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# Architecture
How gasm-devkit is put together and why.
Repository: [sourcedock.dev/petrbalvin/gasm-devkit](https://sourcedock.dev/petrbalvin/gasm-devkit)
## Design goals
1. **A real AST, not a grammar hack.** The linter, analyser, assembler and
language server all need to *reason* about assembly — not just colour it.
So the centre of the toolkit is a hand-written lexer and a parser that
produce a typed AST with source positions on every node.
2. **Architecture as data, not code.** Per-architecture differences (amd64,
arm64, riscv64, loong64) live in register and instruction *tables* (`arch`),
never in `if arch == …` branches scattered through the logic. The
instruction tables are generated from the Go toolchain's own assembler
source (`just gen`), so adding or refreshing an architecture is a data
operation, not a coding one.
3. **Open integration surface.** Everything the toolkit can do is reachable
through two vendor-neutral interfaces: a CLI and an LSP server. No editor
owns the toolkit; the toolkit is offered to editors on standard terms.
## Pipeline
```mermaid
graph TD
SRC["source .s"] --> LEX["lexer<br/>token stream"]
LEX --> PAR["parser<br/>AST + diagnostics"]
LEX --> FMT["format<br/>re-space tokens"]
PAR --> LINT["lint<br/>static checks"]
PAR --> LSP["lsp server"]
LEX --> LSP
ARCH["arch tables<br/>amd64 / arm64 / riscv64 / loong64"] --> LINT
ARCH --> LSP
LINT --> LSP
FMT --> CLI["gasm CLI"]
LINT --> CLI
PAR --> CLI
LEX --> CLI
LSP --> EDITOR["any LSP editor"]
```
The lexer is the shared foundation: the parser builds the AST from it, the
formatter re-spaces its tokens directly, and the language server uses it for
semantic highlighting.
## Components
### `token` and `lexer`
The scanner is hand-written and permissive: it never panics and maps anything
it cannot classify to an `Illegal` token, so every downstream tool still works
on malformed input. Newlines are significant tokens, because Plan 9 assembly
is line-oriented and the parser relies on line structure.
The middle dot (`·`, U+00B7) is treated as an identifier character so that
`·funcName(SB)` lexes as one symbol. Multi-character operators (`<<`, `>>`,
`->`) are recognised so arm64 shift operands scan correctly. A backslash
immediately before a newline is a C-preprocessor line continuation (used by
`#define` macros in the runtime `.s` files); the lexer splices the lines
together so a multi-line macro becomes one logical line the parser treats as an
opaque preprocessor directive.
### `ast` and `parser`
The parser is **line-oriented**, matching how the Plan 9 assembler reads a
file: it groups tokens into lines, classifies each line (directive, label,
instruction, comment, preprocessor) and dispatches. A malformed line is
reported and skipped; it never aborts the file.
Operands are parsed into a faithful, flat representation. The amd64
addressing modes — `reg`, `$imm`, `(base)`, `off(base)`, `(base)(index*scale)`,
`name+off(FP)`, `name<>(SB)` — are all captured structurally, and the original
token text is retained for fidelity.
A deliberate boundary: the AST records **syntax only**. Whether a bare
identifier is a register or a label is an *architecture* question, so it is
left to `arch` and resolved in the lint/lsp layers. This keeps the parser
arch-agnostic and its output deterministic.
### `arch`
Register files are generated programmatically (the regular `R8`–`R15`,
`X0`–`X15`, `Y0`–`Y15`, `Z0`–`Z31`, `K0`–`K7` ranges) plus the irregularly
named registers listed explicitly. Instruction names are **generated from the
Go toolchain's own assembler source** (`cmd/internal/obj/<arch>/anames.go`,
plus the common opcodes and the per-architecture front-end aliases such as the
arm64 `B`/`BL` branches and the `.P`/`.W` load-store addressing suffixes) by
`just gen`, so the tables always match what the real assembler accepts. Each
mnemonic maps to a summary and an optional operand-count range; counts are
recorded only where unambiguous (`-1` disables the operand-count lint for that
instruction) so the linter stays silent rather than guess. For architectures
with highly variable operand forms (arm64, riscv64, loong64) only a few
fixed-arity instructions (`RET`, `NOP`, `JMP`, `CALL`) carry counts at all.
### `lint`
Rules are conservative by design — silence beats a false positive. The rules
are `unknown-instruction`, `operand-count`, `undefined-label`,
`duplicate-label`, `missing-ret`, `missing-textflag-include`, `abi-argsize` and
`unreachable-code`. Every diagnostic carries a stable code so callers can
disable rules individually, and arch-specific rules switch off entirely when
the target architecture cannot be inferred from the file name.
