The lcc 4.1 Code-Generation Interface

Christopher W. Fraser and David R. Hanson, Microsoft Research

Contents

  • Introduction
  • 5.1 Type Metrics
  • 5.3 Symbols
  • 5.5 Dag Operators
  • 5.6 Interface Flags
  • 5.8 Definitions
  • 5.9 Constants
  • 5.12 Upcalls
  • Introduction

    Version 4.1 is the latest release of lcc, the ANSI C compiler described in our book A Retargetable C Compiler: Design and Implementation (Addison-Wesley, 1995, ISBN 0-8053-1670-1). This document summarizes the differences between the 4.1 code-generation interface and the 3.x interface described in Chap. 5 of A Retargetable C Compiler.

    Previous versions of lcc supported only three sizes of integers, two sizes of floats, and insisted that pointers fit in unsigned integers (see Sec. 5.1 of A Retargetable C Compiler). These assumptions simplified the compiler, and were suitable for 32-bit architectures. But on 64-bit architectures, such as the DEC ALPHA, it's natural to have four sizes of integers and perhaps three sizes of floats, and on 16-bit architectures, 32-bit pointers don't fit in unsigned integers. Also, the 3.x constaints limited the use of lcc's back ends for other languages, such as Java.

    Version 4.x removes all of these restrictions: It supports any number of sizes for integers and floats, and the size of pointers need not be related to the size of any of the integer types. The major changes in the code-generation interface are:

    In addition, version 4.x is written in ANSI C and uses the standard I/O library and other standard C functions.

    The sections below parallel the subsections of Chap. 5 of A Retargetable C Compiler and summarize the differences between the 3.x and 4.x code-generation interface. Unaffected subsections are omitted. Page citations refer to pages in A Retargetable C Compiler.

    5.1 Type Metrics

    There are now 10 metrics in an interface record:

    Metrics charmetric;
    Metrics shortmetric;
    Metrics intmetric;
    Metrics longmetric;
    Metrics longlongmetric;
    Metrics floatmetric;
    Metrics doublemetric;
    Metrics longdoublemetric;
    Metrics ptrmetric;
    Metrics structmetric;

    Each of these specifies the size and alignment of the corresponding type. ptrmetric describes all pointers.

    5.3 Symbols

    The actual value of a constant is stored in the u.c.v field of a symbol, which holds a Value:

    typedef union value {
    	long i;
    	unsigned long u;
    	long double d;
    	void *p;
    	void (*g)(void);
    } Value;

    The value is stored in the appropriate field according to its type, which is given by the symbol's type field.

    5.5 Dag Operators

    The op field of a node structure holds a dag operator, which consists of a generic operator, a type suffix, and a size indicator. The type suffixes are:

    enum {
    	F=FLOAT,
    	I=INT,
    	U=UNSIGNED,
    	P=POINTER,
    	V=VOID,
    	B=STRUCT
    };
    
    #define sizeop(n) ((n)<<10)

    Given a generic operator o, a type suffix t, and a size s, a type- and size-specific operator is formed by o+t+sizeop(s). For example, ADD+F+sizeop(4) forms the operator ADDF4, which denotes the sum of two 4-byte floats. Similarly, ADD+F+sizeop(8) forms ADDF8, which denotes 8-byte floating addition. In the 3.x code-generation interface, ADDF and ADDD denoted these operations. There was no size indicator in the 3.x operators because the type suffix supplied both a type and a size.

    Table 5.1 lists each generic operator, its valid type suffixes, and the number of kids and syms that it uses; multiple values for kids indicate type-specific variants. The notations in the syms column give the number of syms values and a one-letter code that suggests their uses: 1V indicates that syms[0] points to a symbol for a variable, 1C indicates that syms[0] is a constant, and 1L indicates that syms[0] is a label. For 1S, syms[0] is a constant whose value is a size in bytes; 2S adds syms[1], which is a constant whose value is an alignment. For most operators, the type suffix and size indicator denote the type and size of operation to perform and the type and size of the result.

