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Merged
fs-eire merged 3 commits into from
Jul 30, 2025
Merged

[webgpu] use u32 to represent f16 in uniform #25391

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1d9f089
[webgpu] use u32 to represent f16 in uniform
fs-eire Jul 14, 2025
f994466
bug fix
fs-eire Jul 14, 2025
dd1c56d
bug fix (2)
fs-eire Jul 15, 2025
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31 changes: 22 additions & 9 deletions onnxruntime/core/providers/webgpu/shader_helper.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -491,16 +491,29 @@ Status ShaderHelper::GenerateSourceCode(std::string& code, std::vector<int>& sha
ss << ",";
}

auto alignment = (data_type == ProgramUniformVariableDataType::Float16 && length > 4) ? "@align(16) " : "";
ss << "\n " << alignment << name << ": ";
// The actual variable type for the uniform variable depends on the data type (T) and length (N).
//
// For T in [i32, u32, f32]:
// - If N == 1, the type is simply i32, u32, or f32.
// - If 2 < N <= 4, the type is vecN<i32>, vecN<u32>, or vecN<f32> where N is the length.
// - If N > 4, the type is array<vec4<T>, ceil(N / 4)>.
//
// For T is f16:
// - If N == 1 or N == 2, the type is u32.
// - If 2 < N <= 8, the type is vecX<u32> where X is ceil(N / 2).
// - If N > 8, the type is array<vec4<u32>, X> where X is ceil(N / 8).
//
// Note: Using f16 type in uniforms is not generally supported on all devices. We use a u32 variable to represent
// 2 f16 values.

if (data_type == ProgramUniformVariableDataType::Float16) {
data_type = ProgramUniformVariableDataType::Uint32; // f16 is represented as u32
length = (length + 1) / 2; // each u32 can hold 2 f16 values
}
ss << "\n " << name << ": ";
if (length > 4) {
if (data_type == ProgramUniformVariableDataType::Float16) {
size_t array_size = (length + 7) / 8;
ss << "array<mat2x4<" << data_type << ">, " << array_size << ">";
} else {
size_t array_size = (length + 3) / 4;
ss << "array<vec4<" << data_type << ">, " << array_size << ">";
}
size_t array_size = (length + 3) / 4;
ss << "array<vec4<" << data_type << ">, " << array_size << ">";
} else if (length > 1) {
ss << "vec" << length << "<" << data_type << ">";
} else {
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34 changes: 25 additions & 9 deletions onnxruntime/core/providers/webgpu/shader_variable.h
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Expand Up @@ -17,18 +17,34 @@ template <typename TIdx,
typename TRank,
typename = std::enable_if_t<std::is_same_v<TRank, int> || std::is_same_v<TRank, size_t>>>
std::string GetElementAt(std::string_view var, const TIdx& idx, TRank rank, bool is_f16 = false) {
// "std::string::rfind(str, 0) == 0" is equivalent to "std::string::starts_with(str)" before C++20.
if (var.rfind("uniforms.", 0) == 0) {
if (rank > 4) {
if constexpr (std::is_integral_v<TIdx>) {
if (is_f16) {
return MakeStringWithClassicLocale(var, "[", idx / 8, "][", (idx % 8) / 4, "][", (idx % 8) % 4, "]");
if (var.starts_with("uniforms.")) {
if (is_f16) {
if (rank > 8) {
// array<vec4<u32>, N>
if constexpr (std::is_integral_v<TIdx>) {
return MakeStringWithClassicLocale("bitcast<vec2<f16>>(", var, "[", idx / 8, "][", (idx % 8) / 2, "])[", (idx % 8) % 2, "]");
} else {
return MakeStringWithClassicLocale(var, "[", idx / 4, "][", idx % 4, "]");
return MakeStringWithClassicLocale("bitcast<vec2<f16>>(", var, "[(", idx, ") / 8][((", idx, ") % 8) / 2])[((", idx, ") % 8) % 2]");
}
} else if (rank > 2) {
// vecN<u32>
if constexpr (std::is_integral_v<TIdx>) {
return MakeStringWithClassicLocale("bitcast<vec2<f16>>(", var, "[", idx / 2, "])[", idx % 2, "]");
} else {
return MakeStringWithClassicLocale("bitcast<vec2<f16>>(", var, "[(", idx, ") / 2])[(", idx, ") % 2]");
}
} else {
if (is_f16) {
return MakeStringWithClassicLocale(var, "[(", idx, ") / 8][(", idx, ") % 8 / 4][(", idx, ") % 8 % 4]");
// u32
if constexpr (std::is_integral_v<TIdx>) {
return MakeStringWithClassicLocale("bitcast<vec2<f16>>(", var, ")[", idx % 2, "]");
} else {
return MakeStringWithClassicLocale("bitcast<vec2<f16>>(", var, ")[(", idx, ") % 2]");
}
}
} else {
if (rank > 4) {
if constexpr (std::is_integral_v<TIdx>) {
return MakeStringWithClassicLocale(var, "[", idx / 4, "][", idx % 4, "]");
} else {
return MakeStringWithClassicLocale(var, "[(", idx, ") / 4][(", idx, ") % 4]");
}
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65 changes: 48 additions & 17 deletions onnxruntime/core/providers/webgpu/webgpu_context.cc
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Expand Up @@ -373,26 +373,57 @@ Status WebGpuContext::Run(ComputeContext& context, const ProgramBase& program) {
continue;
}