Two things keep the rules honest on real-world code:
- **Pseudo-ops and macros are not instructions.** `unknown-instruction` knows
the assembler pseudo-ops (`BYTE`, `WORD`, `FUNCDATA`, `PCDATA`, …) and
recognises macro invocations — an in-file `#define` name, or any identifier
containing an underscore (no Plan 9 mnemonic ever does).
- **Macro-heavy files get the label/RET heuristics turned off.** Without a
preprocessor, labels a macro defines are invisible, so `undefined-label` and
`missing-ret` are suppressed for files that use macros (an in-file `#define`
or a `#include` of anything other than `textflag.h`). `missing-ret` also
treats a trailing unconditional jump and `UNDEF` as valid terminators.
The result is validated by `TestGoRuntimeCorpus`, which parses and lints every
`src/runtime/*.s` file the toolchain ships for all four architectures and
asserts zero parse errors and zero error-severity diagnostics.
Two deeper analyses sit on top of the AST:
- **`abi-argsize`.** Hand-written kernels document their signature in a
`// func …` comment above the `TEXT`. The linter parses that signature with
the standard library's Go parser, lays out the parameters and results under
Go's ABI0 stack rules (results begin on a word boundary after the
parameters), and checks the total against the argument size declared in the
`TEXT` directive. It only runs for stack-argument functions (a non-zero
declared arg area that is actually addressed through `FP`), and aborts
silently on a type whose size it cannot determine — so it never guesses.
- **`unreachable-code`.** Code after a `RET` and before the next label is
dead. The check is suppressed for any function whose reachability cannot be
decided statically: those using PC-relative jumps (`JMP 2(PC)`),
register-indirect branches (`JALR`/`JR`/`JIRL`/`BR`/`BLR`), or living in a
file with `#ifdef` conditionals. `UNDEF` is deliberately not a terminator —
code after it is occasionally intentional metadata.
- **`register-clobber` (register liveness).** The linter builds the function's
control-flow graph (basic blocks split at labels and after branches, with
fall-through and jump-target edges), computes a conservative per-instruction
register def/use, and runs the standard backward liveness iteration to a fixed
point. On top of that it flags writes to the registers the **Go ABI** fixes
across calls that are never saved and restored — calibrated from
`cmd/compile/abi-internal.md`, *not* the platform ABI: Go's stack-based ABI0
has no System V style callee-saved registers (amd64 `BX`, `R12`–`R15` and
the like are caller-saved or permanent scratch, and hand-written kernels may
clobber them freely). The audited set is the frame pointer and the
the frame pointer, the goroutine pointer per architecture (amd64 `BP`/`R14`, arm64 `R18`/`R28`/
`R29`, riscv64 `X27`, loong64 `R22`); the goroutine pointer is reported only
when the function can reach the runtime — it is not `NOSPLIT` or makes a
call — since the ABI0 transition machinery restores it on those paths, and
NOSPLIT call-free leaves may use it (the runtime's own assembly does). It
runs only on macro-free files, where
no opaque macro can perform the save/restore.
- **`funcdata-pcdata`.** `FUNCDATA $idx, sym(SB)` and `PCDATA $idx, $val` are
checked for well-formed operands (arity, immediate index and value, symbol
reference) and a literal index is range-checked; a named index constant such
as `$PCDATA_StackMapIndex` is accepted without a range check.
### `format`
The formatter works on the **token stream, not the AST**, so it preserves
every line — comments and blanks included. It normalises indentation, operand
spacing, per-function mnemonic alignment and blank-line layout: a new block
(a label, `TEXT` or `GLOBL`) is preceded by exactly one blank line (comments
leading a block stay with it), runs of blanks collapse to one, and a `RET`
terminates the body so the next function's doc comment stays at column 0. It
is idempotent and its output always round-trips through the parser. With a
directory argument — or none — it reformats every `.s` file below it in
place and lists the files changed, the way `go fmt` does (`.` and `_`
directories are skipped).
### `lsp`
The server speaks JSON-RPC 2.0 with `Content-Length` framing over any
`io.Reader`/`io.Writer` (normally stdin/stdout). It maintains an in-memory
document store, republishes diagnostics on every change, and provides:
- **completion** — instructions, registers, pseudo-registers, textflag macros
and local labels;
- **hover** — instruction summaries and register descriptions from `arch`;
- **document symbols** — `TEXT` functions with their labels, plus `GLOBL`/`DATA`;
- **semantic tokens** — syntax highlighting delivered as LSP semantic tokens,
classified with the lexer plus `arch` (instructions, registers by class,
pseudo-registers, labels, immediates, comments, directives, textflag macros).
