Implementing High-Impact GPU-Specific Optimization Passes

The symptoms you already see are consistent and telling: a kernel set that’s memory-bound and hurting on global loads, sub-50% SM utilization despite high instruction counts, many tiny launches that dominate latency, or clear warp inefficiency numbers from your profiler. Those are compiler opportunities — not just application bugs — because a compiler that understands warp topology, memory transaction granularity, and live ranges can reorganize computation to eliminate needless traffic and serialization.

Contents

• Fusing kernels to eliminate producer-consumer overhead
• Transforming data layout to achieve true memory coalescing
• Quantifying and surgically reducing thread divergence
• Cutting registers and reshaping loops to control occupancy
• Measuring performance and tuning compiler thresholds
• Practical application: from profiler to production GPU pass

Fusing kernels to eliminate producer-consumer overhead

Why it matters — when a producer kernel writes an intermediate array to global memory and a consumer immediately reads it, you pay write + read + kernel-launch overhead. Fusion replaces that global handshake with in-kernel streaming (via registers or shared memory), collapsing two separate scheduling domains into one and extending optimizer visibility across producer-consumer boundaries. Production compilers and DSLs (e.g., Halide, XLA) make this a core transformation for that reason.

What fusion actually does (practical anatomy)

• Remove intermediate global writes by computing producer values into consumer-local storage (registers or __shared__ buffers).
• Re-tile loops so a single thread-block computes the consumer’s output tile and the corresponding producer inputs.
• Optionally duplicate small producers inside consumers to avoid synchronization (trade: extra compute vs saved memory traffic).

Example (illustrative CUDA-style pseudo-code):

// Unfused: producer writes to temp, consumer reads temp
__global__ void prod(float *A, float *T) {
  int i = blockIdx.x * blockDim.x + threadIdx.x;
  T[i] = compute_producer(A[i]);
}
__global__ void cons(float *T, float *B) {
  int i = blockIdx.x * blockDim.x + threadIdx.x;
  B[i] = compute_consumer(T[i]);
}

// Fused: producer values are passed directly to consumer work
__global__ void fused(float *A, float *B) {
  int i = blockIdx.x * blockDim.x + threadIdx.x;
  float t = compute_producer(A[i]); // kept in register
  B[i] = compute_consumer(t);
}

Cost model you should implement in the pass

• SavedBytes = bytes_written_by_producer_that_would_be_eliminated
• SavedLaunchCost = num_launches_removed × launch_overhead
• RegIncrease = estimated additional registers / thread
• SharedMemIncrease = additional shared memory per block
• DivergenceRisk = probability the fusion causes warp divergence or prevents useful ILP

Concrete (linear) scoring function the pass can evaluate per producer-consumer pair:
Score = alpha * SavedBytes + beta * SavedLaunchCost - gamma * RegIncrease - delta * SharedMemIncrease - epsilon * DivergenceRisk

Tune alpha..epsilon to your hardware model. A positive Score → attempt fusion, but validate with register-pressure checks and a simulated occupancy test. XLA and other compilers already use similar profitability tests in their fusion passes.

Trade-offs and contrarian insight

• Fusion often increases register pressure, which can reduce occupancy and cause spills to local memory (catastrophic for bandwidth). Measure --ptxas-options=-v and simulate occupancy before committing fusion.
• For long producer chains, greedy full fusion can create monolithic kernels that are hard to schedule or debug. Consider hierarchical fusion (fuse in small tiles) or multi-output fusion to keep kernels tractable.
• In some cases recomputation inside the fused kernel is cheaper than storing and loading an intermediate — a controlled recompute vs store decision belongs in the cost model. Halide’s schedule model makes this explicit.

Transforming data layout to achieve true memory coalescing

Why layout matters — GPU DRAM is served in aligned segments; warps fetch fixed-size sectors. Misaligned or strided per-thread accesses blow up the number of memory transactions and waste bandwidth. Real-world measurements show coalesced vs scattered patterns can change transaction counts by multiples, producing order-of-magnitude differences in effective memory throughput. Use the hardware coalescing/caching rules as a hard constraint for your passes.

