeInBits(EltTy)
+                                          : MaxVecRegSize;
+      unsigned MaxNumElts = MaxVecRegSize / EltSize;
+      if (MaxNumElts < 2) {
+        ++IncIt;
+        continue;
+      }
+
       while (SameTypeIt != E &&
-             (*SameTypeIt)->getType() == (*IncIt)->getType()) {
+             (*SameTypeIt)->getType() == EltTy &&
+             (SameTypeIt - IncIt) < MaxNumElts) {
         VisitedInstrs.insert(*SameTypeIt);
         ++SameTypeIt;
       }
Index: llvm/test/Transforms/SLPVectorizer/X86/remark_unsupported.ll
===================================================================
--- llvm/test/Transforms/SLPVectorizer/X86/remark_unsupported.ll
+++ llvm/test/Transforms/SLPVectorizer/X86/remark_unsupported.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -mtriple=x86_64-pc-linux-gnu -mcpu=generic -slp-vectorizer -pass-remarks-output=%t < %s | FileCheck %s
+; RUN: opt -S -mtriple=x86_64-pc-linux-gnu -mcpu=generic -slp-vectorizer --slp-max-reg-size=256 -pass-remarks-output=%t < %s | FileCheck %s
 ; RUN: FileCheck --input-file=%t --check-prefix=YAML %s
 
 ; This type is not supported by SLP
Index: llvm/test/Transforms/SLPVectorizer/slp-max-phi-size.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SLPVectorizer/slp-max-phi-size.ll
@@ -0,0 +1,135 @@
+; RUN: opt -slp-vectorizer -S -slp-max-reg-size=32 < %s | FileCheck -check-prefixes=FUNC,MAX32 %s
+; RUN: opt -slp-vectorizer -S -slp-max-reg-size=128 < %s | FileCheck -check-prefixes=FUNC,MAX128 %s
+; RUN: opt -slp-vectorizer -S -slp-max-reg-size=1024 < %s | FileCheck -check-prefixes=FUNC,MAX1024 %s
+
+; FUNC-LABEL: @phi_float32()
+; MAX32-COUNT32: phi float
+; MAX128-COUNT8: phi <4 x float>
+; MAX128-COUNT2: phi <32 x float>
+
+define void @phi_float32() {
+entry:
+  br i1 undef, label %.thread3941, label %bb
+
+bb:                                               ; preds = %entry
+  br label %.thread3941
+
+.thread3941:                                      ; preds = %bb, %entry
+  %i = fpext half undef to float
+  %i1 = fmul contract float %i, undef
+  %i2 = fadd contract float 0.000000e+00, %i1
+  %i3 = fpext half undef to float
+  %i4 = fmul contract float %i3, undef
+  %i5 = fadd contract float 0.000000e+00, %i4
+  %i6 = fpext half undef to float
+  %i7 = fmul contract float %i6, undef
+  %i8 = fadd contract float 0.000000e+00, %i7
+  %i9 = fpext half undef to float
+  %i10 = fmul contract float %i9, undef
+  %i11 = fadd contract float 0.000000e+00, %i10
+  %i12 = fmul contract float %i, undef
+  %i13 = fadd contract float 0.000000e+00, %i12
+  %i14 = fmul contract float %i3, undef
+  %i15 = fadd contract float 0.000000e+00, %i14
+  %i16 = fmul contract float %i6, undef
+  %i17 = fadd contract float 0.000000e+00, %i16
+  %i18 = fmul contract float %i9, undef
+  %i19 = fadd contract float 0.000000e+00, %i18
+  %i20 = fmul contract float %i, undef
+  %i21 = fadd contract float 0.000000e+00, %i20
+  %i22 = fmul contract float %i3, undef
+  %i23 = fadd contract float 0.000000e+00, %i22
+  %i24 = fmul contract float %i6, undef
+  %i25 = fadd contract float 0.000000e+00, %i24
+  %i26 = fmul contract float %i9, undef
+  %i27 = fadd contract float 0.000000e+00, %i26
+  %i28 = fmul contract float %i, undef
+  %i29 = fadd contract float 0.000000e+00, %i28
+  %i30 = fmul contract float %i3, undef
+  %i31 = fadd contract float 0.000000e+00, %i30
+  %i32 = fmul contract float %i6, undef
+  %i33 = fadd contract float 0.000000e+00, %i32
+  %i34 = fmul contract float %i9, undef
+  %i35 = fadd contract float 0.000000e+00, %i34
+  %i36 = fmul contract float %i, undef
+  %i37 = fadd contract float 0.000000e+00, %i36
+  %i38 = fmul contract float %i3, undef
+  %i39 = fadd contract float 0.000000e+00, %i38
+  %i40 = fmul contract float %i6, undef
+  %i41 = fadd contract float 0.000000e+00, %i40
+  %i42 = fmul contract float %i9, undef
+  %i43 = fadd contract float 0.000000e+00, %i42
+  %i44 = fmul contract float %i, undef
+  %i45 = fadd contract float 0.000000e+00, %i44
+  %i46 = fmul contract float %i3, undef
+  %i47 = fadd contract float 0.000000e+00, %i46
+  %i48 = fmul contract float %i6, undef
+  %i49 = fadd contract float 0.000000e+00, %i48
+  %i50 = fmul contract float %i9, undef
+  %i51 = fadd contract float 0.000000e+00, %i50
+  %i52 = fmul contract float %i, undef
+  %i53 = fadd contract float 0.000000e+00, %i52