Semantic tokens are the key to editor-agnostic highlighting: the editor renders
them from the standard LSP legend, so no editor-specific grammar is needed.
### `asm`
The standalone assembler (Phase 2). Its core is an amd64 instruction encoder:
a REX/ModR-M/SIB/displacement/immediate engine plus the scalar instruction set,
with the Plan 9 operand order (source first) mapped onto the x86 encoding.
Every encoding is validated by decoding it again with `golang.org/x/arch` — the
one module dependency, used in tests only and never linked into the binary.
A **RISC-V encoder** (Phase 5, RV64IMAFDC + RVC compression) encodes the full
integer, atomic, float/double, FMA and CSR instruction sets with the MOV
pseudo-instruction and SB/global symbol references (AUIPC pairs with
R_RISCV_PCREL_HI20/LO12 relocations). The encoder compresses eligible
instructions to 16-bit RVC forms and is validated byte-for-byte against
`GOARCH=riscv64 go tool asm`.
A **LoongArch encoder** (Phase 5, LoongArch64) encodes the integer and
floating-point instruction sets with the dual-form arithmetic mnemonics (3R
vs 2RI12), the 16/21-bit branch families, the MOV pseudo-instruction and its
constant materialisation (the dcon classification driving lu12i.w/ori/lu32i.d/
lu52i.d expansions), the FP/SP frame mapping (autosize = align8(frame+8),
prologue storing the link register before and after the SP decrement) and
SB/global symbol references (pcalau12i pairs with R_LOONG64_ADDR_HI/LO
relocations). Like the RISC-V encoder it is validated byte-for-byte against
`GOARCH=loong64 go tool asm`, and its GOOBJ output is proven end-to-end by
substituting it into a cross-compiled `go build` and linking with `cmd/link`.
An **AArch64 encoder** (Phase 5, arm64) encodes the integer instruction set
with the data-processing (shifted register and immediate forms), load/store
(scaled unsigned immediate and unscaled9-bit immediate), conditional and
unconditional branches, the MOV pseudo-instruction and its constant
materialisation (MOVZ/MOVN/MOVK for wide immediates, ORR with logical bitmask
encoding for values like `$1`), the FP/SP frame mapping (autosize =
align16(frame+8), prologue using pre-index store for small frames and
STP+SUB for large frames) and SB/global symbol references (ADRP+ADD pairs with
R_ADDRARM64 relocations). Like the other encoders it is validated
byte-for-byte against `GOARCH=arm64 go tool asm`.
On top of the encoder, `Assemble` walks a parsed `TEXT` body, converts each
operand to an encoder operand, and lays the instructions out so local labels
resolve to relative jump offsets: jumps start in the short (rel8) form and
expand to rel32 when the settled displacement does not fit, iterating to a
fixed point, and jump-to-jump chains are folded (a conditional jump to a label
whose only instruction is an unconditional jump is redirected to the ultimate
target) exactly as the Go toolchain's linker does before it encodes branches.
The `FP`/`SP` pseudo-
registers are translated onto the hardware stack pointer — `x+N(FP)` becomes
`(N+8)(SP)` for a zero-frame function and `(N+frame+16)(SP)` once a frame
pointer is set up, with the matching Go prologue/epilogue generated — so the
output is byte-identical to the Go assembler for these cases. SIMD is handled
by a VEX (AVX/AVX2) encoder — the two- and three-byte VEX prefixes with XMM/YMM
registers — across eight operand forms: the three-operand NDS form, the
two-operand reg/rm form, the immediate-shift form (plus the variable-count
shifts, which share the NDS shape with the count in an XMM register or
memory), the immediate shuffle form (`VPSHUFD`, `VPERMQ`), the
three-operand-plus-immediate form (`VSHUFPD`,
`VPERM2I128`, `VINSERTI128`), the lane-extract form (`VEXTRACTI128`,
`VEXTRACTF128`, where the YMM source occupies the reg field and the XMM or
memory destination r/m), the direction-sensitive moves (`VMOVDQU`, `VMOVUPD`,
`VMOVD`, `VMOVQ`, `VMOVSD`), the floating-point and FMA arithmetic — the
packed double operations (`VADDPD`/`VSUBPD`/`VMULPD`/`VDIVPD`/`VMINPD`/
`VMAXPD`), the unpacks (`VUNPCKHPD`/`VUNPCKLPD`), the scalar SD and SS
operations, `VMOVDDUP`, `VXORPD`, the width-changing conversions
