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perf: pure FP8 fwd/bwd and lean non-transposed GEMM

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- drop the fused kernel; forward/backward are quantize + a pre-quantized GEMM
- rename module fp8_mm -> fp8_ops (mm.cu -> ops.cu)
- kernels/launchers fp8_gemm_kernel / launch_fp8_gemm; drop PqTraits/gather_trans/pack_fp8x4_vector
- remove the in-kernel transposed-operand branches (TransA/TransB)
- backward: quantize g once (amax_g here), explicit fp8 transposes, fast non-transposed GEMMs (dX = g@w^T, dW = g^T@x^T)
- each pass uses a single FP8 format (E4M3 fwd / E5M2 bwd)
This commit is contained in:
ViperEkura
2026-08-23 15:38:30 +08:00
parent a29bdfae46
commit 4244df2785
8 changed files with 160 additions and 519 deletions
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csrc/kernels/fp8/ops.cu
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// FP8 GEMM torch binding: tensor validation, FP8Params packing, template
// dispatch and pybind. Device code lives in gemm.cuh (pure CUDA) —
// mirroring the attn_*.cu / attn_*_mma.cuh split of the attention kernels.
#include <torch/extension.h>
#include <ATen/cuda/CUDAContext.h>
#include <c10/cuda/CUDAGuard.h>
#include <cuda_bf16.h>
#include <cstdint>
#include <mutex>
#include <tuple>
#include <unordered_map>
#include "gemm.cuh"
#include "../common/device.cuh"
namespace {
// FP8Format / FP8Params live in the global namespace (common.h); the
// launchers live in fp8:: (gemm.cuh).
void check_fp8_device(const torch::Tensor& tensor) {
static std::mutex mutex;
static std::unordered_map<int, bool> supported;
const int device = tensor.device().index();
{
std::lock_guard<std::mutex> lock(mutex);
auto cached = supported.find(device);
if (cached != supported.end()) {
TORCH_CHECK(cached->second,
"fused FP8 MMA requires compute capability 8.9 or newer");
return;
}
}
const auto* properties = at::cuda::getDeviceProperties(device);
const bool is_supported =
astrai::sm_at_least(properties->major, properties->minor,
astrai::kMinSmForFp8Major,
astrai::kMinSmForFp8Minor);
{
std::lock_guard<std::mutex> lock(mutex);
supported.emplace(device, is_supported);
}
TORCH_CHECK(is_supported,
"fused FP8 MMA requires compute capability 8.9 or newer");
}
void check_scale(const torch::Tensor& scale, const torch::Tensor& input,
const char* name) {
TORCH_CHECK(scale.is_cuda() && scale.device() == input.device() &&
scale.scalar_type() == torch::kFloat32 && scale.numel() == 1,
name, " must be a CUDA float32 scalar on the input device");
}
// ---- FP8Params packing (mirrors attention/entry_utils.cuh pack_* helpers) ----
void pack_gemm_params(FP8Params& p, const void* a, const void* b, void* out,
const torch::Tensor& sa, const torch::Tensor& sb,
const torch::Tensor* out_scale, int64_t m, int64_t n,
int64_t k) {
p.a_ptr = a;
p.b_ptr = b;
p.out_ptr = out;
p.scale_a = sa.data_ptr<float>();
p.scale_b = sb.data_ptr<float>();
p.out_scale = out_scale ? out_scale->data_ptr<float>() : nullptr;
p.bias = nullptr;
p.amax_a = nullptr;
p.amax_b = nullptr;
p.m = m;
p.n = n;
p.k = k;
p.total = 0;
}
void pack_quantize_params(FP8Params& p, const void* x, void* x8,
const torch::Tensor& scale, torch::Tensor* amax,
int64_t total) {
p.a_ptr = x;
p.b_ptr = nullptr;
p.out_ptr = x8;
p.scale_a = scale.data_ptr<float>();
p.scale_b = nullptr;
p.out_scale = nullptr;
p.bias = nullptr;
p.amax_a = amax ? amax->data_ptr<float>() : nullptr;
p.amax_b = nullptr;
p.m = p.n = p.k = 0;
p.total = total;
}
} // namespace
// ---------------------------------------------------------------------------
// Entry points
// ---------------------------------------------------------------------------
std::tuple<torch::Tensor, torch::Tensor> quantize_bf16(torch::Tensor x,
torch::Tensor scale,
int64_t fmt) {
// BF16 -> FP8 quantize with fused amax. fmt: 0 = E4M3, 1 = E5M2.
// Returns (x8, amax); the caller never clears amax (zero-initialized here).