bool is_f16 = uniform.data_type == ProgramUniformVariableDataType::Float16;

size_t element_size = ProgramUniformVariableDataTypeSize[static_cast<int>(uniform.data_type)];
// Calculate the size and alignment of the uniform variable.
//
// https://www.w3.org/TR/WGSL/#alignof
size_t base_alignment = is_f16
? (length > 4 ? 16 : length > 2 ? 8
: length * element_size)
: (length > 2 ? 16 : length * element_size);
size_t struct_size = is_f16 && length <= 4 ? length * element_size : 16;

current_offset = (current_offset + base_alignment - 1) / base_alignment * base_alignment;
//
// For f16:
// - length > 8 : array<vec4<u32>, N> (align 16) (size 16 * N, N = ceil(length / 8))
// - length == 7 or 8: vec4<u32> (align 16) (size 16)
// - length == 5 or 6: vec3<u32> (align 16) (size 12)
// - length == 3 or 4: vec2<u32> (align 8) (size 8)
// - length == 1 or 2: u32 (align 4) (size 4)
//
// For other types (i32, u32, f32):
// - length > 4 : array<vec4<T>, N> (align 16) (size 16 * N, N = ceil(length / 4))
// - length == 4 : vec4<T> (align 16) (size 16)
// - length == 3 : vec3<T> (align 16) (size 12)
// - length == 2 : vec2<T> (align 8) (size 8)
// - length == 1 : T (align 4) (size 4)
//

const bool is_f16 = uniform.data_type == ProgramUniformVariableDataType::Float16;

size_t variable_alignment = 4; // default alignment for scalar types
size_t variable_size = 4; // default size for scalar types

if (is_f16) {
if (length > 6) {
variable_alignment = 16;
variable_size = 16 * ((length + 7) / 8);
} else if (length > 4) {
variable_alignment = 16;
variable_size = 12;
} else if (length > 2) {
variable_alignment = 8;
variable_size = 8;
}
} else {
if (length > 3) {
variable_alignment = 16;
variable_size = 16 * ((length + 3) / 4);
} else if (length > 2) {
variable_alignment = 16;
variable_size = 12;
} else if (length > 1) {
variable_alignment = 8;
variable_size = 8;
}
}
current_offset = (current_offset + variable_alignment - 1) / variable_alignment * variable_alignment;
uniform_and_offsets.emplace_back(uniform, current_offset);

// For non-float16 type, when length > 4, the uniform variable is of type array<vec4<i32|u32|f32>,N>, where
// N = ceil(data.length / 4) and SizeOf(vec4<i32|u32|f32>) = 16. The total byte length is N * SizeOf(vec4<i32|u32|f32>).
// For float16 type, when length > 4, the uniform variable is of type array<mat2x4<f16>,N>, where
// N = ceil(data.length / 8) and SizeOf(mat2x4<f16>) = 16. The total byte length is N * SizeOf(mat2x4<f16>).
size_t element_per_struct = is_f16 ? 8 : 4;
current_offset +=
length > 4 ? (length + element_per_struct - 1) / element_per_struct * struct_size : length * element_size;
current_offset += variable_size;
}

// Meet alignment of struct here: https://www.w3.org/TR/WGSL/#alignment-and-size. For simplicity, set
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