"Zicond" Extension for Integer Conditional Operations, Version 1.0.0
Introduction
The Zicond extension defines a simple solution that provides most of the benefit and all of the flexibility one would desire to support conditional arithmetic and conditional-select/move operations, while remaining true to the RISC-V design philosophy. The instructions follow the format for R-type instructions with 3 operands (i.e., 2 source operands and 1 destination operand). Using these instructions, branchless sequences can be implemented (typically in two-instruction sequences) without the need for instruction fusion, special provisions during the decoding of architectural instructions, or other microarchitectural provisions.
One of the shortcomings of RISC-V, compared to competing instruction set architectures, is the absence of conditional operations to support branchless code-generation: this includes conditional arithmetic, conditional select and conditional move operations. The design principles of RISC-V (e.g. the absence of an instruction-format that supports 3 source registers and an output register) make it unlikely that direct equivalents of the competing instructions will be introduced.
Yet, low-cost conditional instructions are a desirable feature as they allow the replacement of branches in a broad range of suitable situations (whether the branch turns out to be unpredictable or predictable) so as to reduce the capacity and aliasing pressures on BTBs and branch predictors, and to allow for longer basic blocks (for both the hardware and the compiler to work with).
Zicond specification
The "Conditional" operations extension provides a simple solution that provides most of the benefit and all of the flexibility one would desire to support conditional arithmetic and conditional-select/move operations, while remaining true to the RISC-V design philosophy. The instructions follow the format for R-type instructions with 3 operands (i.e., 2 source operands and 1 destination operand). Using these instructions, branchless sequences can be implemented (typically in two-instruction sequences) without the need for instruction fusion, special provisions during the decoding of architectural instructions, or other microarchitectural provisions.
The following instructions comprise the Zicond extension:
| RV32 | RV64 | Mnemonic | Instruction |
|---|---|---|---|
✓ |
✓ |
czero.eqz rd, rs1, rs2 |
|
✓ |
✓ |
czero.nez rd, rs1, rs2 |
|
Architecture Comment: defining additional comparisons, in addition to equal-to-zero and not-equal-to-zero, does not offer a benefit due to the lack of immediates or an additional register operand that the comparison takes place against. |
Based on these two instructions, synthetic instructions (i.e., short instruction sequences) for the following conditional arithmetic operations are supported:
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conditional add, if zero
-
conditional add, if non-zero
-
conditional subtract, if zero
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conditional subtract, if non-zero
-
conditional bitwise-and, if zero
-
conditional bitwise-and, if non-zero
-
conditional bitwise-or, if zero
-
conditional bitwise-or, if non-zero
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conditional bitwise-xor, if zero
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conditional bitwise-xor, if non-zero
Additionally, the following conditional select instructions are supported:
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conditional-select, if zero
-
conditional-select, if non-zero
More complex conditions, such as comparisons against immediates, registers, single-bit tests, comparisons against ranges, etc. can be realized by composing these new instructions with existing instructions.
Instructions (in alphabetical order)
czero.eqz
- Synopsis
-
Moves zero to a register rd, if the condition rs2 is equal to zero, otherwise moves rs1 to rd.
- Mnemonic
-
czero.eqz rd, rs1, rs2
- Encoding
- Description
-
If rs2 contains the value zero, this instruction writes the value zero to rd. Otherwise, this instruction copies the contents of rs1 to rd.
This instruction carries a syntactic dependency from both rs1 and rs2 to rd. Furthermore, if the Zkt extension is implemented, this instruction’s timing is independent of the data values in rs1 and rs2.
- SAIL code
let condition = X(rs2);
result : xlenbits = if (condition == zeros()) then zeros()
else X(rs1);
X(rd) = result;czero.nez
- Synopsis
-
Moves zero to a register rd, if the condition rs2 is nonzero, otherwise moves rs1 to rd.
- Mnemonic
-
czero.nez rd, rs1, rs2
- Encoding
- Description
-
If rs2 contains a nonzero value, this instruction writes the value zero to rd. Otherwise, this instruction copies the contents of rs1 to rd.
This instruction carries a syntactic dependency from both rs1 and rs2 to rd. Furthermore, if the Zkt extension is implemented, this instruction’s timing is independent of the data values in rs1 and rs2.
- SAIL code
let condition = X(rs2);
result : xlenbits = if (condition != zeros()) then zeros()
else X(rs1);
X(rd) = result;Usage examples
The instructions from this extension can be used to construct sequences that perform conditional-arithmetic, conditional-bitwise-logical, and conditional-select operations.
Instruction sequences
| Operation | Instruction sequence | Length |
|---|---|---|
Conditional add, if zero |
czero.nez rd, rs2, rc add rd, rs1, rd |
2 insns |
Conditional add, if non-zero |
czero.eqz rd, rs2, rc add rd, rs1, rd |
|
Conditional subtract, if zero |
czero.nez rd, rs2, rc sub rd, rs1, rd |
|
Conditional subtract, if non-zero |
czero.eqz rd, rs2, rc sub rd, rs1, rd |
|
Conditional bitwise-or, if zero |
czero.nez rd, rs2, rc or rd, rs1, rd |
|
Conditional bitwise-or, if non-zero |
czero.eqz rd, rs2, rc or rd, rs1, rd |
|
Conditional bitwise-xor, if zero |
czero.nez rd, rs2, rc xor rd, rs1, rd |
|
Conditional bitwise-xor, if non-zero |
czero.eqz rd, rs2, rc xor rd, rs1, rd |
|
Conditional bitwise-and, if zero |
and rd, rs1, rs2 czero.eqz rtmp, rs1, rc or rd, rd, rtmp |
3 insns |
Conditional bitwise-and, if non-zero |
and rd, rs1, rs2 czero.nez rtmp, rs1, rc or rd, rd, rtmp |
|
Conditional select, if zero |
czero.nez rd, rs1, rc czero.eqz rtmp, rs2, rc or rd, rd, rtmp |
|
Conditional select, if non-zero |
czero.eqz rd, rs1, rc czero.nez rtmp, rs2, rc or rd, rd, rtmp |