Reverse engineering NVIDIA SASS from controlled kernels to production audits.

Article 1 · Article 2 · Knowledge base · Pattern library · SM120 instruction glossary · Encoding notes · Start here · Project structure · Tensor-core chapters · Contributing

SASS King is a systematic reverse-engineering project for NVIDIA SASS, the native GPU instruction set emitted inside compiled CUDA binaries. The project starts with SM120 / SM120a consumer Blackwell hardware and expands toward a full cross-architecture ISA and pattern library over time.

The goal is practical: help a kernel engineer open a SASS dump, recognize compiler patterns, identify performance-relevant structures, and connect the binary back to source-level optimization decisions.

The project has completed its initial Phase 3 pattern library: 29 reusable SASS signatures are now formalized under patterns/, with knowledge/FINDINGS.md kept as the full evidence trail. The next major step is Phase 4: applying those patterns to real production kernels.

The repository is organized as an evidence pipeline:

corpus/      controlled kernels and raw SASS evidence
knowledge/   project-wide findings, instruction notes, and encoding notes
patterns/    reusable Phase 3 audit signatures
production/  Phase 4 real-kernel audits

The last broad public SASS reverse-engineering work comparable in spirit was Jia et al. on Volta and Turing in 2018. Ampere, Hopper, and Blackwell have changed the instruction mix substantially: async copy paths, tensor-core families, matrix load/store instructions, sparse and scaled MMA forms, and new uniform-register flows.

SASS King fills that gap by combining controlled micro-kernels, raw SASS reading, runtime probes, and production-kernel audits.

The formal pattern library is the main output of Phase 3. It turns the chapter-local evidence into reusable audit signatures, so an audit can cite a named pattern instead of rewriting the full research trail every time.

Phase 3 is considered complete because:

• the repeated structures found in chapters 01-25 have been promoted into 29 named pattern pages;
• every pattern has a plain-English explanation, SASS signature, variants, anti-patterns, open gaps, and confidence level;
• claim tags remain bounded to the source evidence in knowledge/FINDINGS.md;
• audit-facing navigation now starts from patterns/README.md;
• unresolved items are explicitly carried forward as gaps instead of being hidden inside the pattern text.

Each pattern page includes:

• plain-English meaning;
• SASS signature;
• observed variants;
• interpretation boundaries;
• anti-patterns;
• open gaps;
• confidence level.

Use patterns/README.md as the audit-facing index. Use knowledge/FINDINGS.md when you need the longer research context behind a pattern.

Phase 3 does not claim that every NVIDIA SASS behavior is decoded. It establishes a reusable SM120 / SM120a pattern layer good enough to begin manual production audits. Runtime layout decode, full control-code bit placement, automated cubin reporting, and cross-architecture replay remain future work.

• New to the project: read Start Here.
• Want the project-wide map: read the knowledge base index.
• Want the current instruction map: read SASS instructions on SM120 / SM120a.
• Want encoding notes: read encoding notes.
• Want the raw source of truth: read findings.
• Want reusable audit signatures: read the pattern library.
• Want tensor-core evidence: start with tensor-core chapters.
• Want to contribute dumps or corrections: read contributing.
• Want the v0.1 boundary: read release notes.

Public writeups:

• Part 1 - Reading NVIDIA SASS from First Principles
• Part 2 - Reading the Compiler's Mind

Controlled variation. Two kernels differ by exactly one variable: dtype, operand order, unroll factor, memory layout, or compilation target. The SASS diff isolates the compiler decision.

Strict claim tags. Every technical claim uses a tag:

Top-down and bottom-up together. Micro-kernels isolate individual instructions and compiler decisions. Production-like kernels show which patterns matter in real code.

Pattern-first audits. A production audit should cite a formal PATTERN-NN page only after matching the visible SASS signature and carrying over its confidence limits, anti-patterns, and open gaps.

The first pass focuses on the SM120 tensor-core and memory pipeline:

• HMMA, QMMA, OMMA
• LDSM, STSM
• LDGSTS, LDGDEPBAR, DEPBAR
• LDG, STG, LDS, STS, REDG
• BRA, EXIT, BSSY, BSYNC, WARPSYNC
• SHFL, VOTE, REDUX
• uniform-register flow: S2UR, R2UR, UMOV, ULEA, LDCU

The project does not pretend the ISA is complete yet. The public glossary tracks what is observed and explained; deeper pages under knowledge/encoding/ track families with enough evidence for matcher-style documentation.

SASS King does not compete with bit-level SASS disassemblers. The project uses local dumps as primary evidence and may use redplait/denvdis as a cross-check for instruction fields, scheduling tables, predicates, and register tracking. denvdis can validate low-level encoding interpretations; SASS King owns the controlled-variation evidence, semantic pattern layer, and production-audit interpretation.

flowchart LR
    P1["Phase 1<br/>Teaching kernels<br/>01-12"] --> P2["Phase 2<br/>SM120 tensor-core corpus<br/>13-25"]
    P2 --> P25["Phase 2.5<br/>denvdis cross-validation<br/>bit-level backend"]
    P25 --> P3["Phase 3<br/>Pattern library<br/>compiler signatures"]
    P3 --> P4["Phase 4<br/>Production audits<br/>real kernels"]
    P4 --> P5["Phase 5<br/>Audit tool<br/>cubin reports"]
    P5 --> P6["Phase 6<br/>Cross-architecture replay<br/>SM80/86/89/90a/100a/120"]

    classDef done fill:#0b6d55,color:#fff,stroke:#0b6d55;
    classDef active fill:#f4c95d,color:#111,stroke:#b89422;
    classDef planned fill:#1f2937,color:#fff,stroke:#6b7280;
    class P1,P2,P25,P3 done;
    class P4 active;
    class P5,P6 planned;

Kernels 01-12 establish baseline SASS concepts: FMA fusion, scoreboard behavior, loop lowering, shared memory, global memory, warp primitives, slow-path math, and local-memory spills.

