Register Convention

寄存器约定是所有RISCV架构都需要遵守的，无法通过mabi选择修改。

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

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

Procedure Calling Convention

所有的ABI选项如下:

ILP32: Integer calling-convention only, hardware floating-point calling convention is not used (i.e. ELFCLASS32 and EF_RISCV_FLOAT_ABI_SOFT).
ILP32F: ILP32 with hardware floating-point calling convention for FLEN=32 (i.e. ELFCLASS32 and EF_RISCV_FLOAT_ABI_SINGLE).
ILP32D: ILP32 with hardware floating-point calling convention for FLEN=64 (i.e. ELFCLASS32 and EF_RISCV_FLOAT_ABI_DOUBLE).
ILP32E: ILP32E calling-convention only, hardware floating-point calling convention is not used (i.e. ELFCLASS32, EF_RISCV_FLOAT_ABI_SOFT, and EF_RISCV_RVE).
LP64: Integer calling-convention only, hardware floating-point calling convention is not used (i.e. ELFCLASS64 and EF_RISCV_FLOAT_ABI_SOFT).
LP64F: LP64 with hardware floating-point calling convention for FLEN=32 (i.e. ELFCLASS64 and EF_RISCV_FLOAT_ABI_SINGLE).
LP64D: LP64 with hardware floating-point calling convention for FLEN=64 (i.e. ELFCLASS64 and EF_RISCV_FLOAT_ABI_DOUBLE).
LP64Q: LP64 with hardware floating-point calling convention for FLEN=128 (i.e. ELFCLASS64 and EF_RISCV_FLOAT_ABI_QUAD).

选项主要分为ILP32和LP64两类，分别对应RV32和RV64，他们的差别主要在long和void *的数据长度上。

LP64, LP64F, LP64D, and LP64Q: use the following type sizes and alignments (based on the LP64 convention):

| 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 |
| float | 4 | 4 |
| double | 8 | 8 |
| long double | 16 | 16 |
| float _Complex | 8 | 4 |
| double _Complex | 16 | 8 |
| long double _Complex | 32 | 16 |
ILP32, ILP32F, ILP32D, and ILP32E: use the following type sizes and alignments (based on the ILP32 convention):

| 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 |
| float | 4 | 4 |
| double | 8 | 8 |
| long double | 16 | 16 |
| float _Complex | 8 | 4 |
| double _Complex | 16 | 8 |
| long double _Complex | 32 | 16 |

选项还分为empty \ f \ d ，他们的差别体现在浮点参数传递上。

x	需要浮点扩展指令？	float参数	double参数
ilp32/lp64	不需要	通过整数寄存器（a0-a1）传递	通过整数寄存器（a0-a3）传递
ilp32f/lp64f	需要F扩展	通过浮点寄存器（fa0-fa1）传递	通过整数寄存器（a0-a3）传递
ilp32d/lp64d	需要F扩展和D扩展	通过浮点寄存器（fa0-fa1）传递	通过浮点寄存器（fa0-fa1）传递

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