CB (immediate)

Compare register with immediate and branch

This instruction compares the value in a register with an immediate, and conditionally branches to a label at a PC-relative offset if the comparison is true. It provides a hint that this is not a subroutine call or return. This instruction does not affect the condition flags.

Encoding: Branch

Variants: FEAT_CMPBR (ARMv9.6)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
	1	1	1	0	1	0	1										0
sf								cc			imm6							imm9									Rt

32-bit greater than (sf == 0 && cc == 000)

CBGT <Wt>, #<imm>, <label>

32-bit less than (sf == 0 && cc == 001)

CBLT <Wt>, #<imm>, <label>

32-bit higher (sf == 0 && cc == 010)

CBHI <Wt>, #<imm>, <label>

32-bit lower (sf == 0 && cc == 011)

CBLO <Wt>, #<imm>, <label>

32-bit equal (sf == 0 && cc == 110)

CBEQ <Wt>, #<imm>, <label>

32-bit not equal (sf == 0 && cc == 111)

CBNE <Wt>, #<imm>, <label>

64-bit greater than (sf == 1 && cc == 000)

CBGT <Xt>, #<imm>, <label>

64-bit less than (sf == 1 && cc == 001)

CBLT <Xt>, #<imm>, <label>

64-bit higher (sf == 1 && cc == 010)

CBHI <Xt>, #<imm>, <label>

64-bit lower (sf == 1 && cc == 011)

CBLO <Xt>, #<imm>, <label>

64-bit equal (sf == 1 && cc == 110)

CBEQ <Xt>, #<imm>, <label>

64-bit not equal (sf == 1 && cc == 111)

CBNE <Xt>, #<imm>, <label>

Decoding algorithm

if !IsFeatureImplemented(FEAT_CMPBR) then EndOfDecode(Decode_UNDEF);
constant integer datasize = 32 << UInt(sf);
constant integer t = UInt(Rt);
constant bits(64) offset = SignExtend(imm9:'00', 64);
CmpOp op;
boolean unsigned;

case cc of
    when '000' op = Cmp_GT; unsigned = FALSE;
    when '001' op = Cmp_LT; unsigned = FALSE;
    when '010' op = Cmp_GT; unsigned = TRUE;
    when '011' op = Cmp_LT; unsigned = TRUE;
    when '110' op = Cmp_EQ; unsigned = TRUE;
    when '111' op = Cmp_NE; unsigned = TRUE;
    otherwise EndOfDecode(Decode_UNDEF);
constant integer value2 = UInt(imm6);

Operation

constant bits(datasize) operand1 = X[t, datasize];
constant boolean branch_conditional = TRUE;

constant integer value1 = Int(operand1, unsigned);
boolean cond;
case op of
    when Cmp_EQ cond = value1 == value2;
    when Cmp_NE cond = value1 != value2;
    when Cmp_GE cond = value1 >= value2;
    when Cmp_LT cond = value1 <  value2;
    when Cmp_GT cond = value1 >  value2;
    when Cmp_LE cond = value1 <= value2;

if cond then
    BranchTo(PC64 + offset, BranchType_DIR, branch_conditional);
else
    BranchNotTaken(BranchType_DIR, branch_conditional);

Explanations

<Wt>: Is the 32-bit name of the general-purpose register to be tested, encoded in the "Rt" field.
<imm>: Is an unsigned immediate, in the range 0 to 63, encoded in the "imm6" field.
<label>: Is the program label to be conditionally branched to. Its offset from the address of this instruction, in the range -1024 to 1020, is encoded as "imm9" times 4.
<Xt>: Is the 64-bit name of the general-purpose register to be tested, encoded in the "Rt" field.