[[riscv-cc]]
= RISC-V Calling Conventions
ifeval::["{docname}" == "riscv-cc"]
include::prelude.adoc[]
endif::[]

== Register Convention

=== Integer Register Convention

.Integer register convention
[cols="1,1,2,2"]
|===
| Name      | ABI Mnemonic | Meaning                | Preserved across calls?

| x0        | zero         | Zero                   | -- (Immutable)
| x1        | ra           | Return address         | No
| x2        | sp           | Stack pointer          | Yes
| x3        | gp           | Global pointer         | -- (Unallocatable)
| x4        | tp           | Thread pointer         | -- (Unallocatable)
| x5 - x7   | t0 - t2      | Temporary registers    | No
| x8 - x9   | s0 - s1      | Callee-saved registers | Yes
| x10 - x17 | a0 - a7      | Argument registers     | No
| x18 - x27 | s2 - s11     | Callee-saved registers | Yes
| x28 - x31 | t3 - t6      | Temporary registers    | No
|===

In the standard ABI, procedures should not modify the integer registers tp and
gp, because signal handlers may rely upon their values.

The presence of a frame pointer is optional.  If a frame pointer exists,
it must reside in x8 (s0); the register remains callee-saved.

If a platform requires use of a dedicated general-purpose register for a
platform-specific purpose, it is recommended to use gp (x3). The platform ABI
specification must document the use of this register. For such platforms, care
must be taken to ensure all code (compiler generated or otherwise) avoids using
gp in a way incompatible with the platform specific purpose, and that global
pointer relaxation is disabled in the toolchain.

=== Frame Pointer Convention

The presence of a frame pointer is optional.  If a frame pointer exists,
it must reside in x8 (s0); the register remains callee-saved.

Code that uses a frame pointer will construct a linked list of stack frames,
where each frame links to its caller using a "frame record". A frame record
consists of two XLEN values on the stack; the return address and the link to
the next frame record. The frame pointer register will point to the innermost
frame, thereby starting the linked list. By convention, the lowest XLEN value
shall point to the previous frame, while the next XLEN value shall be the
return address. The end of the frame record chain is indicated by the address
zero appearing as the next link in the chain.

After the prologue, the frame pointer register will point to the Canonical
Frame Address or CFA, which is the stack pointer value on entry to the current
procedure. The previous frame pointer and return address pair will reside just
prior to the current stack address held in `fp`. This puts the return address
at `fp - XLEN/8`, and the previous frame pointer at `fp - 2 * XLEN/8`.

It is left to the platform to determine the level of conformance with this
convention. A platform may choose:

- not to maintain a frame chain and use the frame pointer register as a general
  purpose callee-saved register.

- to allow the frame pointer register be used as a general purpose callee-saved
  register, but provide a platform specific mechanism to reliably detect this
  condition.

- to use a frame pointer to address a valid frame record at all times, but
  allow any procedure to choose to forgo creating a frame record.

- to use the frame pointer to address a valid frame record at all times, except
  leaf functions, who may elect to forgo creating a frame record.

=== Floating-point Register Convention

.Floating-point register convention
[cols="1,1,2,2"]
|===
| Name      | ABI Mnemonic | Meaning                | Preserved across calls?

| f0 - f7   | ft0 - ft7    | Temporary registers    | No
| f8 - f9   | fs0 - fs1    | Callee-saved registers | Yes*
| f10 - f17 | fa0 - fa7    | Argument registers     | No
| f18 - f27 | fs2 - fs11   | Callee-saved registers | Yes*
| f28 - f31 | ft8 - ft11   | Temporary registers    | No
|===

*: Floating-point values in callee-saved registers are only preserved across
calls if they are no larger than the width of a floating-point register in the
targeted ABI. Therefore, these registers can always be considered temporaries
if targeting the base integer calling convention.

The Floating-Point Control and Status Register (fcsr) must have thread storage
duration in accordance with C11 section 7.6 "Floating-point environment
<fenv.h>".