Kernels 13-25 cover the current SM120 tensor-core path:

Validate redplait/denvdis as the bit-level cross-check backend for SM120 / SM120a before production audits depend on the pattern library. The pass runs nvd -O, nvd -S, nvd -p, and where useful nvd -T on representative local cubins or dumps covering HMMA, QMMA, QMMA.SF, QMMA.SP, OMMA, LDSM, STSM b16/b8, LDGSTS, DEPBAR, and divergence markers.

The output is knowledge/DENVDIS_INTEGRATION.md: a factual compatibility table from family to denvdis recognition status, modifier coverage, exposed control-code fields, and the SASS King action. denvdis output is supporting evidence, not a replacement for local dump observations.

Formalized recurring structures into reusable signatures:

• LDGSTS -> DEPBAR -> LDSM -> MMA
• chained HMMA / QMMA / OMMA
• STSM -> BAR -> LDS -> STG
• warp reductions and cross-lane collectives
• register-spill signatures
• scalar and uniform control-flow patterns

The initial Phase 3 library contains 29 pattern pages under patterns/. knowledge/FINDINGS.md remains the research log and source of truth; patterns/ is the audit-facing entry point.

Phase 3 is complete at the initial library level. Remaining items such as runtime layout decode, full control-code bit placement, and cross-architecture replay are tracked as gaps or future phases, not blockers for starting Phase 4 production audits.

Apply the pattern library to real kernels from libraries such as FlashAttention, CUTLASS, xFormers, Transformer Engine, FlashInfer, llama.cpp / ggml, tinygrad, and related projects. The goal is representative coverage by algorithmic pattern, not one markdown file per kernel.

The first Phase 4 deliverable should be a manual audit report that:

• segments one real kernel into SASS regions;
• cites matching PATTERN-NN pages;
• assigns confidence levels to each conclusion;
• records unexplained regions as new gaps;
• avoids building an audit tool until at least one manual report is stable.

Build a pipeline that takes a cubin, detects known patterns, and emits an optimization-oriented report.

Replay the methodology on additional targets:

.
├── corpus/                                # Controlled kernels, dumps, and chapter writeups
│   ├── basics/                            # Kernels 01-08: scalar/vector and memory basics
│   ├── warp_collectives/                  # Kernels 09-10: shuffle, vote, reduction
│   ├── math_and_spills/                   # Kernels 11-12: slow paths and spills
│   └── tensor_cores/                      # Kernels 13-25: tensor-core studies
├── knowledge/                              # Findings, glossary, encoding notes
│   ├── FINDINGS.md
│   ├── SASS_INSTRUCTIONS_SM120.md
│   └── encoding/
├── patterns/                               # Formal Phase 3 pattern library
├── production/                             # Phase 4 production-kernel audits
├── docs/                                   # Onboarding, structure, and release-facing notes
└── guide/                                  # External SASS reading guide submodule

Each chapter folder contains source kernels, compiled artifacts when relevant, SASS dumps when they are part of the validated evidence set, and a conclusion<N>.md writeup.

For a fuller explanation of what belongs in each directory, read Project Structure.

• cuobjdump --dump-sass for raw disassembly.
• gpuasm.com for scoreboards, stalls, pressure, and dependency arrows.
• Nsight Compute for profiling and stall attribution.
• %clock microbenchmarks for instruction latency probes.
• nvcc -Xptxas -v for register and spill metadata.

• Jia et al. 2018, "Dissecting the NVIDIA Volta GPU Architecture via Microbenchmarking" for the empirical microbenchmarking discipline behind latency, throughput, and dependency validation.
• kuterdinel.com/nv_isa for fuzzed NVIDIA ISA encoding work, especially the idea of deriving machine-readable encoding rules from disassembler behavior.
• redplait/denvdis for opcode tables, bit-level disassembly, encoding-field inspection, scheduling analysis, register tracking, and cubin manipulation. The extracted SM120 data12 tables are used as a low-level cross-check while local dumps remain primary evidence.
• Redplait tooling and notes: ced cubin editor, SASS disassembly Perl bindings, SASS latency analysis, and libcuda/nvasm_internal notes.
• Huerta et al. 2025 for reverse-engineering compiler-guided scheduling, control codes, dependency counters, reuse flags, and yield behavior.
• Yan et al. 2026 for the driver-layer launch and pushbuffer analysis below SASS.
• MaxAS and TuringAS as prior public SASS assembler efforts for older NVIDIA architectures.
• NVIDIA CUDA Binary Utilities documentation for official cubin, fatbin, and disassembly tooling.

SASS King operates at the algorithmic pattern layer: recognizing how compiled kernels are structured and connecting those structures to source-level optimization decisions.

Contributions are welcome, especially:

• raw SASS dumps from hardware not directly available here;
• controlled kernel studies that isolate one compiler decision;
• corrections to existing observations;
• new production-kernel pattern proposals;
• cross-architecture comparisons.

See CONTRIBUTING.md for the expected metadata and writing standard.

Florian Mattana. florianmattana.com