Canonical layout transforms

• AoS → SoA (structure-of-arrays): turns strided access into contiguous per-thread loads.
• Vectorized loads/stores: use float4 / int4 loads where lane alignment guarantees fetch aggregation.
• Tiling + shared-memory transpose: gather strided tiles into __shared__ then distribute coalesced loads/stores to DRAM.
• Stride normalization: remap array indices via loop interchange or index linearization so thread i reads address base + i.

Compiler implementation sketch

1. Analyze all memory access functions: render index expressions as affine forms (use polyhedral analysis or MLIR linalg/affine utilities).
2. Detect common patterns: unit-stride in one dimension, constant stride in another, or complex gather patterns.
3. Propose transformations: loop interchange, tile sizes (tile dims that align to warp and cache-line boundaries), or layout rewrite (AoS→SoA) and insert pack/unpack as needed.
4. Bufferize and schedule pack/unpack to happen inside warps/blocks (shared memory or registers) to avoid extra global traffic. MLIR’s bufferization and tiling/fusion toolchain is designed for exactly this workflow.

Rule-of-thumb for tile sizes

• Make tile width a multiple of warpSize (commonly 32) and align to the device’s memory transaction size (architectures vary between 32B and 128B effective segments). Quantify with your profiler — the CUDA Best Practices Guide shows the relevant segment sizes and alignment rules.

Quick comparison

Quantifying and surgically reducing thread divergence

How divergence kills throughput — when threads in a warp take different control paths, hardware serializes the different paths and wastes execution slots. Compilers must both detect divergence likelihood and transform control flow to minimize observed warp splits. The hardware reconvergence behavior (SIMT stack, early reconvergence heuristics) is an architectural reality that your pass must respect.

Analysis techniques

• Static thread-variant analysis: mark instructions or basic blocks that depend on threadIdx, lane_id, or per-thread data. Those are potential divergence sources.
• Profile-guided probability: instrument branches to measure per-warp uniformity; many branches are uniform in practice and can be left alone.
• Build a per-branch divergence score: DivergenceScore = fraction_of_warps_diverging × cost_of_serialization.

Transformations (programmable)

• If-conversion (predication): convert short branches into predicated instructions; good for small bodies and low divergence probability. Classic compiler if-conversion frameworks remain relevant; there is a trade: predication executes extra instructions across all lanes.
• Tail merging / block reordering: reorder basic blocks to increase the chance of early reconvergence or reduce active-mask fragmentation.
• Warp specialization / dynamic splitting: emit two kernels specialized for hot path and cold path (or use __ballot_sync-based compaction to compress active threads into denser execution groups).
• Use warp-level intrinsics: __ballot_sync, __any_sync, __activemask, and shuffle operations to implement masked loops that pack work for active lanes into contiguous lanes, execute, then unpack.

Example: compress-and-run idiom (pseudo-CUDA)

unsigned mask = __ballot_sync(0xffffffff, cond);
while (mask) {
  unsigned i = __ffs(mask) - 1;           // lane index to run
  // compute only for this lane (or use shuffles to compact)
  // update mask to clear bit i
  mask &= ~(1u << i);
}

Contrarian note — predication is not a silver bullet. For long or complex branch bodies predication increases instruction count and register pressure and can regress performance; the compiler needs a cost function to prefer predication only when body weight < threshold or branch probability is near 0 or 1. On modern GPUs the backend will itself choose between predication and branch; a good divergence pass supplies the backend with a more favorable CFG and hoists uniform tests out of warps where possible.

Cutting registers and reshaping loops to control occupancy

Why register pressure matters — registers are the fastest storage, but they’re a scarce, block-scoped resource. The per-thread register count interacts with the SM’s register file to determine how many blocks/warps can be resident (occupancy). High register usage per-thread can reduce resident warps, reducing latency-hiding capacity; too many registers and the allocation rounds up (hardware granularity) which exaggerates the occupancy loss. The CUDA Best Practices Guide documents these relationships and tooling (--ptxas-options=-v, __launch_bounds__, cudaOccupancyMaxActiveBlocksPerMultiprocessor) you should use while tuning.