=== Vector Register Convention

==== Standard calling convention
.Standard vector register calling convention
[%autowidth]
|===
| Name    | ABI Mnemonic | Meaning                      | Preserved across calls?

| v0-v31  |              | Temporary registers          | No
| vl      |              | Vector length                | No
| vtype   |              | Vector data type register    | No
| vxrm    |              | Vector fixed-point rounding mode register    | No
| vxsat   |              | Vector fixed-point saturation flag register  | No
|===

NOTE: Vector registers are not used for passing arguments or return values in this 
calling convention. Use the calling convention variant to pass arguments and return 
values in vector registers.

The `vxrm` and `vxsat` fields of `vcsr` are not preserved across calls and their
values are unspecified upon entry.

Procedures may assume that `vstart` is zero upon entry. Procedures may assume
that `vstart` is zero upon return from a procedure call.

NOTE: Application software should normally not write `vstart` explicitly.
Any procedure that does explicitly write `vstart` to a nonzero value must zero
`vstart` before either returning or calling another procedure.

==== Calling convention variant
.Variant vector register calling convention*
[%autowidth]
|===
| Name    | ABI Mnemonic | Meaning                      | Preserved across calls?

| v0      |              | Argument register            | No
| v1-v7   |              | Callee-saved registers       | Yes
| v8-v23  |              | Argument registers           | No
| v24-v31 |              | Callee-saved registers       | Yes
| vl      |              | Vector length                | No
| vtype   |              | Vector data type register    | No
| vxrm    |              | Vector fixed-point rounding mode register    | No
| vxsat   |              | Vector fixed-point saturation flag register  | No
|===

*: Functions that use vector registers to pass arguments and return values must
follow this calling convention. Some programming languages can require extra
functions to follow this calling convention (e.g. C/C++ functions with
attribute `riscv_vector_cc`).

Please refer to the <<Standard Vector Calling Convention Variant>> section for
more details about standard vector calling convention variant.

NOTE: The `vxrm` and `vxsat` fields of `vcsr` follow the same behavior as the
standard calling convention.

== Procedure Calling Convention

This chapter defines standard calling conventions and standard calling
convention variants, and describes how to pass arguments and return values.

Functions must follow the register convention defined in calling convention: the
contents of any register without specifying it as an argument register
in the calling convention are unspecified upon entry, and the content of any
register without specifying it as a return value register or callee-saved in
the calling convention are unspecified upon exit, the contents of all
callee-saved registers must be restored to what was set on entry, and the
contents of any fixed registers like `gp` and `tp` never change,


NOTE: Calling convention for big-endian is *NOT* included in this specification
yet, we intend to define that in future version of this specification.

[#integer-cc]
=== Integer Calling Convention

The base integer calling convention provides eight argument registers,
a0-a7, the first two of which are also used to return values.

Scalars that are at most XLEN bits wide are passed in a single argument
register, or on the stack by value if none is available.
When passed in registers or on the stack, integer scalars narrower than XLEN
bits are widened according to the sign of their type up to 32 bits, then
sign-extended to XLEN bits.
When passed in registers or on the stack, floating-point types narrower than
XLEN bits are widened to XLEN bits, with the upper bits undefined.

Scalars that are 2×XLEN bits wide are passed in a pair of argument registers,
with the low-order XLEN bits in the lower-numbered register and the high-order
XLEN bits in the higher-numbered register.  If no argument registers are
available, the scalar is passed on the stack by value.  If exactly one
register is available, the low-order XLEN bits are passed in the register and
the high-order XLEN bits are passed on the stack.

Scalars wider than 2×XLEN bits are passed by reference and are replaced in the
argument list with the address.

Aggregates whose total size is no more than XLEN bits are passed in
a register, with the fields laid out as though they were passed in memory. If
no register is available, the aggregate is passed on the stack.
Aggregates whose total size is no more than 2×XLEN bits are passed in a pair
of registers; if only one register is available, the first XLEN bits are passed
in a register and the remaining bits are passed on the stack. If no registers are
available, the aggregate is passed on the stack. Bits unused due to
padding, and bits past the end of an aggregate whose size in bits is not
divisible by XLEN, are undefined.