+  %i54 = fmul contract float %i3, undef
+  %i55 = fadd contract float 0.000000e+00, %i54
+  %i56 = fmul contract float %i6, undef
+  %i57 = fadd contract float 0.000000e+00, %i56
+  %i58 = fmul contract float %i9, undef
+  %i59 = fadd contract float 0.000000e+00, %i58
+  %i60 = fmul contract float %i, undef
+  %i61 = fadd contract float 0.000000e+00, %i60
+  %i62 = fmul contract float %i3, undef
+  %i63 = fadd contract float 0.000000e+00, %i62
+  %i64 = fmul contract float %i6, undef
+  %i65 = fadd contract float 0.000000e+00, %i64
+  %i66 = fmul contract float %i9, undef
+  %i67 = fadd contract float 0.000000e+00, %i66
+  switch i32 undef, label %bb70 [
+    i32 0, label %bb71
+    i32 1, label %bb68
+    i32 2, label %bb69
+  ]
+
+bb68:                                             ; preds = %.thread3941
+  br label %bb71
+
+bb69:                                             ; preds = %.thread3941
+  br label %bb71
+
+bb70:                                             ; preds = %.thread3941
+  br label %bb71
+
+bb71:                                             ; preds = %bb70, %bb69, %bb68, %.thread3941
+  %.sroa.436.3 = phi float [ %i19, %bb68 ], [ %i19, %bb69 ], [ undef, %bb70 ], [ %i19, %.thread3941 ]
+  %.sroa.374.3 = phi float [ %i17, %bb68 ], [ undef, %bb69 ], [ %i17, %bb70 ], [ %i17, %.thread3941 ]
+  %.sroa.312.3 = phi float [ %i15, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.250.3 = phi float [ %i13, %bb68 ], [ %i13, %bb69 ], [ %i13, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.188.3 = phi float [ %i11, %bb68 ], [ %i11, %bb69 ], [ undef, %bb70 ], [ %i11, %.thread3941 ]
+  %.sroa.126.3 = phi float [ %i8, %bb68 ], [ undef, %bb69 ], [ %i8, %bb70 ], [ %i8, %.thread3941 ]
+  %.sroa.64.3 = phi float [ %i5, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.02268.3 = phi float [ %i2, %bb68 ], [ %i2, %bb69 ], [ %i2, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.498.3 = phi float [ %i21, %bb68 ], [ %i21, %bb69 ], [ %i21, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.560.3 = phi float [ %i23, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.622.3 = phi float [ %i25, %bb68 ], [ undef, %bb69 ], [ %i25, %bb70 ], [ %i25, %.thread3941 ]
+  %.sroa.684.3 = phi float [ %i27, %bb68 ], [ %i27, %bb69 ], [ undef, %bb70 ], [ %i27, %.thread3941 ]
+  %.sroa.746.3 = phi float [ %i29, %bb68 ], [ %i29, %bb69 ], [ %i29, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.808.3 = phi float [ %i31, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.870.3 = phi float [ %i33, %bb68 ], [ undef, %bb69 ], [ %i33, %bb70 ], [ %i33, %.thread3941 ]
+  %.sroa.932.3 = phi float [ %i35, %bb68 ], [ %i35, %bb69 ], [ undef, %bb70 ], [ %i35, %.thread3941 ]
+  %.sroa.994.3 = phi float [ %i37, %bb68 ], [ %i37, %bb69 ], [ %i37, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1056.3 = phi float [ %i39, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1118.3 = phi float [ %i41, %bb68 ], [ undef, %bb69 ], [ %i41, %bb70 ], [ %i41, %.thread3941 ]
+  %.sroa.1180.3 = phi float [ %i43, %bb68 ], [ %i43, %bb69 ], [ undef, %bb70 ], [ %i43, %.thread3941 ]
+  %.sroa.1242.3 = phi float [ %i45, %bb68 ], [ %i45, %bb69 ], [ %i45, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1304.3 = phi float [ %i47, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1366.3 = phi float [ %i49, %bb68 ], [ undef, %bb69 ], [ %i49, %bb70 ], [ %i49, %.thread3941 ]
+  %.sroa.1428.3 = phi float [ %i51, %bb68 ], [ %i51, %bb69 ], [ undef, %bb70 ], [ %i51, %.thread3941 ]
+  %.sroa.1490.3 = phi float [ %i53, %bb68 ], [ %i53, %bb69 ], [ %i53, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1552.3 = phi float [ %i55, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1614.3 = phi float [ %i57, %bb68 ], [ undef, %bb69 ], [ %i57, %bb70 ], [ %i57, %.thread3941 ]
+  %.sroa.1676.3 = phi float [ %i59, %bb68 ], [ %i59, %bb69 ], [ undef, %bb70 ], [ %i59, %.thread3941 ]
+  %.sroa.1738.3 = phi float [ %i61, %bb68 ], [ %i61, %bb69 ], [ %i61, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1800.3 = phi float [ %i63, %bb68 ], [ undef, %bb69 ], [ undef, %bb70 ], [ undef, %.thread3941 ]
+  %.sroa.1862.3 = phi float [ %i65, %bb68 ], [ undef, %bb69 ], [ %i65, %bb70 ], [ %i65, %.thread3941 ]
+  %.sroa.1924.3 = phi float [ %i67, %bb68 ], [ %i67, %bb69 ], [ undef, %bb70 ], [ %i67, %.thread3941 ]
+  ret void
+}