Passes and techniques

• Live-range shrinking: perform local block reordering and value rematerialization for cheap values to reduce their live ranges (remat trades compute for register pressure).
• Partial unrolling and software pipelining: tune unrolling to expose vectorization/ILP without exploding register usage.
• Scalar replacement and store forwarding: convert memory-resident temporaries to registers only when live ranges are small.
• Spill mitigation: use shared memory as a "fast spill" area in some designs (careful — shared memory is also a constrained resource and affects occupancy).
• Use __launch_bounds__ and compile-time maxrregcount as defensive caps for specific kernels when register explosion creates failures.

Occupancy formula (conceptual)

resident_blocks_per_SM = min(
  floor(registers_per_SM / (regs_per_thread * threads_per_block)),
  floor(shared_mem_per_SM / shared_mem_per_block),
  hardware_max_blocks_per_SM
)
occupancy = (resident_blocks_per_SM * threads_per_block) / max_threads_per_SM

Compute this after each transformation to check the impact of register/shared-memory increases.

Contrarian observation — higher occupancy is not always faster. Low-occupancy kernels with more registers per thread can expose ILP that hides latency; the pass should not blindly maximize occupancy but target effective pipeline utilization tracked by warp_execution_efficiency and overall instruction throughput.

Measuring performance and tuning compiler thresholds

Measurement framework

1. Baseline capture: collect a clean profile of the application using nsys (Nsight Systems) for a timeline view and ncu (Nsight Compute) for kernel-level metrics. Capture counters such as gld_efficiency, gst_efficiency, dram_read_throughput, sm_efficiency, achieved_occupancy, and warp_execution_efficiency.
2. Roofline placement: compute operational intensity (FLOPs / DRAM bytes) and plot kernels on a Roofline chart to decide memory-bound vs compute-bound optimization focus. The Roofline model remains the most practical visualization to prioritize memory vs compute work.
3. Controlled experiments: change one pass or parameter at a time (fusion yes/no, layout transform on/off, predication threshold changed) and collect the same metrics to attribute gains.
4. Microbenchmarks: create small, deterministic inputs that fit known working set sizes to isolate L1/L2 vs DRAM behavior.

Parameter tuning

• Fusion budget parameters: tune SavedBytes threshold, allowed RegIncrease fraction, and occupancy floor. Start conservative: require at least >64KB saved global writes and <15% register increase for initial automatic fusion; relax after validating correctness. Use autotuning (parameter sweep) on a small representative dataset to generate a Pareto frontier for each kernel.
• Layout tile sizes: pick tile dimensions that align to cacheline sizes; test powers-of-two around warp-size multiples (e.g., 32, 64, 128 threads per tile).
• Divergence thresholds: for if-conversion, use static body-size heuristics + dynamic branch uniformity (predicated if branch is uniform > 95% of the time or body is < N instructions).

Sample CLI snippets (measurement)

# Nsight Systems timeline (system-level)
nsys profile --output=run1 --trace=cuda,nvtx ./app

# Nsight Compute kernel metrics for a specific kernel
ncu --kernel-name-regex "myKernel" --metrics gld_efficiency,sm_efficiency ./app

Interpretation checklist

• Large gains in gld_efficiency after an AoS→SoA or tiling pass confirm successful coalescing.
• dram_read_throughput approaching measured peak indicates a memory-bound kernel; fusion may not help compute-bound kernels.
• Rising local_replay_overhead or l1tex stalls after fusion suggests register spills or bank conflicts.

