Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -5369,6 +5369,25 @@
     return new ShuffleVectorInst(NewCmp, UndefValue::get(NewCmp->getType()), M);
   }
 
+  // Try to canonicalize compare with splatted operand and splat constant.
+  Constant *C;
+  if (!LHS->hasOneUse() || !match(RHS, m_Constant(C)))
+    return nullptr;
+
+  // Length-changing splats are ok, so adjust the constants as needed:
+  // cmp (shuffle V1, M), C --> shuffle (cmp V1, C'), M
+  Constant *ScalarM = M->getSplatValue(/* AllowUndefs */ true);
+  Constant *ScalarC = C->getSplatValue(/* AllowUndefs */ true);
+  if (ScalarM && ScalarC) {
+    // We allow undefs in matching, but this transform removes those for safety.
+    // Demanded elements analysis should be able to recover some/all of that.
+    C = ConstantVector::getSplat(V1Ty->getVectorNumElements(), ScalarC);
+    M = ConstantVector::getSplat(M->getType()->getVectorNumElements(), ScalarM);
+    Value *NewCmp = IsFP ? Builder.CreateFCmp(Pred, V1, C)
+                         : Builder.CreateICmp(Pred, V1, C);
+    return new ShuffleVectorInst(NewCmp, UndefValue::get(NewCmp->getType()), M);
+  }
+
   return nullptr;
 }
 
Index: llvm/test/Transforms/InstCombine/gep-inbounds-null.ll
===================================================================
--- llvm/test/Transforms/InstCombine/gep-inbounds-null.ll
+++ llvm/test/Transforms/InstCombine/gep-inbounds-null.ll
@@ -91,8 +91,8 @@
 ; CHECK-LABEL: @test_vector_index(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i8*> undef, i8* [[BASE:%.*]], i32 0
-; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <2 x i8*> [[DOTSPLATINSERT]], <2 x i8*> undef, <2 x i32> zeroinitializer
-; CHECK-NEXT:    [[CND:%.*]] = icmp eq <2 x i8*> [[DOTSPLAT]], zeroinitializer
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq <2 x i8*> [[DOTSPLATINSERT]], zeroinitializer
+; CHECK-NEXT:    [[CND:%.*]] = shufflevector <2 x i1> [[TMP0]], <2 x i1> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[CND]]
 ;
 entry:
Index: llvm/test/Transforms/InstCombine/getelementptr.ll
===================================================================
--- llvm/test/Transforms/InstCombine/getelementptr.ll
+++ llvm/test/Transforms/InstCombine/getelementptr.ll
@@ -217,8 +217,8 @@
 ; CHECK-LABEL: @test13_vector2(
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i64> undef, i64 [[X:%.*]], i32 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 2, i64 undef>
-; CHECK-NEXT:    [[A_IDX:%.*]] = shufflevector <2 x i64> [[TMP1]], <2 x i64> undef, <2 x i32> zeroinitializer
-; CHECK-NEXT:    [[C:%.*]] = icmp eq <2 x i64> [[A_IDX]], <i64 -4, i64 -4>
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq <2 x i64> [[TMP1]], <i64 -4, i64 -4>
+; CHECK-NEXT:    [[C:%.*]] = shufflevector <2 x i1> [[TMP2]], <2 x i1> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
 ;
   %A = getelementptr inbounds %S, <2 x %S*> %P, <2 x i64> zeroinitializer, <2 x i32> <i32 1, i32 1>, i64 %X
@@ -232,8 +232,8 @@
 ; CHECK-LABEL: @test13_vector3(
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i64> undef, i64 [[X:%.*]], i32 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 2, i64 undef>
-; CHECK-NEXT:    [[A_IDX:%.*]] = shufflevector <2 x i64> [[TMP1]], <2 x i64> undef, <2 x i32> zeroinitializer
-; CHECK-NEXT:    [[C:%.*]] = icmp eq <2 x i64> [[A_IDX]], <i64 4, i64 4>
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq <2 x i64> [[TMP1]], <i64 4, i64 4>
+; CHECK-NEXT:    [[C:%.*]] = shufflevector <2 x i1> [[TMP2]], <2 x i1> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
 ;
   %A = getelementptr inbounds %S, <2 x %S*> %P, <2 x i64> zeroinitializer, <2 x i32> <i32 1, i32 1>, i64 %X
Index: llvm/test/Transforms/InstCombine/icmp-vec.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp-vec.ll
+++ llvm/test/Transforms/InstCombine/icmp-vec.ll
@@ -291,8 +291,8 @@
 