    Table 5.1|Node Operators.
    syms kids Operator Type Suffixes Sizes Operation
    1V 0 ADDRF ...P.. p address of a parameter
    1V 0 ADDRG ...P.. p address of a global
    1V 0 ADDRL ...P.. p address of a local
    1C 0 CNST FIUP.. fdx csilh p constant
    |
    1 BCOM .IU... ilh bitwise complement
    1S 1 CVF FI.... fdx ilh convert from float
    1S 1 CVI FIU... fdx csilh csilhp convert from signed integer
    1S 1 CVP ..U.. p convert from pointer
    1S 1 CVU .IUP.. csilh p convert from unsigned integer
    1 INDIR FIUP.B fdx csilh p fetch
    1 NEG FI.... fdx ilh negation
    |
    2 ADD FIUP.. fdx ilh ilhp p addition
    2 BAND .IU... ilh bitwise AND
    2 BOR .IU... ilh bitwise inclusive OR
    2 BXOR .IU... ilh bitwise exclusive OR
    2 DIV FIU... fdx ilh division
    2 LSH .IU... ilh left shift
    2 MOD .IU... ilh modulus
    2 MUL FIU... fdx ilh multiplication
    2 RSH .IU... ilh right shift
    2 SUB FIUP.. fdx ilh ilhp p subtraction
    |
    2S 2 ASGN FIUP.B fdx csilh p assignment
    1L 2 EQ FIU... fdx ilh ilhp jump if equal
    1L 2 GE FIU... fdx ilh ilhp jump if greater than or equal
    1L 2 GT FIU... fdx ilh ilhp jump if greater than
    1L 2 LE FIU... fdx ilh ilhp jump if less than or equal
    1L 2 LT FIU... fdx ilh ilhp jump if less than
    1L 2 NE FIU... fdx ilh ilhp jump if not equal
    2S 1 ARG FIUP.B fdx ilh p argument
    1 1 or 2 CALL FIUPVB fdx ilh p function call
    1 RET FIUPV. fdx ilh p return from function
    |
    1 JUMP ....V. unconditional jump
    1L 0 LABEL ....V. label definition

    The entries in the Sizes column indicate sizes of the operators that back ends must implement. Letters denote the size of float (f), double (d), long double (x), character (c), short integer (s), integer (i), long integer (l), "long long" integer (h) , and pointer (p). These sizes are separated into sets for each type suffix, except that a single set is used for both I and U when the set for I is identical to the set for U.

    The actual values for the size indicators, fdxcsilhp, depend on the target. A specification like ADDFf denotes the operator ADD+F+sizeop(f), where "f" is replaced by a target-dependent value, e.g., ADDF4 and ADDF8. For example, back ends must implement the following CVI and MUL operators.

    CVIFf CVIFd CVIFx
    CVIIc CVIIs CVIIi CVIIl CVIIh
    CVIUc CVIUs CVIUi CVIUl CVIUh CVIUp

    MULFf MULFd MULFx
    MULIi MULIl MULIh
    MULUi MULUl MULUh

    On most platforms, there are fewer than three sizes of floats and six sizes of integers, and pointers are usually the same size as one of the integers. And lcc doesn't support the "long long" type, so h is not currently used. So the set of platform-specific operators is usually smaller than the list above suggests. For example, the X86, SPARC, and MIPS back ends implement the following CVI and MUL operators.

    CVIF4 CVIF8
    CVII1 CVII2 CVII4
    CVIU1 CVIU2 CVIU4

    MULF4 MULF8
    MULI4
    MULU4

    The set of operators is thus target-dependent; for example, ADDI8 appears only if the target supports an 8-byte integer type. ops.c is a program that, given a set of sizes, prints the required operators and their values, e.g.,

    % ops c=1 s=2 i=4 l=4 h=4 f=4 d=8 x=8 p=4
    ...
     CVIF4=4225 CVIF8=8321
     CVII1=1157 CVII2=2181 CVII4=4229
     CVIU1=1158 CVIU2=2182 CVIU4=4230
    ...
     MULF4=4561 MULF8=8657
     MULI4=4565
     MULU4=4566
    ...
    131 operators

    The type suffix for a conversion operator denotes the type of the result and the size indicator gives the size of the result. For example, CVUI4 converts an unsigned (U) to a 4-byte signed integer (I4). The syms[0] field points to a symbol-table entry for an integer constant that gives the size of the source operand. For example, if syms[0] in a CVUI4 points to a symbol-table entry for 2, the conversion widens a 2-byte unsigned integer to a 4-byte signed integer. Conversions that widen unsigned integers zero-extend; those that widen signed integers sign-extend.

