This instruction calculates a 2-bit value by rotating the source register right by `imm32` bits and taking the 2 least significant bits (the value of the source register is unaffected). The result is stored in the `fprc` register. This changes the rounding mode of all subsequent floating point instructions.
This instruction calculates a 2-bit value by rotating the source register right by `imm32` bits and taking the 2 least significant bits (the value of the source register is unaffected). The result is stored in the `fprc` register. This changes the rounding mode of all subsequent floating point instructions.
#### 5.4.2 CBRANCH
#### 5.4.2 CBRANCH
This instruction performs a conditional jump in the Program Buffer. It uses an implicit integer register operand `creg`. This register is determined based on preceding instructions. For this purpose, the VM assigns each integer register two tag values:
This instruction adds an immediate value `cimm` (constructed from `imm32`, see below) to the destination register and then performs a conditional jump in the Program Buffer based on the value of the destination register. The target of the jump is the instruction following the instruction when register `dst` was last modified.
* `lastUsed` - the index of the instruction when the register was last modified. The initial value at the start of each program iteration is `-1`, meaning the register is unmodified.
At the beginning of each program iteration, all registers are considered to be unmodified. A register is considered as modified by an instruction in the following cases:
* `count` - the number of times the register has been selected as the operand of a CBRANCH instruction. The initial value at the start of each program iteration is `0`.
A register is considered as modified by an instruction in the following cases:
* It is the destination register of an integer instruction except IMUL_RCP and ISWAP_R.
* It is the destination register of an integer instruction except IMUL_RCP and ISWAP_R.
* It is the destination register of IMUL_RCP and `imm32` is not zero or a power of 2.
* It is the destination register of IMUL_RCP and `imm32` is not zero or a power of 2.
* It is the source or the destination register of ISWAP_R and the destination and source registers are distinct.
* It is the source or the destination register of ISWAP_R and the destination and source registers are distinct.
* The CBRANCH instruction is considered to modify all integer registers.
* The CBRANCH instruction is considered to modify all integer registers.
There are 3 rules for the selection of the `creg` register, evaluated in this order:
If register `dst` has not been modified yet, the jump target is the first instruction in the Program Buffer.
1. The register with the lowest value of `lastUsed` tag is selected.
1. In case multiple registers have the same value of the `lastUsed` tag, the register with the lowest value of the `count` tag is selected from them.
1. In case multiple registers have the same values of both `lastUsed` and `count` tags, the register with the lowest index is selected (`r0` before `r1` etc.) from them.
Whenever a register is selected as the operand of a CBRANCH instruction, its `count` tag is increased by 1.
The CBRANCH instruction performs the following steps:
The CBRANCH instruction performs the following steps:
1. A constant `b` is calculated as `mod.cond + RANDOMX_JUMP_OFFSET`.
1. A constant `b` is calculated as `mod.cond + RANDOMX_JUMP_OFFSET`.
1. A constant `cimm` is constructed as sign-extended `imm32` with bit `b` set to 1 and bit `b-1` set to 0 (if `b > 0`).
1. A constant `cimm` is constructed as sign-extended `imm32` with bit `b` set to 1 and bit `b-1` set to 0 (if `b > 0`).
1. `cimm` is added to `creg`.
1. `cimm` is added to the destination register.
1. If bits `b` to `b + RANDOMX_JUMP_BITS - 1` of `creg` are zero, execution jumps to instruction `creg.lastUsed + 1` (the instruction following the instruction where `creg` was last modified).
1. If bits `b` to `b + RANDOMX_JUMP_BITS - 1` of the destination register are zero, the jump is executed (target is the instruction following the instruction where `dst` was last modified).
Bits in immediate and register values are numbered from 0 to 63 with 0 being the least significant bit. For example, for `b = 10` and `RANDOMX_JUMP_BITS = 8`, the bits are arranged like this:
Bits in immediate and register values are numbered from 0 to 63 with 0 being the least significant bit. For example, for `b = 10` and `RANDOMX_JUMP_BITS = 8`, the bits are arranged like this:
`S` is a copied sign bit from `imm32`. `M` denotes bits of `imm32`. The 9th bit is set to 0 and the 10th bit is set to 1. This value would be added to `creg`.
`S` is a copied sign bit from `imm32`. `M` denotes bits of `imm32`. The 9th bit is set to 0 and the 10th bit is set to 1. This value will be added to `dst`.
The second line uses `X` to mark bits of `creg` that would be checked by the condition. If all these bits are 0 after adding `cimm`, the jump is executed.
The second line uses `X` to mark bits of `dst` that will be checked by the condition. If all these bits are 0 after adding `cimm`, the jump is executed.
The construction of the CBRANCH instruction ensures that no inifinite loops are possible in the program.
The construction of the CBRANCH instruction ensures that no inifinite loops are possible in the program.
tevador
5 years ago