factor/basis/compiler/cfg/alias-analysis/alias-analysis.factor

! Copyright (C) 2008, 2010 Slava Pestov.
! See http://factorcode.org/license.txt for BSD license.
USING: kernel math namespaces assocs hashtables sequences arrays
accessors words vectors combinators combinators.short-circuit
sets classes layouts fry locals cpu.architecture
compiler.cfg
compiler.cfg.rpo
compiler.cfg.def-use
compiler.cfg.liveness
compiler.cfg.registers
compiler.cfg.utilities
compiler.cfg.comparisons
compiler.cfg.instructions
compiler.cfg.representations.preferred ;
FROM: namespaces => set ;
IN: compiler.cfg.alias-analysis

! We try to eliminate redundant slot operations using some simple heuristics.
! 
! All heap-allocated objects which are loaded from the stack, or
! other object slots are pessimistically assumed to belong to
! the same alias class.
!
! Freshly-allocated objects get their own alias class.
!
! Simple pseudo-C example showing load elimination:
! 
! int *x, *y, z: inputs
! int a, b, c, d, e: locals
! 
! Before alias analysis:
!
! a = x[2]
! b = x[2]
! c = x[3]
! y[2] = z
! d = x[2]
! e = y[2]
! f = x[3]
!
! After alias analysis:
!
! a = x[2]
! b = a /* ELIMINATED */
! c = x[3]
! y[2] = z
! d = x[2] /* if x=y, d=z, if x!=y, d=b; NOT ELIMINATED */
! e = z /* ELIMINATED */
! f = c /* ELIMINATED */
!
! Simple pseudo-C example showing store elimination:
!
! Before alias analysis:
!
! x[0] = a
! b = x[n]
! x[0] = c
! x[1] = d
! e = x[0]
! x[1] = c
!
! After alias analysis:
!
! x[0] = a /* dead if n = 0, live otherwise; NOT ELIMINATED */
! b = x[n]
! x[0] = c
! /* x[1] = d */  /* ELIMINATED */
! e = c
! x[1] = c

! Local copy propagation
SYMBOL: copies

: resolve ( vreg -- vreg ) copies get ?at drop ;

: record-copy ( ##copy -- )
    [ src>> resolve ] [ dst>> ] bi copies get set-at ; inline

! Map vregs -> alias classes
SYMBOL: vregs>acs

ERROR: vreg-ac-not-set vreg ;

: vreg>ac ( vreg -- ac )
    #! Only vregs produced by ##allot, ##peek and ##slot can
    #! ever be used as valid inputs to ##slot and ##set-slot,
    #! so we assert this fact by not giving alias classes to
    #! other vregs.
    vregs>acs get ?at [ vreg-ac-not-set ] unless ;

! Map alias classes -> sequence of vregs
SYMBOL: acs>vregs

: ac>vregs ( ac -- vregs ) acs>vregs get at ;

: aliases ( vreg -- vregs )
    #! All vregs which may contain the same value as vreg.
    vreg>ac ac>vregs ;

: each-alias ( vreg quot -- )
    [ aliases ] dip each ; inline

: merge-acs ( vreg into -- )
    [ vreg>ac ] dip
    2dup eq? [ 2drop ] [
        [ ac>vregs ] dip
        [ vregs>acs get '[ [ _ ] dip _ set-at ] each ]
        [ acs>vregs get at push-all ]
        2bi
    ] if ;

! Map vregs -> slot# -> vreg
SYMBOL: live-slots

! Maps vreg -> slot# -> insn# of last store or f
SYMBOL: recent-stores

! A set of insn#s of dead stores
SYMBOL: dead-stores

: dead-store ( insn# -- ) dead-stores get adjoin ;

:: set-ac ( vreg ac -- )
    #! Set alias class of newly-seen vreg.
    H{ } clone vreg recent-stores get set-at
    H{ } clone vreg live-slots get set-at
    ac vreg vregs>acs get set-at
    vreg ac acs>vregs get push-at ;

: live-slot ( slot#/f vreg -- vreg' )
    #! If the slot number is unknown, we never reuse a previous
    #! value.
    over [ live-slots get at at ] [ 2drop f ] if ;

ERROR: vreg-has-no-slots vreg ;

: load-constant-slot ( value slot# vreg -- )
    live-slots get ?at [ vreg-has-no-slots ] unless set-at ;

: load-slot ( value slot#/f vreg -- )
    over [ load-constant-slot ] [ 3drop ] if ;

: record-constant-slot ( slot# vreg -- )
    #! A load can potentially read every store of this slot#
    #! in that alias class.
    [ recent-stores get at delete-at ] with each-alias ;

: record-computed-slot ( vreg -- )
    #! Computed load is like a load of every slot touched so far
    [ recent-stores get at clear-assoc ] each-alias ;

:: remember-slot ( value slot# vreg -- )
    slot# [
        slot# vreg record-constant-slot
        value slot# vreg load-constant-slot
    ] [ vreg record-computed-slot ] if ;

SYMBOL: ac-counter

: next-ac ( -- n )
    ac-counter [ dup 1 + ] change ;

! Alias class for objects which are loaded from the data stack
! or other object slots. We pessimistically assume that they
! can all alias each other.
SYMBOL: heap-ac

: set-heap-ac ( vreg -- ) heap-ac get set-ac ;

: set-new-ac ( vreg -- ) next-ac set-ac ;