Aggregates or scalars passed on the stack are aligned to the greater of the
type alignment and XLEN bits, but never more than the stack alignment.

Aggregates larger than 2×XLEN bits are passed by reference and are replaced in
the argument list with the address, as are {Cpp} aggregates with nontrivial copy
constructors, destructors, or vtables.

Fixed-length vectors are treated as aggregates.

Empty structs or union arguments or return values are ignored by C compilers
which support them as a non-standard extension.  This is not the case for {Cpp},
which requires them to be sized types.

Arguments passed by reference may be modified by the callee.

Floating-point reals are passed the same way as aggregates of the same size;
complex floating-point numbers are passed the same way as a struct containing
two floating-point reals. (This constraint changes when the integer calling
convention is augmented by the hardware floating-point calling convention.)

In the base integer calling convention, variadic arguments are passed in the
same manner as named arguments, with one exception.  Variadic arguments with
2×XLEN-bit alignment and size at most 2×XLEN bits are passed in an
*aligned* register pair (i.e., the first register in the pair is
even-numbered), or on the stack by value if none is available. After a
variadic argument has been passed on the stack, all future arguments will also
be passed on the stack (i.e. the last argument register may be left unused due
to the aligned register pair rule).

Values are returned in the same manner as a first named argument of the same
type would be passed.  If such an argument would have been passed by
reference, the caller allocates memory for the return value, and passes the
address as an implicit first parameter.

NOTE: There is no requirement that the address be returned from the function
and so software should not assume that a0 will hold the address of the return
value on return.

The stack grows downwards (towards lower addresses) and the stack pointer shall
be aligned to a 128-bit boundary upon procedure entry.
The first argument passed on the stack is located at offset zero of the stack pointer
on function entry; following arguments are stored at correspondingly
higher addresses.

In the standard ABI, the stack pointer must remain
aligned throughout procedure execution. Non-standard ABI code must realign the
stack pointer prior to invoking standard ABI procedures.  The operating system
must realign the stack pointer prior to invoking a signal handler; hence,
POSIX signal handlers need not realign the stack pointer.  In systems that
service interrupts using the interruptee's stack, the interrupt service
routine must realign the stack pointer if linked with any code that uses
a non-standard stack-alignment discipline, but need not realign the stack
pointer if all code adheres to the standard ABI.

Procedures must not rely upon the persistence of
stack-allocated data whose addresses lie below the stack pointer.

Registers s0-s11 shall be preserved across procedure calls.
No floating-point registers, if present, are preserved across calls. (This
property changes when the integer calling convention is augmented by the
hardware floating-point calling convention.)

=== Hardware Floating-point Calling Convention

The hardware floating-point calling convention adds eight floating-point
argument registers, fa0-fa7, the first two of which are also used to return
values.  Values are passed in floating-point registers whenever possible,
whether or not the integer registers have been exhausted.

The remainder of this section applies only to named arguments.  Variadic
arguments are passed according to the integer calling convention.

ABI_FLEN refers to the width of a floating-point register in the ABI.
The ABI_FLEN must be no wider than the ISA's FLEN.  The ISA might have wider
floating-point registers than the ABI.

For the purposes of this section, "struct" refers to a C struct with its
hierarchy flattened, including any array fields.  That is, `struct { struct
{ float f[1]; } a[2]; }` and `struct { float f0; float f1; }` are
treated the same.  Fields containing empty structs or unions are ignored while
flattening, even in {Cpp}, unless they have nontrivial copy constructors or
destructors.  Fields containing zero-length bit-fields or zero-length arrays are
ignored while flattening.  Attributes such as `aligned` or `packed` do not
interfere with a struct's eligibility for being passed in registers according
to the rules below, i.e. `struct { int i; double d; }` and `+struct
__attribute__((__packed__)) { int i; double d }+` are treated the same, as are
`struct { float f; float g; }` and `+struct { float f; float g __attribute__
((aligned (8))); }+`.

