Havm 0.25a

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This manual is for HAVM (version 0.25a, 16 May 2012), the Tree Virtual Machine.

Copyright © 2003, 2004, 2005, 2006 Robert Anisko.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”

(a) The FSF's Back-Cover Text is: “You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.”

HAVM is a virtual machine designed to execute simple register based high level intermediate code. It is based on the intermediate representations ("canonicalized" or not) defined by Andrew Appel in his "Modern Compiler Implementation". It is nevertheless generic enough so that any (student) compiler could target its intermediate language to HAVM's language.

Its features are:

It was written by Robert Anisko as an LRDE member, so that EPITA students could exercise their compiler projects before the final jump to assembly code. It is implemented in Haskell, a pure non strict functional language very well suited for this kind of symbolic processing. HAVM was coined on both Haskell, and vm standing for Virtual Machine.

Information about HAVM can be found on HAVM Home Page, and feedback can be sent to lrde's Projects Address. LRDE stands for Laboratoire de Recherche et Dévelopment de l'EPITA, i.e., the Research and Development Labof EPITA, the Ecole Pour l'Informatique et les Techniques Avancées.

Andrew Appel's home page includes links to material related to compilers theory, and some information about the Modern Compiler Implementation book series.

More information on Haskell can be found on Haskell Home Page. HAVM requires a specific Haskell compiler, GHC, the Glasgow Haskell Compiler.

--- The Detailed Node Listing ---

The HAVM Language

Known Problems

Copying This Manual

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1 Invoking HAVM

To invoke havm run

     havm options file

where file is a simple text file, and options is any combination of the following options:

Display a help message and exit successfully.
Display the version number and exit successfully.
Unparse the content of the file on the file descriptor fd, defaulting to 2 (standard error output).
-p [fd]
Report simple profiling information on fd, defaulting to 2 (stderr).
-t [fd]
Display each instruction before execting it on fd, defaulting to 2 (stderr).
-l [fd]
Reject high level constructs. See LIR, for a description of the valid subset.

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2 The HAVM Language

HAVM supports two different source languages, HIR and LIR, the second being a subset of the first one.

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2.1 HIR

In both languages, white spaces are ignored, and comments are introduced by # and end at the end of line, or opened by /* and closed by the next */. HIR is defined by the following grammar:

     Exp ::= "const" int
          |  "name" Label
          |  "temp" Temp
          |  "binop" Oper Exp Exp
          |  "mem" Exp
          |  "call" Exp [{Exp}] "call end"
          |  "eseq" Stm Exp
     Stm ::= "move" Exp Exp
          |  "sxp" Exp
          |  "jump" Exp Label
          |  "cjump" Relop Exp Exp Label Label
          |  "seq" [{Stm}] "seq end"
          |  "label" Label
          |  "label" Label Literal
     Oper  ::= "add" | "sub" | "mul" | "div" | "mod"
     Relop ::= "eq" | "ne" | "lt" | "gt" | "le" | "ge"
     Label ::= Ident
     Temp  ::= fp | rv | sp | Ident
     fp    ::= "fp" | "$fp"
     sp    ::= "sp" | "$sp"
     rv    ::= "rv" | "$v0"
     Ident ::= [$a-zA-Z_][$a-zA-Z_0-9]*

In addition, the following alternative syntax for operators is supported, but deprecated.

     Oper  ::= "(+)" | "(-)" | "(*)" | "(/)" | "(%)"
     Relop ::= "(=)" | "(<>)" | "(<)" | "(>)" | "(<=)" | "(>=)"

A Literal is almost a Tiger string: it is enclosed by ‘"’, with support for the following escapes:

\a’, ‘\b’, ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\v
control characters.
The character which code is num in hexadecimal (upper case or lower case or mixed). num is composed of exactly 2 hexadecimal characters.
A single backslash.
A simple quote.
A double quote.
If no rule above applies, this is an error.

In addition the following restriction must be respected:

A call has always the form ‘move (name l, ...)’.
Destinations must be valid: ‘cjump (op, left, right, name l1, name l1)’.
As for cjump: ‘jump (name l)’.
A move has always the form ‘move (temp t, ...)’ or ‘move (mem e, ...)’.