TORCH_CHECK(x.is_cuda() && scale.is_cuda(), "CUDA tensors required");
TORCH_CHECK(x.scalar_type() == torch::kBFloat16, "x must be bf16");
check_scale(scale, x, "scale");
check_fp8_device(x);
const at::cuda::OptionalCUDAGuard guard(x.device());
auto stream = at::cuda::getCurrentCUDAStream();
auto x_c = x.contiguous();
auto x8 = torch::empty_like(
x_c, x_c.options().dtype(fmt ? torch::kFloat8_e5m2
: torch::kFloat8_e4m3fn));
auto amax = torch::zeros({1}, x_c.options().dtype(torch::kFloat32));
FP8Params p;
pack_quantize_params(p, x_c.data_ptr(), x8.data_ptr(), scale, &amax,
x_c.numel());
if (fmt) {
fp8::launch_fp8_quantize<FP8Format::E5M2>(p, stream.stream());
} else {
fp8::launch_fp8_quantize<FP8Format::E4M3>(p, stream.stream());
}
C10_CUDA_CHECK(cudaGetLastError());
return {x8, amax};
}
torch::Tensor mm_fp8(torch::Tensor a, torch::Tensor b, torch::Tensor sa,
torch::Tensor sb, int64_t out_dtype,
c10::optional<torch::Tensor> out_scale) {
// Pre-quantized FP8 GEMM: out = a @ b^T * (sa * sb), FP32 accumulation.
// out_dtype: 0 = BF16 (default), 1 = FP8 E4M3 (requires out_scale, the
// quantization step for the output — mirrors torch._scaled_mm's
// out_dtype / scale_result). Both operands share the same FP8 format.
TORCH_CHECK(a.is_cuda() && b.is_cuda(), "CUDA tensors required");
TORCH_CHECK(a.scalar_type() == torch::kFloat8_e4m3fn ||
a.scalar_type() == torch::kFloat8_e5m2,
"a and b must be fp8 (e4m3fn or e5m2)");
TORCH_CHECK(a.scalar_type() == b.scalar_type(),
"a and b must share the same fp8 format");
TORCH_CHECK(a.dim() == 2 && b.dim() == 2, "a and b must be 2D");
TORCH_CHECK(a.device() == b.device(), "a and b must be on the same device");
TORCH_CHECK(a.size(1) == b.size(1), "inner dim mismatch");
check_scale(sa, a, "sa");
check_scale(sb, a, "sb");
check_fp8_device(a);
const at::cuda::OptionalCUDAGuard guard(a.device());
auto stream = at::cuda::getCurrentCUDAStream();
auto a_c = a.contiguous();
auto b_c = b.contiguous();
int64_t m = a_c.size(0), k = a_c.size(1), n = b_c.size(0);
const bool out_fp8 = (out_dtype == 1);
TORCH_CHECK(out_dtype == 0 || out_fp8,
"out_dtype must be 0 (bf16) or 1 (fp8 e4m3)");
torch::Tensor os;
if (out_fp8) {
TORCH_CHECK(out_scale.has_value(), "fp8 output requires out_scale");
os = out_scale.value();
check_scale(os, a, "out_scale");
}
auto out = torch::empty(
{m, n},
out_fp8 ? a_c.options().dtype(torch::kFloat8_e4m3fn)
: a_c.options().dtype(torch::kBFloat16));
FP8Params p;
pack_gemm_params(p, a_c.data_ptr(), b_c.data_ptr(), out.data_ptr(), sa, sb,
out_fp8 ? &os : nullptr, m, n, k);
if (a.scalar_type() == torch::kFloat8_e4m3fn) {
if (out_fp8) {
fp8::launch_fp8_gemm<FP8Format::E4M3, true>(p, stream.stream());
} else {
fp8::launch_fp8_gemm<FP8Format::E4M3>(p, stream.stream());
}
} else {
if (out_fp8) {
fp8::launch_fp8_gemm<FP8Format::E5M2, true>(p, stream.stream());
} else {
fp8::launch_fp8_gemm<FP8Format::E5M2>(p, stream.stream());
}
}
C10_CUDA_CHECK(cudaGetLastError());
return out;
}
std::tuple<torch::Tensor, torch::Tensor, torch::Tensor> linear_forward_fp8(
torch::Tensor x, torch::Tensor w, torch::Tensor bias, torch::Tensor sx,
torch::Tensor sw, int64_t fmt) {
// Pure FP8 forward: quantize x/w (fmt: 0 = E4M3, 1 = E5M2), then the
// pre-quantized GEMM; the dequantized BF16 output gets the bias added.
// amax_x / amax_w come from the quantize kernels (zero-initialized here).