    The front end composes conversions between types T1 and T2 by widening T1 to its "supertype", if necessary, converting that result to T2's supertype, then narrowing the result to T2, if necessary. The following table lists the supertypes; omitted entries are their own supertypes.

    Type | Supertype
    signed char int
    signed short int
    unsigned char int, if sizeof (char) < sizeof (int)
    unsigned, otherwise
    unsigned short int, if sizeof (short) < sizeof (int)
    unsigned, otherwise
    void * an unsigned type as large as a pointer

    Pointers are converted to an unsigned type of the same size, even when that type is not one of the integer types.

    For example, the front end converts a signed short to a float by first converting it to an int and then to a float. It converts an unsigned short to an int with a single CVUIi conversion, when shorts are smaller than ints.

    There are now signed and unsigned variants of ASGN, INDIR, BCOM, BOR, BXOR, BAND, ARG, CALL, and RET to simplify code generation on platforms that use different instructions or register set for signed and unsigned operations. Likewise there are now pointer variants of ASGN, INDIR, ARG, CALL, and RET.

    5.6 Interface Flags

    unsigned unsigned_char:1;

    tells the front end whether plain characters are signed or unsigned. If it's zero, char is a signed type; otherwise, char is an unsigned type.

    All the interface flags can be set by command-line options, e.g., -Wf-unsigned_char=1 causes plain characters to be unsigned.

    5.8 Definitions

    The front end announces local variables by calling

    void (*local)(Symbol);

    It announces temporaries likewise; these have the symbol's temporary flag set, which indicates that the symbol will be used only in the next call to gen. If a temporary's u.t.cse field is nonnull, it points to the node that computes the value assigned to the temporary; see page 346.

    The front end calls

    void (*address)(Symbol p, Symbol q, long n);

    to initialize q to a symbol that represents an address of the form x+n, where x is the address represented by p and the long integer n is positive or negative.

    5.9 Constants

    The interface function

    void (*defconst)(int suffix, int size, Value v);

    initializes constants. defconst emits directives to define a cell and initialize it to a constant value. v is the constant value, suffix identifies the type of the value, and size is the size of the value in bytes. The value of suffix indicates which field of v holds the value, as shown in the following table.

    suffix | v Field | size
    F v.d float, double, long double
    I v.i signed char, signed short, signed int, signed long
    U v.u unsigned char, unsigned short, unsigned int, unsigned long
    P v.p void *

    defconst must narrow v.x when size is less than sizeof v.x; e.g., to emit an unsigned char, defconst should emit (unsigned char)v.i.

    5.12 Upcalls

    lcc 4.x uses standard I/O and its I/O functions have been changed accordingly. lcc reads input from the standard input, emits code to the standard output, and writes diagnostics to the standard error output. It uses freopen to redirect these streams to explicit files, when necessary.

    bp, outflush, and outs have been eliminated.

    extern void fprint(FILE *f, const char *fmt, ...);
    extern void  print(const char *fmt, ...);

    print formatted data to file f (fprint) or the standard output (print). These functions are like standard C's printf and fprintf, but support only some of the standard conversion specifiers and do not support flags, precision, and field-width specifications. They support the following new conversion specifiers in addition to those described on page 99.

    Specifiers | Corresponding printf Specifiers
    %c %c
    %d %D %d %ld
    %u %U %u %lu
    %x %X %x %lx
    %f %e %g %e %f %g
    %p Converts the corresponding void * argument to unsigned long and prints it with the printf %#x specifier or just %x when the argument is null.
    %I Prints the number of spaces given by the corresponding argument.
    #define generic(op)  ((op)&0x3F0)
    #define specific(op) ((op)&0x3FF)

    generic(op) returns the generic variant of op; that is, without its type suffix and size indicator. specific(op) returns the type-specific variant of op; that is, without its size indicator.

    newconst has been replaced by

    extern Symbol intconst(int n);

    which installs the integer constant n in the symbol table, if necessary, and returns a pointer to the symbol-table entry.


    Chris Fraser / cwfraser@microsoft.com
    David Hanson / drh@microsoft.com
    $Revision: 145 $ $Date: 2001-10-17 16:53:10 -0500 (Wed, 17 Oct 2001) $