RISC-V GCC通过-mabi选项指定数据模型和浮点参数传递规则。有效的选项值包括ilp32、ilp32f、ilp32d、lp64、lp64f 和 lp64d。前半部分指定数据模型，后半部分指定浮点参数传递规则。
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i指int，l指long，p指pointer即指针，32/64指前面给出的类型是32/64位的；f指float，指float型浮点数参数通过浮点数寄存器传递；d指double，指float型和double型浮点数参数通过浮点数寄存器传递。
数据模型：
x | int字长 | long字长 | 指针字长 |
ilp32/ilp32f/ilp32d | 32bits | 32bits | 32bits |
lp64/lp64f/lp64d | 32bits | 64bits | 64bits |
浮点参数传递规则：
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 |
浮点参数传递规则只跟-mabi选项有关，和-march选项没有直接关系，但是部分-mabi选项需要浮点寄存器，浮点寄存器是通过浮点扩展指令引入的，这就需要在-march选项中指定浮点扩展。
RISC-V GCC 使用手册对-mabi选项的说明：
-mabi=ABI-string
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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.
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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.
