diff --git a/src/mono/mono/arch/amd64/amd64-codegen.h b/src/mono/mono/arch/amd64/amd64-codegen.h
index ccd82d1ac172d8..9ac73b9853c466 100644
--- a/src/mono/mono/arch/amd64/amd64-codegen.h
+++ b/src/mono/mono/arch/amd64/amd64-codegen.h
@@ -1206,6 +1206,7 @@ typedef union {
 #define amd64_alu_membase8_imm_size(inst,opc,basereg,disp,imm,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),0,0,(basereg)); x86_alu_membase8_imm((inst),(opc),((basereg)&0x7),(disp),(imm)); amd64_codegen_post(inst); } while (0)
 #define amd64_alu_mem_reg_size(inst,opc,mem,reg,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),0,0,(reg)); x86_alu_mem_reg((inst),(opc),(mem),((reg)&0x7)); amd64_codegen_post(inst); } while (0)
 #define amd64_alu_membase_reg_size(inst,opc,basereg,disp,reg,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),(reg),0,(basereg)); x86_alu_membase_reg((inst),(opc),((basereg)&0x7),(disp),((reg)&0x7)); amd64_codegen_post(inst); } while (0)
+#define amd64_alu_membase8_reg_size(inst,opc,basereg,disp,reg,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),(reg),0,(basereg)); x86_alu_membase8_reg((inst),(opc),((basereg)&0x7),(disp),((reg)&0x7)); amd64_codegen_post(inst); } while (0)
 //#define amd64_alu_reg_reg_size(inst,opc,dreg,reg,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),(dreg),0,(reg)); x86_alu_reg_reg((inst),(opc),((dreg)&0x7),((reg)&0x7)); amd64_codegen_post(inst); } while (0)
 #define amd64_alu_reg8_reg8_size(inst,opc,dreg,reg,is_dreg_h,is_reg_h,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),(dreg),0,(reg)); x86_alu_reg8_reg8((inst),(opc),((dreg)&0x7),((reg)&0x7),(is_dreg_h),(is_reg_h)); amd64_codegen_post(inst); } while (0)
 #define amd64_alu_reg_mem_size(inst,opc,reg,mem,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),0,0,(reg)); x86_alu_reg_mem((inst),(opc),((reg)&0x7),(mem)); amd64_codegen_post(inst); } while (0)
diff --git a/src/mono/mono/arch/x86/x86-codegen.h b/src/mono/mono/arch/x86/x86-codegen.h
index 74f96f81af4077..aca2b659ca058b 100644
--- a/src/mono/mono/arch/x86/x86-codegen.h
+++ b/src/mono/mono/arch/x86/x86-codegen.h
@@ -682,6 +682,13 @@ mono_x86_patch_inline (guchar* code, gpointer target)
 		x86_membase_emit ((inst), (reg), (basereg), (disp));	\
 	} while (0)
 
+#define x86_alu_membase8_reg(inst,opc,basereg,disp,reg)	\
+	do {	\
+		x86_codegen_pre(&(inst), 1 + kMaxMembaseEmitPadding); \
+		x86_byte (inst, (((unsigned char)(opc)) << 3));	\
+		x86_membase_emit ((inst), (reg), (basereg), (disp));	\
+	} while (0)
+
 #define x86_alu_reg_reg(inst,opc,dreg,reg)	\
 	do {	\
 		x86_codegen_pre(&(inst), 2); \
diff --git a/src/mono/mono/mini/mini-amd64.c b/src/mono/mono/mini/mini-amd64.c
index 3f52a94732440d..bc4eedd8f86c5e 100644
--- a/src/mono/mono/mini/mini-amd64.c
+++ b/src/mono/mono/mini/mini-amd64.c
@@ -5511,7 +5511,7 @@ mono_arch_output_basic_block (MonoCompile *cfg, MonoBasicBlock *bb)
 		}
 		case OP_CHECK_THIS:
 			/* ensure ins->sreg1 is not NULL */
-			amd64_alu_membase_imm_size (code, X86_CMP, ins->sreg1, 0, 0, 4);
+			amd64_alu_membase8_reg_size (code, X86_CMP, ins->sreg1, 0, ins->sreg1, 1);
 			break;
 		case OP_ARGLIST: {
 			amd64_lea_membase (code, AMD64_R11, cfg->frame_reg, cfg->sig_cookie);
diff --git a/src/mono/mono/mini/mini-llvm.c b/src/mono/mono/mini/mini-llvm.c
index 3d7a809c2173a8..e6224e598f6701 100644
--- a/src/mono/mono/mini/mini-llvm.c
+++ b/src/mono/mono/mini/mini-llvm.c
@@ -6856,7 +6856,7 @@ MONO_RESTORE_WARNING
 		}
 
 		case OP_CHECK_THIS:
-			LLVMBuildLoad2 (builder, IntPtrType (), convert (ctx, lhs, pointer_type (IntPtrType ())), "");
+			LLVMBuildLoad2 (builder, LLVMInt8Type (), convert (ctx, lhs, pointer_type (LLVMInt8Type ())), "");
 			break;
 		case OP_OUTARG_VTRETADDR:
 			break;
diff --git a/src/mono/mono/mini/mini-x86.c b/src/mono/mono/mini/mini-x86.c
index 6680b08f72d287..1006eabf989e54 100644
--- a/src/mono/mono/mini/mini-x86.c
+++ b/src/mono/mono/mini/mini-x86.c
@@ -3192,11 +3192,8 @@ mono_arch_output_basic_block (MonoCompile *cfg, MonoBasicBlock *bb)
 			break;
 		}
 		case OP_CHECK_THIS:
-			/* ensure ins->sreg1 is not NULL
-			 * note that cmp DWORD PTR [eax], eax is one byte shorter than
-			 * cmp DWORD PTR [eax], 0
-		         */
-			x86_alu_membase_reg (code, X86_CMP, ins->sreg1, 0, ins->sreg1);
+			/* ensure ins->sreg1 is not NULL */
+			x86_alu_membase8_reg (code, X86_CMP, ins->sreg1, 0, ins->sreg1);
 			break;
 		case OP_ARGLIST: {
 			int hreg = ins->sreg1 == X86_EAX? X86_ECX: X86_EAX;