codegen-notes.md

RISC-V Assembly Programmer's Manual

Calling conventions

• ra : return address
• sp : stack pointer
• fp : frame pointer
• a0 : return value or first argument
• a1 - a7 : arguments

How does the call/ret works and the ra register

call just uses auipc and jalr.

  78:	00000097          	auipc	ra,0x0
  7c:	000080e7          	jalr	ra # 78 <main+0x18>

Why the auipc offset is always 0x0 ?

See this stack-overflow

Add -r to objdump to see the relocation info.

  78:	00000097          	auipc	ra,0x0
			78: R_RISCV_CALL	fn
			78: R_RISCV_RELAX	*ABS*
  7c:	000080e7          	jalr	ra # 78 <main+0x18>

auipc,jal and jalr

• auipc : add upper immediate to pc
• jal : jump and link
• jalr : jump and link register

What's the difference between jal and jalr ? They all save the pc + 4 in a register, and jump to a new address.

• jal : pc + offset
• jalr : x[rs1] + offset

That is because the offset is not large enough.

Caller saved and callee saved registers

Call frame infromation