(`VCVTDQ2PS`, `VCVTPS2PD`, `VCVTDQ2PD`, and the `VCVTPD2DQX`/`Y` and
`VCVTTPD2DQX`/`Y` spellings, whose length follows the wider source) and
`VFMADD231PD` — and the no-operand `VZEROUPPER`, together with `VPERMD` and
the scalar families (`CMOVcc`, `SETcc`, `LZCNT`/`TZCNT`, the extending moves,
`CVTSx2SD`, `IMUL3`) and the EVEX (AVX-512) prefix — the four-byte prefix with
5-bit register fields (Z0–Z31, X/Y 16–31, with the mod=11 quirk that carries
rm[4] in X̄), opmask registers (K0–K7 as operands, mask destinations and
explicit merging/zeroing masks — written the way Go writes them, as a K
operand among the operands plus a `.Z` mnemonic suffix), and the compressed
disp8×N displacement, whose multiplier follows the memory operand's size —
covering every instruction the go-flac and go-lz4 AVX2/AVX-512 kernels use,
plus the common AVX-512 F/BW integer set, the floating-point and conversion
set (the packed double and single arithmetic, the scalar SD/SS forms —
whose EVEX encodings serve masked and zeroing use — `VMOVDDUP`, the
replicating moves, and the width-changing conversions, including the
`VCVTPD2DQ`/`VCVTTPD2DQ` family whose length follows the wider source
operand), and the wider AVX-512 set: ternary logic, lane shuffles, inserts
and extracts, compares with an opmask destination, the permutes, the
expand/compress family, the broadcasts, the opmask-register instructions
(KAND/KOR/KXNOR/KADD/KUNPCK/KNOT/KSHIFTL/KORTEST and KMOVQ), the aligned
moves and the remaining extending/narrowing moves, the floating-point
helper and conversion tail (VRCP14*, VRSQRT14*, VGETEXP*, VGETMANT*,
VSCALEF*, VRNDSCALE*, VREDUCE*, VFIXUPIMM*, VRANGE*, VFPCLASS* with an
opmask destination, and the VCVT* conversions — signed, unsigned and
truncating, including the length-suffixed X/Y spellings and the
mask/vector conversions VPMOVM2*/VPMOV*2M, and the scalar conversions
between vector and general-purpose registers (VCVT{,T}S{D,S}2SI{,Q} and
the unsigned forms, VCVTSI2*/VCVTUSI2*), and gather/scatter with VSIB addressing — both the
VEX spelling with a vector mask register and the EVEX spelling with an
explicit K mask, where the EVEX length follows the VSIB index register,
not the data register. The EVEX mnemonic
suffixes — rounding modes (.RN_SAE/.RD_SAE/.RU_SAE/.RZ_SAE),
suppress-all-exceptions (.SAE) and memory broadcast (.BCST) — set the EVEX
b bit and the L'L rounding-control field (broadcast keeps the vector length
and scales disp8 by the element size), and combine with the .Z zeroing
suffix. Every encoding is validated two ways: by
round-trip decoding through `golang.org/x/arch`, and byte-for-byte against
the machine code the real Go assembler emits — a comparison that holds for
whole functions: all 27 functions of both kernels assemble to exactly the Go
toolchain's bytes, the lone exception being the displacements of the
static-constant loads, which the Go linker fills at link time.
File-level assembly (`AssembleFile`) goes beyond single functions: it
materialises the file's static symbols (`GLOBL`/`DATA`) in a data section
behind the code and resolves references to them (`mask<>(SB)`) to
RIP-relative loads whose displacements point inside the resulting image, so
the bytes are self-consistent at any base address. References to symbols no
`GLOBL` defines are kept as relocations on the function layout, and the
object-file emitters turn the whole image into a linkable object: the ELF
writer (`gasm asm --format elf`) lays the code and data out as `.text`/`.data`
sections, exports a symbol per
`TEXT` and `GLOBL` (the `<>` ones local, the rest global) and emit one
PC-relative relocation per static-symbol reference — undefined external
symbols included, so the output links with the system toolchain. The GOOBJ
emitter (`gasm asm --format goobj`) writes the format the Go linker consumes
directly: the functions as non-package symbols (the way `cmd/asm` records
assembly symbols), the `GLOBL` data, one `FuncInfo` per function and the
pc-value tables — `pcsp` built from the prologue and epilogue stack
boundaries, plus flat `pcfile`, `pcline` and `pcinline` tables — so a
gasm-assembled object drops into a `go build` in place of the toolchain's.