NOTE: One exceptional case for the flattening rule is an array of empty
structs or unions; C treats it as an empty field, but {Cpp}
treats it as a non-empty field since {Cpp} defines the size of an empty struct
or union as 1. i.e. for `struct { struct {} e[1]; float f; }` as the first
argument, C will treat it like `struct { float f; }` and pass `f` in `fa0` as
described below, whereas {Cpp} will pass the pass the entire aggregate in `a0` 
(XLEN = 64) or `a0` and `a1` (XLEN = 32), as described in the integer calling
convention.
Zero-length arrays of empty structs or union will be
ignored for both C and {Cpp}. i.e. For `struct { struct {} e[0]; float f; };`,
as the first argument, C and {Cpp} will treat it like `struct { float f; }`
and pass `f` in `fa0` as described below.

A real floating-point argument is passed in a floating-point argument
register if it is no more than ABI_FLEN bits wide and at least one floating-point
argument register is available.  Otherwise, it is passed according to the
integer calling convention.
When a floating-point argument narrower than FLEN bits is passed in a
floating-point register, it is 1-extended (NaN-boxed) to FLEN bits.

A struct containing just one floating-point real is passed as though it were
a standalone floating-point real.

A struct containing two floating-point reals is passed in two floating-point
registers, if neither real is more than ABI_FLEN bits wide and at least two floating-point
argument registers are available.  (The registers need not be an aligned pair.)
Otherwise, it is passed according to the integer calling convention.

A complex floating-point number, or a struct containing just one complex
floating-point number, is passed as though it were a struct containing two
floating-point reals.

A struct containing one floating-point real and one integer (or bitfield), in
either order, is passed in a floating-point register and an integer register,
provided the floating-point real is no more than ABI_FLEN bits wide and the
integer is no more than XLEN bits wide, and at least one floating-point
argument register and at least one integer argument register is available.
If the struct is passed in this manner, and the integer is narrower than XLEN
bits, the remaining bits are unspecified.
If the struct is not passed in this manner, then it is passed according to the
integer calling convention.

Unions are never flattened and are always passed according to the integer
calling convention.

Values are returned in the same manner as a first named argument of the same
type would be passed.

Floating-point registers fs0-fs11 shall be preserved across procedure calls,
provided they hold values no more than ABI_FLEN bits wide.

=== Standard Vector Calling Convention Variant

The _RISC-V V Vector Extension_<<riscv-v-extension>> defines a set of thirty-two
vector registers, v0-v31. The _RISC-V Vector Extension Intrinsic
Document_<<rvv-intrinsic-doc>> defines vector types which include vector mask
types, vector data types, and tuple vector data types. A value of vector type can
be stored in vector register groups.

The remainder of this section applies only to named vector arguments, other
named arguments and return values follow the standard calling convention.
Variadic vector arguments are passed by reference.

v0 is used to pass the first vector mask argument to a function, and to return
vector mask result from a function. v8-v23 are used to pass vector data
arguments, tuple vector data arguments and the rest vector mask arguments to a
function, and to return vector data and vector tuple results from a function.

It must ensure that the entire contents of v1-v7 and v24-v31 are preserved
across the call.

Each vector data type and vector tuple type has an LMUL attribute that
indicates a vector register group. The value of LMUL indicates the number of
vector registers in the vector register group and requires the first vector
register number in the vector register group must be a multiple of it. For
example, the LMUL of `vint64m8_t` is 8, so v8-v15 vector register group can be
allocated to this type, but v9-v16 can not because the v9 register number is
not a multiple of 8. If LMUL is less than 1, it is treated as 1. If it is a
vector mask type, its LMUL is 1.

Each vector tuple type also has an NFIELDS attribute that indicates how many
vector register groups the type contains. Thus a vector tuple type needs to
take up LMUL×NFIELDS registers.