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2.2 Special Temporaries

Some of the temporaries have a special meaning for HAVM:

The frame pointer.
The stack pointer. One cannot read beyond sp.
The result register. Functions should store their result there. This is the strongest dependency on registers, since an expression such as ‘1 + call (fact, 2)’ needs to “know” that the result of ‘call (fact, 2’) is to be “read” in rv.

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2.3 The HAVM Runtime

HAVM provides a set of predefined functions, modeled after the Tiger runtime.

— string: chr (code : int)

Return the one character long string containing the character which code is code. If code does not belong to the range [0..255], raise a runtime error: ‘chr: character out of range’.

— string: concat (first: string, second: string)

Concatenate first and second.

— void: exit (status: int)

Exit the program with exit code status. Note that contrary to SPIM 6.5, HAVM's own exit status is status.

— void: flush ()

Flush the output buffer.

— string: getchar ()

Read a character on input. Return an empty string on an end of file.

— pointer: init_array (size: int, init: pointer)

Return the address of a freshly allocated array of size 4 byte elements, initialized to init.

— pointer: malloc (size: int)

Return the address of a freshly allocated block of memory of size size.

— int: not (boolean: int)

Return 1 if boolean = 1, else return 0.

— int: _not (boolean: int)

Same as not, but provided under two names so that people using _not in their MIPS output don't have to change the HAVM name. This is because the SPIM scanner cannot tell the difference between not as an instruction and as a label.

— int: ord (string: string)

Return the ascii code of the first character in string and -1 if the given string is empty.

— void: print (string: string)

Print string on the standard output.

— void: print_err (string: string)

Note: this is an EPITA extension. Same as print, but the output is written to the standard error.

— void: print_int (int: int)
— void: printint (int: int)

Note: this is an EPITA extension. Output int in its decimal canonical form (equivalent to ‘%d’ for printf).

— int: size (string: string)

Return the size in characters of the string.

— int: strcmp (a: string, b: string)

Note: this is an EPITA extension. Compare the strings a and b: return -1 if a < b, 0 if equal, and 1 otherwise.

— int: streq (a: string, b: string)
— int: stringEqual (a: string, b: string)

Note: this is an EPITA extension. Return 1 if the strings a and b are equal, 0 otherwise. Often faster than strcmp to test string equality.

— string: substring (string: string, first: int, length: int)

Return a string composed of the characters of string starting at the first character (0 being the origin), and composed of length characters (i.e., up to and including the character first + length).

Let size be the size of the string, the following assertions must hold:

otherwise a runtime failure is raised: ‘substring: arguments out of bounds’.

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2.4 LIR

A valid LIR program is a valid HIR program that in addition verifies the following constraints:

no nested seq
The HIR tree must be flattened in a single thread of execution. Therefore, there must be at most one seq per function.
no eseq
Similarly, the instruction eseq must not be used.
no nested call
Calls cannot be embedded within other calls. Actually, the restriction is even stronger than this: a call can only appear in the following patterns:
move dest call ...
A function call.
sxp call ...
A procedure call.

one way cjump
cjumps must be normalized in such a way that they are always followed by their negative destination.

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3 Known Problems

Unfortunately, because HAVM is currently implemented with a naive recursive function, it is not robust to violations of the recursion due to jumps in high-level representation. Because in low-level intermediate representation there are no nested jumps, the problem does not arise.

This unique problem comes in several flavors, depending on how you generated the jump.

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3.1 Ineffective break

The following example in Tiger uses a break within an expression.

     while 1 do
       print_int ((break; 1))

The generated high level intermediate representation is:

     /tmp % cat foo.hir
     /* == High Level Intermediate representation. == */
     # Routine: main
     label main
     # Prologue
     # Body
       label l1
       cjump ne
         const 1
         const 0
         name l2
         name l0
       label l2
           name print_int
             jump  ###########   the break
               name l0
             const 1
         call end
         name l1
       label l0
     seq end
     # Epilogue
     label end

If you run HAVM, it will loop.

The problem is the recursive evaluation: it recurses to evaluate the arguments of print_int, falls on the jump, executes it, and then finds the end of the control flow (seq end). Then... it goes back to its previous recursive evaluation: that of the arguments of print_int, evaluates it, and honors the jump of the loop.

