I get quite a few emails about just what's involved in writing a compiler, so thought I'd throw this together.
It takes a 'simple BASIC' source program and converts it into x86 assembler.
The x86 wont even work 'as-is'. In particular, the condition code stuff in x86 is downright bizarre and I didn't bother implementing it properly, but it shouldn't take too much hacking to get working.
If someone's really keen, they might want to fix this up, stick the output through 'NASM' (a cool, free assembler) and see what happens! The included print command is implemented as a library call, so it'll have to be linked with something that exports a 'print' function.
This is designed (hopefully) for simplicity - not efficiency! Blitz code generation does considerably better than this, and will improve further over time.
Have fun! |
;Very simple compiler.
;
;Blitz does NOT generate code like this! But the parsing is similar...
;
;Statements supported: let, print, end, if, then, else
;
;Operators supported: binary +, -, *, /, <, =, >, <=, <>, >= and unary -
;
;*, / have precedence over +, -, which have precendence over <,=,>,<=,<>,>=
;
;Parenthesis can be used to group subexpressions.
;**** The program *****
;
Data "let a=1"
Data "let b=2"
Data "let c=a*b+a/b"
Data "if aTOKE_IDENT SynErr()
id$=toke$
AddGlob( toke$ )
If GetToke()<>TOKE_EQ SynErr()
t=ParseExpr()
Print " mov [_"+id$+"],eax"
Case TOKE_IF
If ParseExpr()<>TOKE_THEN SynErr()
label=label+1:lab=label
Print " and eax,eax"
Print " jz __"+lab
t=ParseStmt( GetToke() )
If t=TOKE_ELSE
label=label+1:lab2=label
Print " jmp __"+lab2
Print "__"+lab+":"
t=ParseStmt( GetToke() )
lab=lab2
EndIf
Print "__"+lab+":"
Case TOKE_PRINT
t=ParseExpr()
Print " call PRINT"
Case TOKE_END
Print " ret"
Print
For gl.Glob=Each Glob
Print "_"+gl\name+":"
Print " dd 0"
Next
Print:Print "Done!":WaitKey
End
Default
SynErr()
End Select
Return t
End Function
Function ParseExpr()
Return ParseComp()
End Function
Function ParseComp()
t=ParseTerm()
Repeat
Select t
Case TOKE_LT op$="lt" ;these ops are wrong! x86 is weird!
Case TOKE_GT op$="gt"
Case TOKE_LE op$="le"
Case TOKE_GE op$="ge"
Case TOKE_EQ op$="eq"
Case TOKE_NE op$="ne"
Default Return t
End Select
Print " push eax"
t=ParseTerm()
Print " pop ecx"
Print " cmp ecx,eax"
Print " s"+op$+" eax"
Print " and eax,255"
Forever
End Function
Function ParseTerm()
t=ParseFact()
Repeat
Select t
Case TOKE_ADD
Print " push eax"
t=ParseFact()
Print " pop ecx"
Print " add eax,ecx"
Case TOKE_SUB
Print " push eax"
t=ParseFact()
Print " mov ecx,eax"
Print " pop eax"
Print " sub eax,ecx"
Default
Return t
End Select
Forever
End Function
Function ParseFact()
t=ParseLeaf()
Repeat
Select t
Case TOKE_MUL
Print " push eax"
t=ParseLeaf()
Print " pop ecx"
Print " imul eax,ecx"
Case TOKE_DIV
Print " push eax"
t=ParseLeaf()
Print " mov ecx,eax"
Print " pop ecx"
Print " idiv eax,ecx"
Default
Return t
End Select
Forever
End Function
Function ParseLeaf()
t=GetToke()
Select t
Case TOKE_SUB
t=ParseLeaf()
Print " neg eax"
Case TOKE_OPENPAR
If ParseExpr() <> TOKE_CLOSEPAR SynErr()
t=GetToke()
Case TOKE_IDENT
Print " mov eax,[_"+toke$+"]"
AddGlob( toke$ )
t=GetToke()
Case TOKE_CONST
Print " mov eax,"+toke$
t=GetToke()
Default
SynErr()
End Select
Return t
End Function
Function AddGlob( name$ )
For gl.Glob=Each Glob
If gl\name=name Return
Next
gl=New Glob
gl\name=name
End Function
Function SynErr()
RuntimeError "Syntax Error"
End Function
Function CurrCh$()
If lin$=""
Read lin$
lin$=Lower$( lin$ )+" "
EndIf
Return Left( lin$,1 )
End Function
Function NextCh()
lin$=Mid$( lin$,2 )
End Function
Function GetToke()
Repeat
toke$=CurrCh$()
NextCh()
Until toke$<>" "
If toke$>="0" And toke$<="9"
Repeat
t$=CurrCh$()
If t$<"0" Or t$>"9" Return TOKE_CONST
toke$=toke$+t$
nextCh()
Forever
EndIf
If toke$>="a" And toke$<="z"
Repeat
t$=CurrCh$()
If t$<"a" Or t$>="z"
Select toke$
Case "let" Return TOKE_LET
Case "print" Return TOKE_PRINT
Case "end" Return TOKE_END
Case "if" Return TOKE_IF
Case "then" Return TOKE_THEN
Case "else" Return TOKE_ELSE
End Select
Return TOKE_IDENT
EndIf
toke$=toke$+t$
NextCh()
Forever
EndIf
Select toke$
Case "+" Return TOKE_ADD
Case "-" Return TOKE_SUB
Case "*" Return TOKE_MUL
Case "/" Return TOKE_DIV
Case "(" Return TOKE_OPENPAR
Case ")" Return TOKE_CLOSEPAR
Case "="
If CurrCh$()="<" NextCh():Return TOKE_LE
If CurrCh$()=">" NextCh():Return TOKE_GE
Return TOKE_EQ
Case "<"
If CurrCh$()="=" NextCh():Return TOKE_LE
If currCh$()=">" NextCh():Return TOKE_NE
Return TOKE_LT
Case ">"
If CurrCh$()="=" NextCh():Return TOKE_GE
If CurrCh$()="<" NextCh():Return TOKE_NE
Return TOKE_GT
End Select
SynErr()
End Function |