Practical application: from profiler to production GPU pass

Step-by-step protocol for a fusion/mem-layout/divergence pipeline (high-level)

1. Profile broadly with nsys/ncu to find top-k kernels by time and bytes transferred. Log gld_efficiency, dram_read_throughput, sm_efficiency, and warp_execution_efficiency.
2. For a given hot kernel, run access-analysis (affine extraction) to find producer-consumer boundaries and per-thread index functions (use MLIR linalg or XLA HLO analysis).
3. Run a proposal generator that emits candidate transforms:
   • Producer-consumer fusion candidates with estimated Score.
   • Layout transforms (AoS→SoA, pad/align) and tiled variants.
   • If-conversion or warp-specialization candidates for hot branches.
4. Cost-model evaluation: compute Score for each candidate, reject those that violate reg/shared resource budgets, or that reduce simulated occupancy below a safe minimum (e.g., 30–40% of max threads for latency hiding).
5. Apply transformation in a sandboxed IR (e.g., MLIR linalg → tile/fuse → bufferize) and run functional tests to verify correctness (unit tests + randomized checks).
6. Micro-benchmark the transformed kernel under profiler automation; compare metrics and commit only when performance improves according to a specified policy (e.g., >2% wall-clock improvement and no regressions in gld_efficiency or sm_efficiency).
7. Add the transform as a tunable pass with conservative defaults; gather telemetry from CI/perf regression harnesses and expand coverage as confidence grows.

Pass skeleton (MLIR/LLVM-style pseudocode)

// Pseudo-structure for a producer-consumer fusion pass
struct ProducerConsumerFusionPass : public Pass {
  void runOnModule() override {
    auto module = getModuleOp();
    analyzeAffineAccesses(module);
    for (auto &candidate : findProducersConsumers(module)) {
      auto score = computeFusionScore(candidate);
      if (score < threshold) continue;
      auto fused = attemptFuse(candidate);
      if (!validateRegisterBudget(fused)) { revert(); continue; }
      if (!unitTestsPass(fused)) { revert(); continue; }
      commitChange(fused);
    }
  }
};

Validation checklist before commit

• Correctness: unit tests + randomized differential tests.
• Performance: repeatable improvement in wall-clock + favorable micro-metrics.
• Resource safety: no register or shared-memory explosion; acceptable occupancy.
• Maintenability: readable IR for debugging and a de-fusion path if needed.

Important: Automating these passes requires a robust cost model and a regression harness — avoid pushing transformations blindly into a release compiler without a path to revert or to limit scope per-kernel.

Sources

CUDA C++ Best Practices Guide (CUDA 12.5) - Rules and explanations for memory coalescing, occupancy math, register pressure, and best-practice heuristics used when evaluating trade-offs.

Unlock GPU Performance: Global Memory Access in CUDA (NVIDIA Developer Blog) - Illustrative examples and data showing the large efficiency differences between coalesced and scattered global memory accesses.

Decoupling Algorithms from Schedules for Easy Optimization of Image Processing Pipelines (Halide, SIGGRAPH 2012) - Demonstrates fusion/tiling/schedule separation and how fusion improves locality and performance in practice.

Kernel Weaver: Automatically Fusing Database Primitives for Efficient GPU Computation (Kernel Weaver paper) - Research showing practical kernel fusion benefits (reported multi-× speedups) and producer-consumer fusion design.

XLA Instruction Fusion (source excerpt) - Real-world production compiler fusion logic and profitability checks used in a major ML compiler backend.

MLIR Bufferization and Passes (MLIR official docs) - Reference for bufferization, tiling, fusion, and the recommended sequence of tensor→memref transforms in modern IR pipelines.

Roofline: An Insightful Visual Performance Model for Floating-Point Programs and Multicore Architectures (Williams et al.) - The Roofline model to diagnose memory-bound vs compute-bound kernels and to prioritize optimizations.

NVIDIA Nsight Systems User Guide - System-level profiling and GPU metrics that help correlate CPU/GPU activity and identify kernel launch/IO bottlenecks.

NVIDIA Nsight Compute Documentation (metrics and CLI) - Kernel-level counters (gld_efficiency, sm_efficiency, warp_execution_efficiency, etc.) and guidance for measuring kernel micro-behavior.

General-purpose Graphics Processor Architectures (SIMT control-flow and reconvergence discussion) - Academic treatment of SIMT control flow, reconvergence strategies, and hardware/algorithmic techniques for handling divergence.

Apply these passes surgically: measure first, let cost models veto aggressive transforms, and iterate with microbenchmarks so that each fusion, layout change, or divergence transformation delivers measurable improvements in bandwidth utilization and SM efficiency.