ISASPEC - XML Based ISA Specification — The Mesa 3D Graphics Library latest documentation

# ISASPEC - XML Based ISA Specification

isaspec provides a mechanism to describe an instruction set in XML, and generate a disassembler and assembler. The intention is to describe the instruction set more formally than hand-coded assembler and disassembler, and better decouple the shader compiler from the underlying instruction encoding to simplify dealing with instruction encoding differences between generations of GPU.

Benefits of a formal ISA description, compared to hand-coded assemblers and disassemblers, include easier detection of new bit combinations that were not seen before in previous generations due to more rigorous description of bits that are expect to be ‘0’ or ‘1’ or ‘x’ (dontcare) and verification that different encodings don’t have conflicting bits (i.e. that the specification cannot result in more than one valid interpretation of any bit pattern).

The isaspec tool and XML schema are intended to be generic (not specific to ir3), although there are currently a couple limitations due to short- cuts taken to get things up and running (which are mostly not inherent to the XML schema, and should not be too difficult to remove from the py and decode/disasm utility):

- Maximum “field” size is 64b

- Fixed instruction size

Often times, especially when new functionality is added in later gens while retaining (or at least mostly retaining) backwards compatibility with encodings used in earlier generations, the actual encoding can be rather messy to describe. To support this, isaspec provides many flexible mechanism, such as conditional overrides and derived fields. This not only allows for describing an irregular instruction encoding, but also allows matching an existing disasm syntax (which might not have been design around the idea of disassembly based on a formal ISA description).

## Bitsets

The fundamental concept of matching a bit-pattern to an instruction decoding/encoding is the concept of a hierarchical tree of bitsets. This is intended to match how the HW decodes instructions, where certain bits describe the instruction (and sub-encoding, and so on), and other bits describe various operands to the instruction.

Bitsets can also be used recursively as the type of a field described in another bitset.

The leaves of the tree of instruction bitsets represent every possible instruction. Deciding which instruction a bitpattern is amounts to:

    m = (val & bitsets[n]->mask) & ~bitsets[n]->dontcare;

    if (m == bitsets[n]->match) {
       /* we've found the instruction description */
    }

For example, the starting point to decode an ir3 instruction is a 64b bitset:

    <bitset name="#instruction" size="64">
         <doc>
                 Encoding of an ir3 instruction.  All instructions are 64b.
         </doc>
    </bitset>

In the first level of instruction encoding hierarchy, the high three bits group things into instruction “categories”:

    <bitset name="#instruction-cat2" extends="#instruction">
         <field name="DST" low="32" high="39" type="#reg-gpr"/>
         <field name="REPEAT" low="40" high="41" type="#rptN"/>
         <field name="SAT" pos="42" type="bool" display="(sat)"/>
         <field name="SS" pos="44" type="bool" display="(ss)"/>
         <field name="UL" pos="45" type="bool" display="(ul)"/>
         <field name="DST_CONV" pos="46" type="bool">
                 <doc>
                         Destination register is opposite precision as source, i.e.
                         if {FULL} is true then destination is half precision, and
                         visa versa.
                 </doc>
         </field>
         <derived name="DST_HALF" expr="#dest-half" type="bool" display="h"/>
         <field name="EI" pos="47" type="bool" display="(ei)"/>
         <field name="FULL" pos="52" type="bool">
                 <doc>Full precision source registers</doc>
         </field>
         <field name="JP" pos="59" type="bool" display="(jp)"/>
         <field name="SY" pos="60" type="bool" display="(sy)"/>
         <pattern low="61" high="63">010</pattern>  <!-- cat2 -->
         <!--
                 NOTE, both SRC1_R and SRC2_R are defined at this level because
                 SRC2_R is still a valid bit for (nopN) (REPEAT==0) for cat2
                 instructions with only a single src
          -->
         <field name="SRC1_R" pos="43" type="bool" display="(r)"/>
         <field name="SRC2_R" pos="51" type="bool" display="(r)"/>
         <derived name="ZERO" expr="#zero" type="bool" display=""/>
    </bitset>

The `<pattern>` elements are the part(s) that determine which leaf-node bitset matches against a given bit pattern. The leaf node’s match/mask/ dontcare bitmasks are a combination of those defined at the leaf node and recursively each parent bitclass.