The rules for passing vector arguments are as follows:

1. For the first vector mask argument, use v0 to pass it.

2. For vector data arguments or rest vector mask arguments, starting from the
v8 register, if a vector register group between v8-v23 that has not been
allocated can be found and the first register number is a multiple of LMUL,
then allocate this vector register group to the argument and mark these
registers as allocated. Otherwise, pass it by reference and are replaced in
the argument list with the address.

3. For tuple vector data arguments, starting from the v8 register, if NFIELDS
consecutive vector register groups between v8-v23 that have not been allocated
can be found and the first register number is a multiple of LMUL, then allocate
these vector register groups to the argument and mark these registers as
allocated. Otherwise, pass it by reference and are replaced in the argument list
with the address.

NOTE: The registers assigned to the tuple vector data argument must be
consecutive. For example, for the function
`void foo(vint32m1_t a, vint32m2_t b, vint32m1x2_t c)`, v8 will be allocated
to `a`, v10-v11 will be allocated to `b`, v12-v13 instead of v9 and v12 will
beallocated to `c`.

NOTE: It should be stressed that the search for the appropriate vector register
groups starts at v8 each time and does not start at the next register after the
registers are allocated for the previous vector argument. Therefore, it is
possible that the vector register number allocated to a vector argument can be
less than the vector register number allocated to previous vector arguments.
For example, for the function
`void foo (vint32m1_t a, vint32m2_t b, vint32m1_t c)`, according to the rules
of allocation, v8 will be allocated to `a`, v10-v11 will be allocated to `b`
and v9 will be allocated to `c`. This approach allows more vector registers to
be allocated to arguments in some cases.

Vector values are returned in the same manner as the first named argument of
the same type would be passed.

Vector types are disallowed in struct or union.

Vector arguments and return values are disallowed to pass to an unprototyped
function.

NOTE: Functions that use the standard vector calling convention variant must be
marked with `STO_RISCV_VARIANT_CC`, see <<Dynamic Linking>> for the meaning of
`STO_RISCV_VARIANT_CC`.

NOTE: `setjmp`/`longjmp` follow the standard calling convention, which clobbers
all vector registers. Hence, the standard vector calling convention variant
won't disrupt the `jmp_buf` ABI.

=== ILP32E Calling Convention

IMPORTANT: RV32E is not a ratified base ISA and so we cannot guarantee the
stability of ILP32E, in contrast with the rest of this document. This documents
the current implementation in GCC as of the time of writing, but may be subject
to change.

The ILP32E calling convention is designed to be usable with the RV32E ISA. This
calling convention is the same as the integer calling convention, except for the
following differences.  The stack pointer need only be aligned to a 32-bit
boundary.  Registers x16-x31 do not participate in the calling convention, so
there are only six argument registers, a0-a5, only two callee-saved registers,
s0-s1, and only three temporaries, t0-t2.

If used with an ISA that has any of the registers x16-x31 and f0-f31, then
these registers are considered temporaries.

The ILP32E calling convention is not compatible with ISAs that have registers
that require load and store alignments of more than 32 bits. In particular, this
calling convention must not be used with the D ISA extension.

=== Named ABIs

This specification defines the following named ABIs:

[[abi-ilp32]]
ILP32:: Integer calling-convention only, hardware
floating-point calling convention is not used (i.e. <<ELFCLASS32,ELFCLASS32>> and
<<EF_RISCV_FLOAT_ABI_SOFT,EF_RISCV_FLOAT_ABI_SOFT>>).

[[abi-ilp32f]]
ILP32F:: ILP32 with hardware floating-point calling
convention for ABI_FLEN=32 (i.e. <<ELFCLASS32,ELFCLASS32>> and
<<EF_RISCV_FLOAT_ABI_SINGLE,EF_RISCV_FLOAT_ABI_SINGLE>>).

[[abi-ilp32d]]
ILP32D:: ILP32 with hardware floating-point calling
convention for ABI_FLEN=64 (i.e. <<ELFCLASS32,ELFCLASS32>> and
<<EF_RISCV_FLOAT_ABI_DOUBLE,EF_RISCV_FLOAT_ABI_DOUBLE>>).

