Pass 4 of Twobit generates assembly code for a hypothetical machine, which is confusingly called the MacScheme machine. (It was designed to overcome the shortcomings of a similar intermediate language used in MacScheme.) Its instructions were designed for easy peephole optimization when generating code for real RISC machines.
The MacScheme machine contains a RESULT register
and general registers
REG0,
REG1,
REG2,
and so forth.
REG0 always holds the currently executing procedure,
providing access to its constants and free variables.
Twobit's front end translates Feeley's iterative factorial example
(define fact
(lambda (n)
(let loop ((i n) (ans 1))
(if (= i 0)
ans
(loop (- i 1) (* ans i))))))
into
(let* ((.T12 (lambda (.n|1)
(define .loop|7
(lambda (.i|8 .ans|8)
(let ((.T3 (= .i|8 0)))
(if .T3
.ans|8
(let* ((.REG2 (* .ans|8 .i|8)) (.REG1 (- .i|8 1)))
(.loop|7 .REG1 .REG2))))))
(.loop|7 .n|1 1)))
(.T13 (set! fact .T12)))
'fact)
The code generator and local optimizer translate this into the MacScheme machine code
lambda 1000,0
setreg 7
reg 7,.T12
setglbl fact
const fact
return
L1000
.proc
args= 1
const 1
setreg 2
branch 1001,2
.align 4
L1001
.proc
.proc-doc #(.loop|7 #f 2 #f #f (i ans))
reg 1,.i|8
op2imm =,0
branchf 1005,2
reg 2,.ans|8
return
L1005
reg 2,.ans|8
op2 *,1
setreg 2
reg 1,.i|8
op2imm -,1
setreg 1
branch 1001,2
With peephole optimization and tagged fixnum arithmetic, the corresponding MIPS code might be
L1000: L1000:
args= 1 subi $2,$2,1
beq $2,$0,L1
[arity exception]
L1:
const 1 ori $5,$0,4
setreg 2
branch 1001,2
L1001: L1001:
reg 1 bne $4,$0,L1004
op2imm =,0
branchf 1004,2
reg 2 ori $2,$5,0
return jr $ra
L1004: L1004:
reg 2 asr $3,$4,2
op2 *,1 muls $5,$5,$3
setreg 2
reg 1 subi $4,$4,4
op2imm -,1
setreg 1
branch 1001,2 b L1001
Last updated 27 October 2002.