For example, cat2 instructions (ALU instructions with up to two src registers) can have either one or two source registers:

    <bitset name="#instruction-cat2-1src" extends="#instruction-cat2">
         <override expr="#cat2-cat3-nop-encoding">
                 <display>
                         {SY}{SS}{JP}{SAT}(nop{NOP}) {UL}{NAME} {EI}{DST_HALF}{DST}, {SRC1}
                 </display>
                 <derived name="NOP" expr="#cat2-cat3-nop-value" type="uint"/>
                 <field name="SRC1" low="0" high="15" type="#multisrc">
                         <param name="ZERO" as="SRC_R"/>
                         <param name="FULL"/>
                 </field>
         </override>
         <display>
                 {SY}{SS}{JP}{SAT}{REPEAT}{UL}{NAME} {EI}{DST_HALF}{DST}, {SRC1}
         </display>
         <pattern low="16" high="31">xxxxxxxxxxxxxxxx</pattern>
         <pattern low="48" high="50">xxx</pattern>  <!-- COND -->
         <field name="SRC1" low="0" high="15" type="#multisrc">
                 <param name="SRC1_R" as="SRC_R"/>
                 <param name="FULL"/>
         </field>
    </bitset>

    <bitset name="absneg.f" extends="#instruction-cat2-1src">
         <pattern low="53" high="58">000110</pattern>
    </bitset>

In this example, `absneg.f` is a concrete cat2 instruction (leaf node of the bitset inheritance tree) which has a single src register. At the `#instruction-cat2-1src` level, bits that are used for the 2nd src arg and condition code (for cat2 instructions which use a condition code) are defined as ‘x’ (dontcare), which matches our understanding of the hardware (but also lets the disassembler flag cases where ‘1’ bits show up in places we don’t expect, which may signal a new instruction (sub)encoding).

You’ll notice that `SRC1` refers back to a different bitset hierarchy that describes various different src register encoding (used for cat2 and cat4 instructions), i.e. GPR vs CONST vs relative GPR/CONST. For fields which have bitset types, parameters can be “passed” in via `<param>` elements, which can be referred to by the display template string, and/or expressions. For example, this helps to deal with cases where other fields outside of that bitset control the encoding/decoding, such as in the `#multisrc` example:

    <bitset name="#multisrc" size="16">
         <doc>
                 Encoding for instruction source which can be GPR/CONST/IMMED
                 or relative GPR/CONST.
         </doc>
    </bitset>

    ...

    <bitset name="#multisrc-gpr" extends="#multisrc">
         <display>
                 {ABSNEG}{SRC_R}{HALF}{SRC}
         </display>
         <derived name="HALF" expr="#multisrc-half" type="bool" display="h"/>
         <field name="SRC" low="0" high="7" type="#reg-gpr"/>
         <pattern low="8" high="13">000000</pattern>
         <field name="ABSNEG" low="14" high="15" type="#absneg"/>
    </bitset>

At some level in the bitset inheritance hierarchy, there is expected to be a `<display>` element specifying a template string used during bitset decoding. The display template consists of references to fields (which may be derived fields) specified as `{FIELDNAME}` and other characters which are just echoed through to the resulting decoded bitset.

The special field reference `{NAME}` prints the name of the bitset. This is often useful when the `<display>` element is at a higher level than the leaves of the hierarchy, for example a whole class of similar instructions that only differ in opcode.

Sometimes there may be multiple variants of an instruction that must be different bitsets, for example because they are so different that they must derive from different bitsets, but they have the same name. Because bitset names must be unique in the encoder, this can be a problem, but this can worked around with the `displayname` attribute on the `bitset` which changes how `{NAME}` is displayed but not the name used in the encoder. `displayname` is only useful for leaf bitsets.

It is possible to define a line column alignment value per field to influence the visual output. It needs to be specified as `{FIELDNAME:align=xx}`.

The `<override>` element will be described in the next section, but it provides for both different decoded instruction syntax/mnemonics (when simply providing a different display template string) as well as instruction encoding where different ranges of bits have a different meaning based on some other bitfield (or combination of bitfields). In this example it is used to cover the cases where `SRCn_R` has a different meaning and a different disassembly syntax depending on whether `REPEAT` equals zero.

The `<template>` element can be used to represent a placeholder for a more complex `<display>` substring.

## Overrides

In many cases, a bitset is not convenient for describing the expected disasm syntax, and/or interpretation of some range of bits differs based on some other field or combination of fields. These could be modeled as different derived bitsets, at the expense of a combinatorial explosion of the size of the bitset inheritance tree. For example, every cat2 (and cat3) instruction has both a `(nopN)` interpretation in addition to the `(rptN`)` interpretation.

An `<override>` in a bitset allows to redefine the display string, and/or field definitions from the default case. If the override’s expr(ession) evaluates to non-zero, `<display>`, `<field>`, and `<derived>` elements take precedence over what is defined in the top-level of the bitset (i.e. the default case).