[[abi-ilp32e]]
ILP32E:: <<ILP32E Calling Convention,ILP32E calling-convention>> only,
hardware floating-point calling convention is not used (i.e. <<ELFCLASS32,ELFCLASS32>>,
<<EF_RISCV_FLOAT_ABI_SOFT,EF_RISCV_FLOAT_ABI_SOFT>>, and <<EF_RISCV_RVE,EF_RISCV_RVE>>).

[[abi-lp64]]
LP64:: Integer calling-convention only, hardware
floating-point calling convention is not used (i.e. <<ELFCLASS64,ELFCLASS64>> and
<<EF_RISCV_FLOAT_ABI_SOFT,EF_RISCV_FLOAT_ABI_SOFT>>).

[[abi-lp64f]]
LP64F:: LP64 with hardware floating-point calling
convention for ABI_FLEN=32 (i.e. <<ELFCLASS64,ELFCLASS64>> and
<<EF_RISCV_FLOAT_ABI_SINGLE,EF_RISCV_FLOAT_ABI_SINGLE>>).

[[abi-lp64d]]
LP64D:: LP64 with hardware floating-point calling
convention for ABI_FLEN=64 (i.e. <<ELFCLASS64,ELFCLASS64>> and
<<EF_RISCV_FLOAT_ABI_DOUBLE,EF_RISCV_FLOAT_ABI_DOUBLE>>).

[[abi-lp64q]]
LP64Q:: LP64 with hardware floating-point calling
convention for ABI_FLEN=128 (i.e. <<ELFCLASS64,ELFCLASS64>> and
<<EF_RISCV_FLOAT_ABI_QUAD,EF_RISCV_FLOAT_ABI_QUAD>>).

The ILP32* ABIs are only compatible with RV32* ISAs, and the LP64* ABIs are
only compatible with RV64* ISAs. A future version of this specification may
define an ILP32 ABI for the RV64 ISA, but currently this is not a supported
operating mode.

The *F ABIs require the *F ISA extension, the *D ABIs require the *D ISA
extension, and the LP64Q ABI requires the Q ISA extension.

NOTE: This means code targeting the Zfinx extension always uses the ILP32,
ILP32E or LP64 integer calling-convention only ABIs as there is no dedicated
hardware floating-point register file.

=== Default ABIs

While various different ABIs are technically possible, for software
compatibility reasons it is strongly recommended to use the following
default ABIs for specific architectures:

[horizontal]
on RV32G:: <<abi-ilp32d,ILP32D>>
on RV64G:: <<abi-lp64d,LP64D>>

NOTE: Although RV64GQ systems can technically use <<abi-lp64q,LP64Q>>, it is
strongly recommended to use LP64D on general-purpose RV64GQ systems for
compatibility with standard RV64G software.

== Calling Convention for System Calls

The calling convention for system calls does not fall within the scope of this
document.
Please refer to the documentation of the RISC-V execution environment interface
(e.g OS kernel ABI, SBI).

== C/{Cpp} type details

=== C/{Cpp} type sizes and alignments

There are two conventions for C/{Cpp} type sizes and alignments.

ILP32, ILP32F, ILP32D, and ILP32E:: Use the following type sizes and
alignments (based on the ILP32 convention):
+
.C/{Cpp} type sizes and alignments for RV32
[cols="4,>2,>3"]
[width=60%]
|===
| Type                 | Size (Bytes)  | Alignment (Bytes)

| bool/_Bool           |  1            |  1
| char                 |  1            |  1
| short                |  2            |  2
| int                  |  4            |  4
| long                 |  4            |  4
| long long            |  8            |  8
| void *               |  4            |  4
| +++__bf16+++         |  2            |  2
| _Float16             |  2            |  2
| float                |  4            |  4
| double               |  8            |  8
| long double          | 16            | 16
| float _Complex       |  8            |  4
| double _Complex      | 16            |  8
| long double _Complex | 32            | 16
|===