TORCH_CHECK(x.is_cuda() && w.is_cuda(), "CUDA tensors required");
TORCH_CHECK(x.scalar_type() == torch::kBFloat16 &&
w.scalar_type() == torch::kBFloat16,
"x and w must be bf16");
TORCH_CHECK(x.device() == w.device(), "x and w must be on the same device");
check_scale(sx, x, "sx");
check_scale(sw, x, "sw");
check_fp8_device(x);
const at::cuda::OptionalCUDAGuard guard(x.device());
auto stream = at::cuda::getCurrentCUDAStream();
auto x_c = x.reshape({-1, w.size(1)}).contiguous(); // [M, K]
auto w_c = w.contiguous(); // [N, K]
int64_t m = x_c.size(0), k = x_c.size(1), n = w_c.size(0);
TORCH_CHECK(w_c.dim() == 2 && w_c.size(1) == k, "inner dim mismatch");
const bool has_bias = bias.defined() && bias.numel() > 0;
if (has_bias) {
TORCH_CHECK(bias.is_cuda() && bias.device() == x.device() &&
bias.scalar_type() == torch::kBFloat16 &&
bias.numel() == n,
"bias must be CUDA bf16 with shape [N]");
}
const auto f8opt = fmt ? torch::kFloat8_e5m2 : torch::kFloat8_e4m3fn;
auto x8 = torch::empty({m, k}, x_c.options().dtype(f8opt));
auto w8 = torch::empty({n, k}, x_c.options().dtype(f8opt));
auto amax_x = torch::zeros({1}, x.options().dtype(torch::kFloat32));
auto amax_w = torch::zeros({1}, x.options().dtype(torch::kFloat32));
auto out = torch::empty({m, n}, x_c.options());
auto quantize = [&](const torch::Tensor& src, torch::Tensor& dst,
const torch::Tensor& scale, torch::Tensor* amax) {
FP8Params qp;
pack_quantize_params(qp, src.data_ptr(), dst.data_ptr(), scale, amax,
src.numel());
if (fmt) {
fp8::launch_fp8_quantize<FP8Format::E5M2>(qp, stream.stream());
} else {
fp8::launch_fp8_quantize<FP8Format::E4M3>(qp, stream.stream());
}
};
quantize(x_c, x8, sx, &amax_x);
quantize(w_c, w8, sw, &amax_w);
FP8Params p;
pack_gemm_params(p, x8.data_ptr(), w8.data_ptr(), out.data_ptr(), sx, sw,
nullptr, m, n, k);
if (fmt) {
fp8::launch_fp8_gemm<FP8Format::E5M2>(p, stream.stream());
} else {
fp8::launch_fp8_gemm<FP8Format::E4M3>(p, stream.stream());
}
C10_CUDA_CHECK(cudaGetLastError());
std::vector<int64_t> shape(x.sizes().begin(), x.sizes().end() - 1);
shape.push_back(n);
auto out_r = out.reshape(shape);
if (has_bias) out_r = out_r + bias;
return {out_r, amax_x, amax_w};
}
std::tuple<torch::Tensor, torch::Tensor, torch::Tensor, torch::Tensor>
linear_backward_fp8(torch::Tensor g, torch::Tensor x, torch::Tensor w,
std::vector<int64_t> masks, torch::Tensor sg,
torch::Tensor sw, torch::Tensor sx, int64_t fmt) {
// Pre-quantized FP8 backward: grad is quantized once (E4M3 or E5M2 per
// `fmt`), then dX / dW run as FP8 tensor-core GEMMs sharing g8.
// Returns (grad_input, grad_weight, grad_bias, amax_g).