## Expressions

Both `<override>` and `<derived>` fields make use of `<expr>` elements, either defined inline, or defined and named at the top level and referred to by name in multiple other places. An expression is a simple ‘C’ expression which can reference fields (including other derived fields) with the same `{FIELDNAME}` syntax as display template strings. For example:

    <expr name="#cat2-cat3-nop-encoding">
         (({SRC1_R} != 0) || ({SRC2_R} != 0)) &amp;&amp; ({REPEAT} == 0)
    </expr>

In the case of `<override>` elements, the override applies if the expression evaluates to non-zero. In the case of `<derived>` fields, the expression evaluates to the value of the derived field.

## Branching

isaspec supports a few special field types for printing branch destinations. If `isaspec_decode_options::branch_labels` is true, a pre-pass over the program to be disassembled determines which instructions are branch destinations and then they are printed when disassembling, in addition to printing the name of the destination when printing the field itself.

There are two different types, which affect how the destination is computed. If the field type is `branch`, then the field is interpreted as a signed offset from the current instruction. If the type is `absbranch`, then it is interpreted as an offset from the first instruction to be disassembled. In either case, the offset is multiplied by the instruction size.

For example, here is what a signed-offset unconditional jump instruction might look like:

    <bitset name="jump" extends="#instruction">
       <display>
          jump #{OFFSET}
       </display>
       <pattern low="26" high="31">110010</pattern> <!-- opcode goes here -->
       <field name="OFFSET" low="0" high="25" type="branch"/>
    </bitset>

This would produce a disassembly like `jump #l42` if the destination is 42 instructions after the start of the disassembly. The destination would be preceded by a line with just `l42:`.

`branch` and `absbranch` fields can additionally have a `call="true"` attribute. For now, this just changes the disassembly. In particular the label prefix is changed to `fxn` and an extra empty line before the destination is added to visually separate the disassembly into functions. So, for example, a call instruction defined like this:

    <bitset name="call" extends="#instruction">
       <display>
          call #{OFFSET}
       </display>
       <pattern low="26" high="31">110010</pattern> <!-- opcode goes here -->
       <field name="OFFSET" low="0" high="25" type="branch" call="true"/>
    </bitset>

will disassemble to `call #fxn42`.

Finally, users with special knowledge about where execution may start can define “entrypoints” when disassembling which are printed like function call destinations, with an extra empty line, but with an arbitrary user-defined name. Names that are `fxn` or `l` followed by a number are discouraged because they may clash with automatically-generated names.

## Encoding

To facilitate instruction encoding, `<encode>` elements can be provided to teach the generated instruction packing code how to map from data structures representing the IR to fields. For example:

    <bitset name="#instruction" size="64">
         <doc>
                 Encoding of an ir3 instruction.  All instructions are 64b.
         </doc>
         <gen min="300"/>
         <encode type="struct ir3_instruction *" case-prefix="OPC_">
                 <!--
                         Define mapping from encode src to individual fields,
                         which are common across all instruction categories
                         at the root instruction level

                         Not all of these apply to all instructions, but we
                         can define mappings here for anything that is used
                         in more than one instruction category.  For things
                         that are specific to a single instruction category,
                         mappings should be defined at that level instead.
                  -->
                 <map name="DST">src->regs[0]</map>
                 <map name="SRC1">src->regs[1]</map>
                 <map name="SRC2">src->regs[2]</map>
                 <map name="SRC3">src->regs[3]</map>
                 <map name="REPEAT">src->repeat</map>
                 <map name="SS">!!(src->flags &amp; IR3_INSTR_SS)</map>
                 <map name="JP">!!(src->flags &amp; IR3_INSTR_JP)</map>
                 <map name="SY">!!(src->flags &amp; IR3_INSTR_SY)</map>
                 <map name="UL">!!(src->flags &amp; IR3_INSTR_UL)</map>
                 <map name="EQ">0</map>  <!-- We don't use this (yet) -->
                 <map name="SAT">!!(src->flags &amp; IR3_INSTR_SAT)</map>
         </encode>
    </bitset>

The `type` attribute specifies that the input to encoding an instruction is a `struct ir3_instruction *`. In the case of bitset hierarchies with multiple possible leaf nodes, a `case-prefix` attribute should be supplied along with a function that maps the bitset encode source to an enum value with the specified prefix prepended to uppercased leaf node name. I.e. in this case, “add.f” becomes `OPC_ADD_F`.

Individual `<map>` elements teach the encoder how to map from the encode source to fields in the encoded instruction.