Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -18128,6 +18128,9 @@
   X86::CondCode X86CC;
   if (isNullConstant(Op1)) {
     X86CC = CC == ISD::SETEQ ? X86::COND_E : X86::COND_NE;
+  } else if (isAllOnesConstant(Op1)) {
+    // C flag is set for all ones.
+    X86CC = CC == ISD::SETEQ ? X86::COND_B : X86::COND_AE;
   } else
     return SDValue();
 
Index: test/CodeGen/X86/avx512-mask-op.ll
===================================================================
--- test/CodeGen/X86/avx512-mask-op.ll
+++ test/CodeGen/X86/avx512-mask-op.ll
@@ -2836,3 +2836,82 @@
 }
 declare void @foo()
 
+; Make sure we don't emit a ktest for signed comparisons.
+define void @ktest_allones(<16 x i32> %x, <16 x i32> %y) {
+; KNL-LABEL: ktest_allones:
+; KNL:       ## %bb.0:
+; KNL-NEXT:    pushq %rax
+; KNL-NEXT:    .cfi_def_cfa_offset 16
+; KNL-NEXT:    vporq %zmm1, %zmm0, %zmm0
+; KNL-NEXT:    vptestnmd %zmm0, %zmm0, %k0
+; KNL-NEXT:    kmovw %k0, %eax
+; KNL-NEXT:    cmpl $65535, %eax ## imm = 0xFFFF
+; KNL-NEXT:    je LBB65_2
+; KNL-NEXT:  ## %bb.1: ## %bb.1
+; KNL-NEXT:    vzeroupper
+; KNL-NEXT:    callq _foo
+; KNL-NEXT:  LBB65_2: ## %bb.2
+; KNL-NEXT:    popq %rax
+; KNL-NEXT:    vzeroupper
+; KNL-NEXT:    retq
+;
+; SKX-LABEL: ktest_allones:
+; SKX:       ## %bb.0:
+; SKX-NEXT:    pushq %rax
+; SKX-NEXT:    .cfi_def_cfa_offset 16
+; SKX-NEXT:    vporq %zmm1, %zmm0, %zmm0
+; SKX-NEXT:    vptestnmd %zmm0, %zmm0, %k0
+; SKX-NEXT:    ktestw %k0, %k0
+; SKX-NEXT:    jb LBB65_2
+; SKX-NEXT:  ## %bb.1: ## %bb.1
+; SKX-NEXT:    vzeroupper
+; SKX-NEXT:    callq _foo
+; SKX-NEXT:  LBB65_2: ## %bb.2
+; SKX-NEXT:    popq %rax
+; SKX-NEXT:    vzeroupper
+; SKX-NEXT:    retq
+;
+; AVX512BW-LABEL: ktest_allones:
+; AVX512BW:       ## %bb.0:
+; AVX512BW-NEXT:    pushq %rax
+; AVX512BW-NEXT:    .cfi_def_cfa_offset 16
+; AVX512BW-NEXT:    vporq %zmm1, %zmm0, %zmm0
+; AVX512BW-NEXT:    vptestnmd %zmm0, %zmm0, %k0
+; AVX512BW-NEXT:    kmovw %k0, %eax
+; AVX512BW-NEXT:    cmpl $65535, %eax ## imm = 0xFFFF
+; AVX512BW-NEXT:    je LBB65_2
+; AVX512BW-NEXT:  ## %bb.1: ## %bb.1
+; AVX512BW-NEXT:    vzeroupper
+; AVX512BW-NEXT:    callq _foo
+; AVX512BW-NEXT:  LBB65_2: ## %bb.2
+; AVX512BW-NEXT:    popq %rax
+; AVX512BW-NEXT:    vzeroupper
+; AVX512BW-NEXT:    retq
+;
+; AVX512DQ-LABEL: ktest_allones:
+; AVX512DQ:       ## %bb.0:
+; AVX512DQ-NEXT:    pushq %rax
+; AVX512DQ-NEXT:    .cfi_def_cfa_offset 16
+; AVX512DQ-NEXT:    vporq %zmm1, %zmm0, %zmm0
+; AVX512DQ-NEXT:    vptestnmd %zmm0, %zmm0, %k0
+; AVX512DQ-NEXT:    ktestw %k0, %k0
+; AVX512DQ-NEXT:    jb LBB65_2
+; AVX512DQ-NEXT:  ## %bb.1: ## %bb.1
+; AVX512DQ-NEXT:    vzeroupper
+; AVX512DQ-NEXT:    callq _foo
+; AVX512DQ-NEXT:  LBB65_2: ## %bb.2
+; AVX512DQ-NEXT:    popq %rax
+; AVX512DQ-NEXT:    vzeroupper
+; AVX512DQ-NEXT:    retq
+  %a = icmp eq <16 x i32> %x, zeroinitializer
+  %b = icmp eq <16 x i32> %y, zeroinitializer
+  %c = and <16 x i1> %a, %b
+  %d = bitcast <16 x i1> %c to i16
+  %e = icmp eq i16 %d, -1
+  br i1 %e, label %bb.2, label %bb.1
+bb.1:
+  call void @foo()
+  br label %bb.2
+bb.2:
+  ret void
+}