Instruction Selection for LLVM-- Aslan Askarov aslan@cs.au.dk The - - PowerPoint PPT Presentation

Compilation 2016

Instruction Selection for LLVM--

Aslan Askarov aslan@cs.au.dk
 
 


The x86 assembly language

• Assembler produces object file that contains

segments (address ranges with a purpose)

• Segments containing runnable code: .text
• Segments containing initialized data: .data
• Remember to specify segment to fit purpose
• Specifying data:
• .ascii .asciz
• Metadata:
• .func .type .endfunc .size .globl
• For examples: check provided *.s

The x86 assembly language

• Example instruction: movq %rax, -48(%rbp)
• General format: <instr><S> <src>, <dst>
• <S> ∈ {b,s,w,l,q,t} specifies size (q: 64 bit)
• $ indicates literal number
• % indicates register
• o(%r1, %r2, n) means o + %r1 + %r2 * n
• Example label: mylabel
• gives the “current address” the name ‘mylabel’
• Example directives:
• .text .data .globl .ascii .asciz


.size .func .type .endfunc

• x86.sml – A friendly module for creating and printing

x86 64 assembly programs

• System V x86 64 Application Binary Interface (ABI)
• https://software.intel.com/sites/default/files/article/

402129/mpx-linux64-abi.pdf

• A document describing (among other things)
• Frame organization
• Calling conventions
• Register purpose

Stack organization – a figure from System V ABI document

Register usage

Note argument calling convention we don’t use these registers

Organizing stack for Tiger programs

0x1018

0x1200

… 0x10B0 0x10A8 0x10A0 0x1080 0x1010

stack grows from high memory addresses to low addresses

… current frame %rsp

saved %rbp value ret addr

0x1200 %rbp previous frame 0x1088

%local1 %local2 %localN

0x1030

alloca

0x1018 result of alloca

saved args

…

Q: why do we need to save args?

backend.sml – LLVM-- to x86 instruction generation

• Simple strategy – compiling all %uids to stack-

allocated slots

• Each LLVM-- instruction may correspond to several

x86 instructions

• Strategy:
• designate temporary registers for storing

intermediate results when translating operands

• e.g., %rax, %rcx
• Q: callee-save vs caller-save registers?

Functions

• Function name: just a label
• Function call:
• Use assembly callq instruction
• Function prologue:
• save %rbp on stack; decrement %rsp
• do not forget to copy arguments on the stack
• Function epilogue:
• invoked by LL terminators

Other LL commands

• Pick your compilation strategy, as long as you are

within the bounds of what x86.sml offers you (there is plenty of space)

• Recommend to tackle GEP last
• Generate test LL programs from your LL module