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MobileGL/MobileGL/MG_IntegrationTest/Scenarios/DoublePrecisionScenario.cpp
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// MobileGL - MobileGL/MG_IntegrationTest/Scenarios/DoublePrecisionScenario.cpp
// Copyright (c) 2025-2026 MobileGL-Dev
// Licensed under the GNU Lesser General Public License v3.0:
// https://www.gnu.org/licenses/gpl-3.0.txt
// https://www.gnu.org/licenses/lgpl-3.0.txt
// SPDX-License-Identifier: LGPL-3.0-only
// End of Source File Header
//
// Scenario - GLSL DOUBLES, RUN AT SINGLE PRECISION.
//
// No mobile GPU has 64-bit floats. Adreno and Mali both report shaderFloat64 == VK_FALSE, so
// Magma cannot build a module that declares the Float64 capability, and ESSL has no fp64 type
// at all, so SPIRV-Cross refuses the module outright on Espryt ("FP64 not supported in ES
// profile") and the program never reaches the driver. MobileGL therefore narrows every 64-bit
// float in a shader to 32 bits (ShaderTranspiler::DemoteFloat64Pass) rather than declining the
// shader: `double` compiles and runs everywhere, at float precision.
//
// The narrowing is only half a contract. The other half is the API side: the global UBO is
// laid out by reflecting the DEMOTED module, so glUniform*d has to store a float where the
// shader reads a float, glGetUniform*v has to read one back, and a dmat4's columns are now
// std140-padded like any other matrix's. Every one of those is a byte offset that fails
// silently - the uniform simply reads as something else - so the cases below set values
// through the API and have the SHADER report what it saw.
//
// What is deliberately NOT asserted: that the values are exact to double precision. They are
// not, and cannot be. Every expectation here is the float value of the double that was set,
// which is the whole point.
#include <cmath>
#include <string>
#include <vector>
#include "../Harness/HeadlessGL.h"
#include "../Harness/ScenarioFixture.h"
#ifdef GLAPI
#undef GLAPI
#endif
#define GL_GLEXT_PROTOTYPES
#include <GL/gl.h>
#include <GL/glcorearb.h>
#undef GL_GLEXT_PROTOTYPES
namespace MGITest {
namespace {
// Doubles in every shape the demotion has to handle - a scalar, a vector, a matrix
// whose column stride changes, an array whose element stride changes - all reported
// through one float SSBO so a single readback says which one moved.
constexpr const char* kComputeSource = R"(#version 430 core
layout(local_size_x = 1) in;
uniform double uScalar;
uniform dvec3 uVector;
uniform dmat4 uMatrix;
uniform double uArray[3];
layout(std430, binding = 0) buffer Output {
float g_out[];
};
void main() {
g_out[0] = float(uScalar);
g_out[1] = float(uVector.x);
g_out[2] = float(uVector.y);
g_out[3] = float(uVector.z);
// Column-major [column][row]. Off-diagonal entries catch a column-stride mistake that a
// diagonal-only check reads straight past.
g_out[4] = float(uMatrix[0][0]);
g_out[5] = float(uMatrix[0][3]);
g_out[6] = float(uMatrix[3][0]);
g_out[7] = float(uMatrix[3][3]);
g_out[8] = float(uArray[0]);
g_out[9] = float(uArray[1]);
g_out[10] = float(uArray[2]);
// Arithmetic on doubles, including an implicit float->double conversion and a literal
// with the fp64 suffix: this is what an application actually writes, and it is the part
// that has to survive the conversion folding.
