Fix leaq corner cases and add a test

encoder.go

@@ -408,10 +408,19 @@ func encode(stmt *Stmt, keySymbol string, srcOp Operand, trgtOp Operand) (code [
 				var modRM, rm, reg byte
 				var displacementBytes []byte
 				if displacement == 0 {
-					mod := ModIndirectionWithNoDisplacement
-					reg = trgtRegi.toBits()
 					rm = srcRegi.toBits()
-					modRM = composeModRM(mod, reg, rm)
+					if rm == regBits("sp") {
+						mod := ModIndirectionWithNoDisplacement
+						reg = trgtRegi.toBits()
+						modRM = composeModRM(mod, reg, rm)
+						// There is no displacementBytes. For optimization ?
+					} else {
+						// same as isInInt8Range case below
+						mod := ModIndirectionWithDisplacement8
+						reg = trgtRegi.toBits()
+						modRM = composeModRM(mod, reg, rm)
+						displacementBytes = Bytes(0)
+					}
 				} else if isInInt8Range(int(displacement)) {
 					mod := ModIndirectionWithDisplacement8
 					reg = trgtRegi.toBits()

main_test.go

@@ -22,7 +22,9 @@ func TestEncodeStringAsText(t *testing.T) {
 
 		{"callq SYMBOL", "callq myfunc", []byte{0xe8, 0, 0, 0, 0}},
 
-		{"leaq IND, REG", "leaq 127(%rbp), %rdi", []byte{0x48, 0x8d, 0b01_111_101, 0x7f}},
+		{"leaq IND8, REG", "leaq 127(%rbp), %rdi", []byte{0x48, 0x8d, 0b01_111_101, 0x7f}},
+		{"leaq IND0, REG", "leaq 0(%rbp), %rdi", []byte{0x48, 0x8d, 0b01_111_101, 0}},
+		{"leaq IND0, REG", " leaq 0(%rsp), %rdi", []byte{0x48, 0x8d, 0b00_111_100, 0b00_100_100}},
 		{"movb REG, IND", "movb %bl, 0(%rdi)", []byte{0x88, 0b00_011_111}},
 		{"movw REG, IND", "movw %bx, 0(%rdi)", []byte{0x66, 0x89, 0b00_011_111}},
 		{"movl IMM32, REG", "movl $2147483647, %ebx", []byte{0xb8 + 0b011, 0xff, 0xff, 0xff, 0x7f}},