LP64, LP64F, LP64D, and LP64Q:: Use the following type sizes and
alignments (based on the LP64 convention):
+
.C/{Cpp} type sizes and alignments for RV64
[cols="4,>2,>3"]
[width=60%]
|===
| Type                 | Size (Bytes)  | Alignment (Bytes)

| bool/_Bool           |  1            |  1
| char                 |  1            |  1
| short                |  2            |  2
| int                  |  4            |  4
| long                 |  8            |  8
| long long            |  8            |  8
| +++__int128+++       | 16            | 16
| void *               |  8            |  8
| +++__bf16+++         |  2            |  2
| _Float16             |  2            |  2
| float                |  4            |  4
| double               |  8            |  8
| long double          | 16            | 16
| float _Complex       |  8            |  4
| double _Complex      | 16            |  8
| long double _Complex | 32            | 16
|===

The alignment of `max_align_t` is 16.

`CHAR_BIT` is 8.

Structs and unions are aligned to the alignment of their most strictly aligned
member. The size of any object is a multiple of its alignment.

=== Fixed-length vector

Various compilers have support for fixed-length vector types, for example GCC
and Clang both support declaring a type with `\\__attribute__\((vector_size(N))`,
where N is a positive number larger than zero.

The alignment requirement for the fixed length vector shall be equivalent to the
alignment requirement of its elemental type.

The size of the fixed length vector is determined by multiplying the size of its
elemental type by the total number of elements within the vector.

=== C/{Cpp} type representations

`char` is unsigned.

Booleans (`bool`/`_Bool`) stored in memory or when being passed as scalar
arguments are either `0` (`false`) or `1` (`true`).

A null pointer (for all types) has the value zero.

`_Float16` is as defined in the C ISO/IEC TS 18661-3 extension.

`__bf16` has the same parameter passing and return rules as for `_Float16`.

`_Complex` types have the same layout as a struct containing two fields of the
corresponding real type (`float`, `double`, or `long double`), with the first
member holding the real part and the second member holding the imaginary part.

The type `size_t` is defined as `unsigned int` for RV32 and `unsigned long` for RV64.

The type `ptrdiff_t` is defined as `int` for RV32 and `long` for RV64.

=== Bit-fields

Bit-fields are packed in little-endian fashion. A bit-field that would span the
alignment boundary of its integer type is padded to begin at the next
alignment boundary. For example, `struct { int x : 10; int y : 12; }` is
a 32-bit type with `x` in bits 9-0, `y` in bits 21-10, and bits 31-22
undefined.  By contrast, `struct { short x : 10; short y : 12; }` is a 32-bit
type with `x` in bits 9-0, `y` in bits 27-16, and bits 31-28 and bits 15-10
undefined.

Bit-fields which are larger than their integer types are only present in {Cpp}
and are defined by the _Itanium {Cpp} ABI_ <<itanium-cxx-abi>>.  The bit-field
and containing struct are aligned on a boundary corresponding to the largest
integral type smaller than the bit-field, up to 64-bit alignment on RV32 or
128-bit alignment on RV64.  Any bits in excess of the size of the declared type
are treated as padding.  For example `struct { char x : 9; char y; }` is a
24-bit type with `x` in bits 7-0, `y` in bit 23-16, and bits 15-8 undefined;
`struct { char x : 9; char y : 2 }` is a 16-bit type with `x` in bits 7-0, `y`
in bit 10-9, and bits 8 and 15-11 undefined.

Unnamed nonzero length bit-fields allocate space in the same fashion as named
bitfields but do not affect the alignment of the containing struct.

Zero length bit-fields are aligned relative to the start of the containing
struct according to their declared type and, since they must be unnamed, do not
affect the struct alignment.  C requires bit-fields on opposite sides of a
zero-length bitfield to be treated as separate memory locations for the
purposes of data races.