The object preamble (the version-and-experiment header the linker compares
verbatim) is captured from the installed `go tool asm`, so the output is
always consistent with the toolchain that links it. RISC-V and LoongArch
GOOBJ emission share this emitter: the loong64 marker with
R_LOONG64_ADDR_HI/LO relocation types, and the riscv64 marker with a single
R_RISCV_PCREL_ITYPE/STYPE relocation per AUIPC pair (plus `R_RISCV_JAL` for
`CALL sym(SB)`) — the model `cmd/asm`
writes, not the ELF HI20/LO12 pair — and both link into a real `go build` for
their `GOARCH`. Per function, the emitter also writes the two DWARF
symbols the linker's DWARF pass reads verbatim — the subprogram DIE
(`SDWARFFCN`) and the `.debug_line` state-machine program (`SDWARFLINES`),
both built the way `cmd/asm` builds them (the DIE carries the
R_DWTXTADDR_U4 address reference; the line program one row per source-line
change, in the same special-opcode encoding) — and the pc-value deltas are
in the architecture's MinLC units, as the runtime's `pcvalue` expects.
External cross-package references remain future work (the amd64 and RISC-V
paths resolve them; LoongArch does not yet); the rest of Phase 2 is those
and the remaining EVEX forms.
### `verify`
The dynamic-analysis substrate (Phase 3). It JIT-loads assembled images into
executable memory and invokes them directly, enabling differential testing,
runtime ABI checks and coverage profiling.
The execution model is pure Go (stdlib only). `Map` copies machine code into
an anonymous `syscall.Mmap` mapping and enforces W^X (write the bytes, then
`mprotect` to read-execute). `Call` prepares a stack whose first word is the
address of an assembly trampoline (`leaveJIT`), lays the ABI0 argument
block after it, switches to that stack via `enterJIT` (which saves the Go
stack pointer in a package global and jumps to the target), and recovers
control when the function RETs into `leaveJIT` (which restores the Go stack
and returns). A 64-byte pad below the return address accommodates the
ABIInternal wrapper that the Go runtime interposes on assembly functions.
`Load` / `LoadSource` / `LoadAST` parse, assemble and map a `.s` file in one
step, returning a `Kernel` whose `CallFunc` method marshals the argument block
by name. The image must be self-contained (no external relocations); the
assembler’s `Image.Bytes()` provides the code-and-data concatenation.
The `gasm verify` CLI subcommand exposes this: it loads a file, reports the
available functions and (with `-smoke`) calls each NOSPLIT function with zeroed
arguments to confirm the trampoline round-trips. `gasm verify --fuzz` combines
ABI checks (sentinel registers, canary, stack bounds) with differential fuzz
testing, comparing the JIT-assembled kernel against the portable Go reference
bit-for-bit while verifying the ABI contract on every iteration. When a fuzz
iteration crashes or mismatches, `FuzzResult.CrashInput` stores the exact input
for reproducibility. `gasm verify --call <func> --buf name:size:pattern`
invokes a single function with user-supplied buffers (patterns: zero, ones,
seq, or hex), printing the ABI0 argument block before and after the call —
useful for partial functions (e.g. decoders) that crash on random input but
should succeed on valid data. `gasm verify --ground-truth` compares the
assembled machine code byte-for-byte against `go tool asm` (relocation sites
masked), reporting any encoding drift.
### `debug`
The interactive debugger (Phase 4, linux/amd64). It launches the target
function in a child process that maps the JIT code, calls
`PTRACE_TRACEME`, and stops; the parent attaches via ptrace and controls
execution. Breakpoints are patched as INT3 bytes through `/proc/pid/mem`
(PTRACE_PEEKTEXT is unreliable with Go's multi-threaded runtime).
The child pins its goroutine to the OS thread with `runtime.LockOSThread`
so the traced thread is the one executing JIT code. The REPL provides
single-step, register inspection (GPR + YMM/XMM via `PTRACE_GETFPREGS`),
label resolution, named buffer allocation with pattern filling
(`--buf name:size:pattern` — zero, ones, seq, or hex), and breakpoint
management.
## Extension points
- **New architecture:** add an entry to the generator in `_gen`, run
`just gen`, and add a `buildXXX()` register file plus a case in `ForArch`.
- **New lint rule:** add a function in `lint` and a rule-code constant.
- **New LSP feature:** add a method case in `dispatch` and a handler.
The phases follow a dependency chain. Phase 1 (static analysis) builds only on
the AST; Phase 2 (the standalone assembler) emits object code; Phases 3
(dynamic analysis) and 4 (the debugger) both consume the execution substrate
that the assembler provides.