TORCH_CHECK(g.is_cuda() && x.is_cuda() && w.is_cuda(), "CUDA tensors required");
TORCH_CHECK(g.scalar_type() == torch::kBFloat16 &&
x.scalar_type() == torch::kBFloat16 &&
w.scalar_type() == torch::kBFloat16,
"g, x, and w must be bf16");
TORCH_CHECK(g.device() == x.device() && g.device() == w.device(),
"g, x, and w must be on the same device");
TORCH_CHECK(masks.size() == 3, "masks must contain three values");
check_fp8_device(g);
const at::cuda::OptionalCUDAGuard guard(g.device());
auto stream = at::cuda::getCurrentCUDAStream();
auto g_c = g.reshape({-1, w.size(0)}).contiguous(); // [M, N]
auto x_c = x.reshape({-1, x.size(-1)}).contiguous(); // [M, K]
auto w_c = w.contiguous(); // [N, K]
int64_t m = g_c.size(0), n = w_c.size(0), k = w_c.size(1);
TORCH_CHECK(x_c.size(0) == m && x_c.size(1) == k && g_c.size(1) == n,
"backward shape mismatch");
auto grad_input = torch::empty_like(x);
auto grad_weight = torch::empty_like(w);
auto grad_bias = torch::empty({0}, g.options());
auto amax_g = torch::zeros({1}, g.options().dtype(torch::kFloat32));
auto f8opt = fmt ? g.options().dtype(torch::kFloat8_e5m2)
: g.options().dtype(torch::kFloat8_e4m3fn);
auto quantize = [&](const torch::Tensor& src, torch::Tensor& dst,
const torch::Tensor& scale, torch::Tensor* amax) {
FP8Params qp;
pack_quantize_params(qp, src.data_ptr(), dst.data_ptr(), scale, amax,
src.numel());
if (fmt) {
fp8::launch_fp8_quantize<FP8Format::E5M2>(qp, stream.stream());
} else {
fp8::launch_fp8_quantize<FP8Format::E4M3>(qp, stream.stream());
}
};
// Explicit-transpose backward: the gradient/activation tensors keep their
// natural row-major layout, which the GEMM consumes transposed (W is
// [N,K] but dX contracts over N; x is [M,K] and g is [M,N] for dW), so
// the fp8 operands are transposed once and run through the fast non-trans
// pre-quantized GEMM. g is quantized once (amax_g measured here); its
// transpose is derived from the same g8 so both GEMMs share the value.
auto pq_n = [&](const torch::Tensor& a8, const torch::Tensor& b8,
torch::Tensor& out, const torch::Tensor& sa,
const torch::Tensor& sb, int64_t mm, int64_t nn,
int64_t kk) {
FP8Params gp;
pack_gemm_params(gp, a8.data_ptr(), b8.data_ptr(), out.data_ptr(), sa,
sb, nullptr, mm, nn, kk);
if (fmt) {
fp8::launch_fp8_gemm<FP8Format::E5M2>(gp, stream.stream());
} else {
fp8::launch_fp8_gemm<FP8Format::E4M3>(gp, stream.stream());
}
};
torch::Tensor g8;
if (masks[0] || masks[1]) {
g8 = torch::empty({m, n}, f8opt);
quantize(g_c, g8, sg, &amax_g);
}
// dX = g @ W: A = g8 [M,N] natural; B = W^T [K,N] (w8 transposed in fp8).
if (masks[0]) {
auto w8 = torch::empty({n, k}, f8opt);
quantize(w_c, w8, sw, nullptr);
auto w8T = w8.transpose(0, 1).contiguous(); // [K, N]
auto grad_input_2d = grad_input.reshape({m, k});
pq_n(g8, w8T, grad_input_2d, sg, sw, m, k, n);
}
// dW = g^T @ x: A = g^T [N,M] (g8 transposed); B = x^T [K,M].
if (masks[1]) {
auto g8T = g8.transpose(0, 1).contiguous(); // [N, M]
auto x8 = torch::empty({m, k}, f8opt);
quantize(x_c, x8, sx, nullptr);
auto x8T = x8.transpose(0, 1).contiguous(); // [K, M]
pq_n(g8T, x8T, grad_weight, sg, sx, n, k, m);
}
if (!masks[0] && !masks[1]) {
amax_g.copy_(g_c.abs().amax().to(torch::kFloat32));
}
C10_CUDA_CHECK(cudaGetLastError());
if (masks[2]) grad_bias = g_c.sum(0).to(g.scalar_type());
return {grad_input, grad_weight, grad_bias, amax_g};
}
PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
m.def("quantize_bf16", &quantize_bf16, py::arg("x"), py::arg("scale"),
py::arg("fmt"),
"BF16 to FP8 (E4M3/E5M2) quantize with fused amax; returns (x8, amax)");
m.def("mm_fp8", &mm_fp8, py::arg("a"), py::arg("b"), py::arg("sa"),
py::arg("sb"), py::arg("out_dtype") = 0,
py::arg("out_scale") = py::none(),
"Pre-quantized FP8 GEMM: a @ b^T * (sa * sb); out_dtype 0=bf16, "
"1=fp8 e4m3 (requires out_scale)");
m.def("linear_forward_fp8", &linear_forward_fp8, py::arg("x"),
py::arg("w"), py::arg("bias"), py::arg("sx"), py::arg("sw"),
py::arg("fmt") = 0,
"Pure FP8 linear forward: quantize x/w, pre-quantized GEMM; "
"returns (out, amax_x, amax_w)");
m.def("linear_backward_fp8", &linear_backward_fp8, py::arg("g"),
py::arg("x"), py::arg("w"), py::arg("masks"), py::arg("sg"),
py::arg("sw"), py::arg("sx"), py::arg("fmt"),
"FP8 linear backward; returns (grad_input, grad_weight, grad_bias, amax_g)");
}