=== va_list, va_start, and va_arg

The `va_list` type has the same representation as `void*` and points to a
sequence of zero or more arguments with preceding padding for alignment,
formatted and aligned as variadic arguments passed on the stack according to
the integer calling convention (<<integer-cc>>). All standard calling conventions use the
same representation for variadic arguments to allow `va_list` types to be
shared between them.

The `va_start` macro in a function initializes its `va_list` argument to point
to the first address at which a variadic argument could be passed to the
function. If all integer argument registers are used for named formal
arguments, the first variadic argument will have been passed on the stack by
the caller, and the `va_list` can point to the address immediately after the
last named argument passed on the stack, or the `sp` value on entry if no named
arguments were passed on the stack. If some integer argument registers were not
used for named formal arguments, then the first variadic argument may have been
passed in a register.  The function is then expected to construct a _varargs
save area_ immediately below the entry `sp` and fill it with the entry values
of all integer argument registers not used for named arguments, in sequence.
The `va_list` value can then be initialized to the start of the varargs save
area, and it will iterate through any variadic arguments passed via registers
before continuing to variadic arguments passed on the stack, if any.

The `va_arg` macro will align its `va_list` argument, fetch a value, and
increment the `va_list` according to the alignment and size of a variadic
argument of the given type, which may not be the same as the alignment and size
of the given type in memory. If the type is passed by reference, the size and
alignment used will be those of a pointer, and the fetched pointer will be used
as the address of the actual argument. The `va_copy` macro is a single pointer
copy and the `va_end` macro performs no operation.

=== Vector type sizes and alignments

This section defines the sizes and alignments for the vector types defined in
the _RISC-V Vector Extension Intrinsic Document_<<rvv-intrinsic-doc>>.
The actual size of each type is determined by the hardware configuration, which
is based on the content of the `vlenb` register.

There are three classes of vector types: the vector mask types, the vector
data types and the vector tuple types.

.Type sizes and alignments for vector mask types
[cols="4,3,>3,>2"]
[width=80%]
|===
| Internal Name              | Type                 | Size (Bytes)       | Alignment (Bytes)

| __rvv_vbool1_t             | vbool1_t             |  VLENB             |  1
| __rvv_vbool2_t             | vbool2_t             |  VLENB / 2         |  1
| __rvv_vbool4_t             | vbool4_t             |  VLENB / 4         |  1
| __rvv_vbool8_t             | vbool8_t             |  ceil(VLENB / 8)   |  1
| __rvv_vbool16_t            | vbool16_t            |  ceil(VLENB / 16)  |  1
| __rvv_vbool32_t            | vbool32_t            |  ceil(VLENB / 32)  |  1
| __rvv_vbool64_t            | vbool64_t            |  ceil(VLENB / 64)  |  1
|===

include::vector_type_infos.adoc[]

NOTE: The vector mask types utilize a portion of the space, while the remaining
content may be undefined, both in the register and in memory.

NOTE: Size must be a positive integer.

[appendix]
== Linux-specific ABI

NOTE: This section of the RISC-V calling convention specification only applies
to Linux-based systems.

In order to ensure compatibility between different implementations of the C
library for Linux, we provide some extra definitions which only apply on those
systems. These are noted in this section.

=== Linux-specific C type sizes and alignments

The following definitions apply for all ABIs defined in this document. Here
there is no differentiation between ILP32 and LP64 ABIs.

.Linux-specific C type sizes and alignments
[cols="2,>1,>1"]
[width=80%]
|===
| Type    | Size (Bytes) | Alignment (Bytes)

| wchar_t |  4           |  4
| wint_t  |  4           |  4
|===

=== Linux-specific C type representations

The following definitions apply for all ABIs defined in this document. Here
there is no differentiation between ILP32 and LP64 ABIs.

`wchar_t` is signed.  `wint_t` is unsigned.

[bibliography]
== References

* [[[riscv-v-extension]]] "RISC-V V vector extension specification"
https://github.com/riscv/riscv-v-spec

* [[[rvv-intrinsic-doc]]] "RISC-V Vector Extension Intrinsic Document"
https://github.com/riscv-non-isa/rvv-intrinsic-doc