The traces demonstrate this:

      call ( name main ) []
     7.2-7.10:  label l1
     9.4-9.11:  const 1
     10.4-10.11:  const 0
     8.2-12.11:  cjump ne 1 0 ( name l2 ) ( name l0 )
     17.6-20.15:  eseq
     18.8-19.17:  jump ( name l0 )
     20.8-20.15:  const 1
     15.4-21.12:  call ( name print_int ) [1]
     15.4-21.12:  end call ( name print_int ) [1] = ()
     14.2-21.12:  sxp ()
     22.2-23.11:  jump ( name l1 )
     9.4-9.11:  const 1
     10.4-10.11:  const 0
     8.2-12.11:  cjump ne 1 0 ( name l2 ) ( name l0 )
     17.6-20.15:  eseq
     18.8-19.17:  jump ( name l0 )
     20.8-20.15:  const 1
     15.4-21.12:  call ( name print_int ) [1]
     15.4-21.12:  end call ( name print_int ) [1] = ()
     14.2-21.12:  sxp ()
     22.2-23.11:  jump ( name l1 )
     9.4-9.11:  const 1
     10.4-10.11:  const 0

etc. etc.

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3.2 Ineffective Boolean Operator

Another means to generate a jump that breaks the recursive evaluation is using optimized if. Consider the following Tiger code:

     print_int (if 0 | 0 then 0 else 1)

which translates in the following High-Level Intermediate code:

     3.0-3.10: label main
     6.0-43.10: sxp
         7.2-43.10: call
             8.4-8.18: name print_int
             9.4-42.13: eseq
                 10.4-41.11: seq
     ### The dispatching seq
                     11.6-29.13: seq
                         12.8-16.17: cjump ne
                             13.10-13.17: const 0
                             14.10-14.17: const 0
                             15.10-15.17: name l3
                             16.10-16.17: name l4
                         17.8-17.16: label l3
                         18.8-22.17: cjump ne
                             19.10-19.17: const 1
                             20.10-20.17: const 0
                             21.10-21.17: name l0
                             22.10-22.17: name l1
                         23.8-23.16: label l4
                         24.8-28.17: cjump ne
                             25.10-25.17: const 0
                             26.10-26.17: const 0
                             27.10-27.17: name l0
                             28.10-28.17: name l1
                     11.6-29.13: seq end
     ### End of the dispatching seq
                     30.6-30.14: label l0
                     31.6-33.15: move
                         32.8-32.15: temp t0
                         33.8-33.15: const 0
                     34.6-35.15: jump
                         35.8-35.15: name l2
                     36.6-36.14: label l1
                     37.6-39.15: move
                         38.8-38.15: temp t0
                         39.8-39.15: const 1
                     40.6-40.14: label l2
                 10.4-41.11: seq end
                 42.6-42.13: temp t0
         7.2-43.10: call end
     45.0-45.9: label end

Its verbose evaluation is:

     % havm --trace /tmp/broken-if.hir; echo
      call ( name main ) []
     9.4-42.13:  eseq
     10.4-41.11:  seq
     11.6-29.13:  seq
     13.10-13.17:  const 0
     14.10-14.17:  const 0
     12.8-16.17:  cjump ne 0 0 ( name l3 ) ( name l4 )
     25.10-25.17:  const 0
     26.10-26.17:  const 0
     24.8-28.17:  cjump ne 0 0 ( name l0 ) ( name l1 )
     39.8-39.15:  const 1
     37.6-39.15:  move ( temp t0 ) 1
     40.6-40.14:  label l2
     # End of the assignment seq.
     11.6-29.13:  seq end
     # Return to the dispatching seq.
     30.6-30.14:  label l0
     33.8-33.15:  const 0
     31.6-33.15:  move ( temp t0 ) 0
     34.6-35.15:  jump ( name l2 )
     10.4-41.11:  seq end
     42.6-42.13:  temp t0 = 0
     7.2-43.10:  call ( name print_int ) [0]
     7.2-43.10:  end call ( name print_int ) [0] = ()
     6.0-43.10:  sxp ()
      end call ( name main ) [] = 0

If you read the example, you will notice that there are several seqs: the innermost branches to the label l1, then when it finishes the evaluation of the outer seq, it... returns to what its call-stack dictates: the evaluation of the inner seq. The latter drives it to execute the wrong assignment, producing an incorrect result.

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Annexe A Copying This Manual

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    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.


    You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.


    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

A.1.1 ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  year  your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
       A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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