: kill-constant-set-slot ( slot# vreg -- )
    [ live-slots get at delete-at ] with each-alias ;

:: record-constant-set-slot ( insn# slot# vreg -- )
    vreg recent-stores get at :> recent-stores
    slot# recent-stores at [ dead-store ] when*
    insn# slot# recent-stores set-at ;

: kill-computed-set-slot ( vreg -- )
    [ live-slots get at clear-assoc ] each-alias ;

:: remember-set-slot ( insn# slot# vreg -- )
    slot# [
        insn# slot# vreg record-constant-set-slot
        slot# vreg kill-constant-set-slot
    ] [ vreg kill-computed-set-slot ] if ;

: init-alias-analysis ( -- )
    H{ } clone vregs>acs set
    H{ } clone acs>vregs set
    H{ } clone live-slots set
    H{ } clone copies set
    H{ } clone recent-stores set
    HS{ } clone dead-stores set
    0 ac-counter set ;

GENERIC: insn-slot# ( insn -- slot#/f )
GENERIC: insn-object ( insn -- vreg )

M: ##slot insn-slot# drop f ;
M: ##slot-imm insn-slot# slot>> ;
M: ##set-slot insn-slot# drop f ;
M: ##set-slot-imm insn-slot# slot>> ;
M: ##alien-global insn-slot# [ library>> ] [ symbol>> ] bi 2array ;
M: ##vm-field insn-slot# offset>> ;
M: ##set-vm-field insn-slot# offset>> ;

M: ##slot insn-object obj>> resolve ;
M: ##slot-imm insn-object obj>> resolve ;
M: ##set-slot insn-object obj>> resolve ;
M: ##set-slot-imm insn-object obj>> resolve ;
M: ##alien-global insn-object drop \ ##alien-global ;
M: ##vm-field insn-object drop \ ##vm-field ;
M: ##set-vm-field insn-object drop \ ##vm-field ;

GENERIC: analyze-aliases ( insn -- insn' )

M: insn analyze-aliases ;

M: vreg-insn analyze-aliases
    ! If an instruction defines a value with a non-integer
    ! representation it means that the value will be boxed
    ! anywhere its used as a tagged pointer. Boxing allocates
    ! a new value, except boxing instructions haven't been
    ! inserted yet.
    dup defs-vreg [
        over defs-vreg-rep { int-rep tagged-rep } member?
        [ set-heap-ac ] [ set-new-ac ] if
    ] when* ;

M: ##phi analyze-aliases
    dup defs-vreg set-heap-ac ;

M: ##allocation analyze-aliases
    #! A freshly allocated object is distinct from any other
    #! object.
    dup dst>> set-new-ac ;

M: ##box-displaced-alien analyze-aliases
    [ call-next-method ]
    [ base>> heap-ac get merge-acs ] bi ;

M: ##read analyze-aliases
    call-next-method
    dup [ dst>> ] [ insn-slot# ] [ insn-object ] tri
    2dup live-slot dup
    [ 2nip <copy> analyze-aliases nip ]
    [ drop remember-slot ]
    if ;

: idempotent? ( value slot#/f vreg -- ? )
    #! Are we storing a value back to the same slot it was read
    #! from?
    live-slot = ;

M:: ##write analyze-aliases ( insn -- insn )
    insn src>> resolve :> src
    insn insn-slot# :> slot#
    insn insn-object :> vreg
    insn insn#>> :> insn#

    src slot# vreg idempotent? [ insn# dead-store ] [
        src heap-ac get merge-acs
        insn insn#>> slot# vreg remember-set-slot
        src slot# vreg load-slot
    ] if

    insn ;

M: ##copy analyze-aliases
    #! The output vreg gets the same alias class as the input
    #! vreg, since they both contain the same value.
    dup record-copy ;

: useless-compare? ( insn -- ? )
    {
        [ cc>> cc= eq? ]
        [ [ src1>> ] [ src2>> ] bi [ resolve vreg>ac ] bi@ = not ]
    } 1&& ; inline

M: ##compare analyze-aliases
    call-next-method
    dup useless-compare? [
        dst>> f \ ##load-reference new-insn
        analyze-aliases
    ] when ;

: reset-alias-analysis ( -- )
    recent-stores get clear-assoc
    vregs>acs get clear-assoc
    acs>vregs get clear-assoc
    live-slots get clear-assoc
    copies get clear-assoc
    dead-stores get table>> clear-assoc

    next-ac heap-ac set
    \ ##vm-field set-new-ac
    \ ##alien-global set-new-ac ;

M: factor-call-insn analyze-aliases
    heap-ac get ac>vregs [
        [ live-slots get at clear-assoc ]
        [ recent-stores get at clear-assoc ] bi
    ] each ;

GENERIC: eliminate-dead-stores ( insn -- ? )

M: ##set-slot-imm eliminate-dead-stores
    insn#>> dead-stores get in? not ;

M: insn eliminate-dead-stores drop t ;

: alias-analysis-step ( insns -- insns' )
    reset-alias-analysis
    [ local-live-in [ set-heap-ac ] each ]
    [ 0 [ [ insn#<< ] [ drop 1 + ] 2bi ] reduce drop ]
    [ [ analyze-aliases ] map! [ eliminate-dead-stores ] filter! ] tri ;

: alias-analysis ( cfg -- cfg )
    init-alias-analysis
    dup [ alias-analysis-step ] simple-optimization ;