 define <4 x i1> @splat_icmp(<4 x i8> %x) {
 ; CHECK-LABEL: @splat_icmp(
-; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <4 x i8> [[X:%.*]], <4 x i8> undef, <4 x i32> <i32 3, i32 3, i32 3, i32 3>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <4 x i8> [[SPLATX]], <i8 42, i8 42, i8 42, i8 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt <4 x i8> [[X:%.*]], <i8 42, i8 42, i8 42, i8 42>
+; CHECK-NEXT:    [[CMP:%.*]] = shufflevector <4 x i1> [[TMP1]], <4 x i1> undef, <4 x i32> <i32 3, i32 3, i32 3, i32 3>
 ; CHECK-NEXT:    ret <4 x i1> [[CMP]]
 ;
   %splatx = shufflevector <4 x i8> %x, <4 x i8> undef, <4 x i32> <i32 3, i32 3, i32 3, i32 3>
@@ -302,8 +302,8 @@
 
 define <4 x i1> @splat_icmp_undef(<4 x i8> %x) {
 ; CHECK-LABEL: @splat_icmp_undef(
-; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <4 x i8> [[X:%.*]], <4 x i8> undef, <4 x i32> <i32 2, i32 undef, i32 undef, i32 2>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult <4 x i8> [[SPLATX]], <i8 undef, i8 42, i8 undef, i8 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <4 x i8> [[X:%.*]], <i8 42, i8 42, i8 42, i8 42>
+; CHECK-NEXT:    [[CMP:%.*]] = shufflevector <4 x i1> [[TMP1]], <4 x i1> undef, <4 x i32> <i32 2, i32 2, i32 2, i32 2>
 ; CHECK-NEXT:    ret <4 x i1> [[CMP]]
 ;
   %splatx = shufflevector <4 x i8> %x, <4 x i8> undef, <4 x i32> <i32 2, i32 undef, i32 undef, i32 2>
@@ -313,8 +313,8 @@
 
 define <4 x i1> @splat_icmp_larger_size(<2 x i8> %x) {
 ; CHECK-LABEL: @splat_icmp_larger_size(
-; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <2 x i8> [[X:%.*]], <2 x i8> undef, <4 x i32> <i32 1, i32 undef, i32 1, i32 undef>
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <4 x i8> [[SPLATX]], <i8 42, i8 42, i8 undef, i8 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 42, i8 42>
+; CHECK-NEXT:    [[CMP:%.*]] = shufflevector <2 x i1> [[TMP1]], <2 x i1> undef, <4 x i32> <i32 1, i32 1, i32 1, i32 1>
 ; CHECK-NEXT:    ret <4 x i1> [[CMP]]
 ;
   %splatx = shufflevector <2 x i8> %x, <2 x i8> undef, <4 x i32> <i32 1, i32 undef, i32 1, i32 undef>
@@ -324,8 +324,8 @@
 
 define <4 x i1> @splat_fcmp_smaller_size(<5 x float> %x) {
 ; CHECK-LABEL: @splat_fcmp_smaller_size(
-; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <5 x float> [[X:%.*]], <5 x float> undef, <4 x i32> <i32 1, i32 undef, i32 1, i32 undef>
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp oeq <4 x float> [[SPLATX]], <float 4.200000e+01, float 4.200000e+01, float undef, float 4.200000e+01>
+; CHECK-NEXT:    [[TMP1:%.*]] = fcmp oeq <5 x float> [[X:%.*]], <float 4.200000e+01, float 4.200000e+01, float 4.200000e+01, float 4.200000e+01, float 4.200000e+01>
+; CHECK-NEXT:    [[CMP:%.*]] = shufflevector <5 x i1> [[TMP1]], <5 x i1> undef, <4 x i32> <i32 1, i32 1, i32 1, i32 1>
 ; CHECK-NEXT:    ret <4 x i1> [[CMP]]
 ;
   %splatx = shufflevector <5 x float> %x, <5 x float> undef, <4 x i32> <i32 1, i32 undef, i32 1, i32 undef>
@@ -333,6 +333,8 @@
   ret <4 x i1> %cmp
 }
 
+; Negative test
+
 define <4 x i1> @splat_icmp_extra_use(<4 x i8> %x) {
 ; CHECK-LABEL: @splat_icmp_extra_use(
 ; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <4 x i8> [[X:%.*]], <4 x i8> undef, <4 x i32> <i32 3, i32 3, i32 3, i32 3>
@@ -346,6 +348,8 @@
   ret <4 x i1> %cmp
 }
 
+; Negative test
+
 define <4 x i1> @not_splat_icmp(<4 x i8> %x) {
 ; CHECK-LABEL: @not_splat_icmp(
 ; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <4 x i8> [[X:%.*]], <4 x i8> undef, <4 x i32> <i32 3, i32 2, i32 3, i32 3>
@@ -357,6 +361,8 @@
   ret <4 x i1> %cmp
 }
 
+; Negative test
+
 define <4 x i1> @not_splat_icmp2(<4 x i8> %x) {
 ; CHECK-LABEL: @not_splat_icmp2(
 ; CHECK-NEXT:    [[SPLATX:%.*]] = shufflevector <4 x i8> [[X:%.*]], <4 x i8> undef, <4 x i32> <i32 2, i32 2, i32 2, i32 2>