RISC-V Options

These command-line options are defined for RISC-V targets:

-mbranch-cost=n

Set the cost of branches to roughly n instructions.

-mplt, -mno-plt

When generating PIC code, do or don’t allow the use of PLTs. Ignored for non-PIC. The default is -mplt.

-mabi=ABI-string

Specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers.

The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There is also the ilp32e ABI that can only be used with the rv32e architecture. This ABI is not well specified at present, and is subject to change.

-mfdiv, -mno-fdiv

Do or don’t use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.

-mdiv, -mno-div

Do or don’t use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.

-misa-spec=ISA-spec-string

Specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for ISA-spec-string are:

2.2

Produce code conforming to version 2.2.

20190608

Produce code conforming to version 20190608.

20191213

Produce code conforming to version 20191213.

The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version.

-march=ISA-string

Generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf.

When -march= is not specified, use the setting from -mcpu.

If both -march and -mcpu= are not specified, the default for this argument is system dependent, users who want a specific architecture extensions should specify one explicitly.

-mcpu=processor-string

Use architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are: sifive-e20, sifive-e21, sifive-e24, sifive-e31, sifive-e34, sifive-e76, sifive-s21, sifive-s51, sifive-s54, sifive-s76, sifive-u54, and sifive-u74.

-mtune=processor-string

Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, thead-c906, size, and all valid options for -mcpu=.

When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both are not specified.

The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size while still giving good performance.

-mpreferred-stack-boundary=num

Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128-bits).

Warning

If you use this switch, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.

-msmall-data-limit=n

Put global and static data smaller than n bytes into a special section (on some targets).

-msave-restore, -mno-save-restore

Do or don’t use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues.

-mshorten-memrefs, -mno-shorten-memrefs

Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of ‘base register + large offset’ with a new load/store of ‘new base + small offset’. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only.

-mstrict-align, -mno-strict-align

Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.

-mcmodel=medlow

Generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model.

-mcmodel=medany

Generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.

-mexplicit-relocs, -mno-exlicit-relocs

Use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization.

-mrelax, -mno-relax

Take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations.

-mriscv-attribute, -mno-riscv-attribute

Emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least binutils 2.32.

-mcsr-check, -mno-csr-check

Enables or disables the CSR checking.

-malign-data=type

Control how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen which uses x register width as the alignment value, and natural which uses natural alignment. xlen is the default.

-mbig-endian

Generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target.

-mlittle-endian

Generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* but not a riscv64be-*-* or riscv32be-*-* target.

-mstack-protector-guard=guard

Generate stack protection code using canary at guard. Supported locations are global for a global canary or tls for per-thread canary in the TLS block.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel.