Decodetree Specification — QEMU documentation

# Decodetree Specification

A decodetree is built from instruction patterns. A pattern may represent a single architectural instruction or a group of same, depending on what is convenient for further processing.

Each pattern has both fixedbits and fixedmask, the combination of which describes the condition under which the pattern is matched:

    (insn & fixedmask) == fixedbits

Each pattern may have fields, which are extracted from the insn and passed along to the translator. Examples of such are registers, immediates, and sub-opcodes.

In support of patterns, one may declare fields, argument sets, and formats, each of which may be re-used to simplify further definitions.

## Fields

Syntax:

    field_def     := '%' identifier ( unnamed_field )* ( !function=identifier )?
    unnamed_field := number ':' ( 's' ) number

For unnamed_field, the first number is the least-significant bit position of the field and the second number is the length of the field. If the ‘s’ is present, the field is considered signed. If multiple `unnamed_fields` are present, they are concatenated. In this way one can define disjoint fields.

If `!function` is specified, the concatenated result is passed through the named function, taking and returning an integral value.

One may use `!function` with zero `unnamed_fields`. This case is called a parameter, and the named function is only passed the `DisasContext` and returns an integral value extracted from there.

A field with no `unnamed_fields` and no `!function` is in error.

Field examples:

Input | Generated code
--- | ---
%disp 0:s16 | sextract(i, 0, 16)
%imm9 16:6 10:3 | extract(i, 16, 6) << 3 | extract(i, 10, 3)
%disp12 0:s1 1:1 2:10 | sextract(i, 0, 1) << 11 | extract(i, 1, 1) << 10 | extract(i, 2, 10)
%shimm8 5:s8 13:1 !function=expand_shimm8 | expand_shimm8(sextract(i, 5, 8) << 1 | extract(i, 13, 1))

## Argument Sets

Syntax:

    args_def    := '&' identifier ( args_elt )+ ( !extern )?
    args_elt    := identifier (':' identifier)?

Each args_elt defines an argument within the argument set. If the form of the args_elt contains a colon, the first identifier is the argument name and the second identifier is the argument type. If the colon is missing, the argument type will be `int`.

Each argument set will be rendered as a C structure “arg_$name” with each of the fields being one of the member arguments.

If `!extern` is specified, the backing structure is assumed to have been already declared, typically via a second decoder.

Argument sets are useful when one wants to define helper functions for the translator functions that can perform operations on a common set of arguments. This can ensure, for instance, that the `AND` pattern and the `OR` pattern put their operands into the same named structure, so that a common `gen_logic_insn` may be able to handle the operations common between the two.

Argument set examples:

    &reg3       ra rb rc
    &loadstore  reg base offset
    &longldst   reg base offset:int64_t

## Formats

Syntax:

    fmt_def      := '@' identifier ( fmt_elt )+
    fmt_elt      := fixedbit_elt | field_elt | field_ref | args_ref
    fixedbit_elt := [01.-]+
    field_elt    := identifier ':' 's'? number
    field_ref    := '%' identifier | identifier '=' '%' identifier
    args_ref     := '&' identifier

Defining a format is a handy way to avoid replicating groups of fields across many instruction patterns.

A fixedbit_elt describes a contiguous sequence of bits that must be 1, 0, or don’t care. The difference between ‘.’ and ‘-’ is that ‘.’ means that the bit will be covered with a field or a final 0 or 1 from the pattern, and ‘-’ means that the bit is really ignored by the cpu and will not be specified.

A field_elt describes a simple field only given a width; the position of the field is implied by its position with respect to other fixedbit_elt and field_elt.

If any fixedbit_elt or field_elt appear, then all bits must be defined. Padding with a fixedbit_elt of all ‘.’ is an easy way to accomplish that.

A field_ref incorporates a field by reference. This is the only way to add a complex field to a format. A field may be renamed in the process via assignment to another identifier. This is intended to allow the same argument set be used with disjoint named fields.

A single args_ref may specify an argument set to use for the format. The set of fields in the format must be a subset of the arguments in the argument set. If an argument set is not specified, one will be inferred from the set of fields.

It is recommended, but not required, that all field_ref and args_ref appear at the end of the line, not interleaving with fixedbit_elf or field_elt.

Format examples:

    @opr    ...... ra:5 rb:5 ... 0 ....... rc:5
    @opi    ...... ra:5 lit:8    1 ....... rc:5

## Patterns

Syntax:

    pat_def      := identifier ( pat_elt )+
    pat_elt      := fixedbit_elt | field_elt | field_ref | args_ref | fmt_ref | const_elt
    fmt_ref      := '@' identifier
    const_elt    := identifier '=' number

The fixedbit_elt and field_elt specifiers are unchanged from formats. A pattern that does not specify a named format will have one inferred from a referenced argument set (if present) and the set of fields.

A const_elt allows a argument to be set to a constant value. This may come in handy when fields overlap between patterns and one has to include the values in the fixedbit_elt instead.

The decoder will call a translator function for each pattern matched.

Pattern examples:

    addl_r   010000 ..... ..... .... 0000000 ..... @opr
    addl_i   010000 ..... ..... .... 0000000 ..... @opi

which will, in part, invoke:

    trans_addl_r(ctx, &arg_opr, insn)

and:

    trans_addl_i(ctx, &arg_opi, insn)

## Pattern Groups

Syntax:

    group            := overlap_group | no_overlap_group
    overlap_group    := '{' ( pat_def | group )+ '}'
    no_overlap_group := '[' ( pat_def | group )+ ']'

A group begins with a lone open-brace or open-bracket, with all subsequent lines indented two spaces, and ending with a lone close-brace or close-bracket. Groups may be nested, increasing the required indentation of the lines within the nested group to two spaces per nesting level.

Patterns within overlap groups are allowed to overlap. Conflicts are resolved by selecting the patterns in order. If all of the fixedbits for a pattern match, its translate function will be called. If the translate function returns false, then subsequent patterns within the group will be matched.

Patterns within no-overlap groups are not allowed to overlap, just the same as ungrouped patterns. Thus no-overlap groups are intended to be nested inside overlap groups.

The following example from PA-RISC shows specialization of the or instruction:

    {
      {
        nop   000010 ----- ----- 0000 001001 0 00000
        copy  000010 00000 r1:5  0000 001001 0 rt:5
      }
      or      000010 rt2:5 r1:5  cf:4 001001 0 rt:5
    }

When the cf field is zero, the instruction has no side effects, and may be specialized. When the rt field is zero, the output is discarded and so the instruction has no effect. When the rt2 field is zero, the operation is `reg[r1] | 0` and so encodes the canonical register copy operation.

The output from the generator might look like:

    switch (insn & 0xfc000fe0) {
    case 0x08000240:
      /* 000010.. ........ ....0010 010..... */
      if ((insn & 0x0000f000) == 0x00000000) {
          /* 000010.. ........ 00000010 010..... */
          if ((insn & 0x0000001f) == 0x00000000) {
              /* 000010.. ........ 00000010 01000000 */
              extract_decode_Fmt_0(&u.f_decode0, insn);
              if (trans_nop(ctx, &u.f_decode0)) return true;
          }
          if ((insn & 0x03e00000) == 0x00000000) {
              /* 00001000 000..... 00000010 010..... */
              extract_decode_Fmt_1(&u.f_decode1, insn);
              if (trans_copy(ctx, &u.f_decode1)) return true;
          }
      }
      extract_decode_Fmt_2(&u.f_decode2, insn);
      if (trans_or(ctx, &u.f_decode2)) return true;
      return false;
    }