double accumulated = uScalar * 2.0lf + 1.5;
g_out[11] = float(accumulated);
}
)";
constexpr int kOutputSlots = 12;
class DoublePrecisionScenario : public ScenarioTest {
protected:
void SetUp() override {
ScenarioTest::SetUp();
if (!Ready()) return;
m_program = CompileComputeProgram(kComputeSource);
ASSERT_NE(m_program, 0u) << m_buildLog;
glGenBuffers(1, &m_output);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, m_output);
const std::vector<float> zeroes(kOutputSlots, 0.0f);
glBufferData(GL_SHADER_STORAGE_BUFFER, kOutputSlots * sizeof(float), zeroes.data(),
GL_DYNAMIC_DRAW);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, m_output);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
}
void TearDown() override {
if (!Ready()) return;
if (m_output != 0) glDeleteBuffers(1, &m_output);
if (m_program != 0) glDeleteProgram(m_program);
}
unsigned int CompileComputeProgram(const char* source) {
const GLuint shader = glCreateShader(GL_COMPUTE_SHADER);
glShaderSource(shader, 1, &source, nullptr);
glCompileShader(shader);
GLint compiled = 0;
glGetShaderiv(shader, GL_COMPILE_STATUS, &compiled);
if (compiled == GL_FALSE) {
char log[2048] = {};
glGetShaderInfoLog(shader, sizeof(log) - 1, nullptr, log);
m_buildLog = std::string("compute shader did not compile: ") + log;
glDeleteShader(shader);
return 0;
}
const GLuint program = glCreateProgram();
glAttachShader(program, shader);
glLinkProgram(program);
glDeleteShader(shader);
GLint linked = 0;
glGetProgramiv(program, GL_LINK_STATUS, &linked);
if (linked == GL_FALSE) {
char log[2048] = {};
glGetProgramInfoLog(program, sizeof(log) - 1, nullptr, log);
m_buildLog = std::string("compute program did not link: ") + log;
glDeleteProgram(program);
return 0;
}
return program;
}
std::vector<float> Dispatch() {
glUseProgram(m_program);
glDispatchCompute(1, 1, 1);
glMemoryBarrier(GL_BUFFER_UPDATE_BARRIER_BIT);
std::vector<float> values(kOutputSlots, -1.0f);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, m_output);
glGetBufferSubData(GL_SHADER_STORAGE_BUFFER, 0, kOutputSlots * sizeof(float), values.data());
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
glUseProgram(0);
return values;
}
unsigned int m_program = 0;
unsigned int m_output = 0;
std::string m_buildLog;
};
TEST_F(DoublePrecisionScenario, ADoubleUniformReachesTheShaderAtFloatPrecision) {
if (!Ready()) return;
glUseProgram(m_program);
const GLint scalar = glGetUniformLocation(m_program, "uScalar");
ASSERT_GE(scalar, 0);
// 0.1 has no exact float (or double) representation, so this only passes if the
// value really travelled through the demoted slot rather than being read out of
// some other four bytes.
glUniform1d(scalar, 0.1);
glUseProgram(0);
const std::vector<float> values = Dispatch();
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_NO_ERROR));
EXPECT_FLOAT_EQ(values[0], static_cast<float>(0.1));
EXPECT_FLOAT_EQ(values[11], static_cast<float>(static_cast<float>(0.1) * 2.0f + 1.5f))
<< "arithmetic on the demoted value, including the folded fp64 literal";
}
TEST_F(DoublePrecisionScenario, EveryDoubleShapeLandsInItsOwnSlot) {
if (!Ready()) return;
glUseProgram(m_program);
const GLint scalar = glGetUniformLocation(m_program, "uScalar");
const GLint vector = glGetUniformLocation(m_program, "uVector");
const GLint matrix = glGetUniformLocation(m_program, "uMatrix");
const GLint array0 = glGetUniformLocation(m_program, "uArray[0]");
const GLint array2 = glGetUniformLocation(m_program, "uArray[2]");
ASSERT_GE(scalar, 0);
ASSERT_GE(vector, 0);
ASSERT_GE(matrix, 0);
ASSERT_GE(array0, 0);
ASSERT_GE(array2, 0);
glUniform1d(scalar, 5.0);
const GLdouble vectorValue[3] = {11.0, 12.0, 13.0};
glUniform3dv(vector, 1, vectorValue);
// Column-major, and every entry distinct so a transposed or mis-strided write
// cannot land on a value that happens to match.
GLdouble matrixValue[16] = {};
for (int i = 0; i < 16; ++i) matrixValue[i] = 100.0 + i;
glUniformMatrix4dv(matrix, 1, GL_FALSE, matrixValue);
const GLdouble arrayValue[3] = {71.0, 72.0, 73.0};
glUniform1dv(array0, 3, arrayValue);
glUseProgram(0);
const std::vector<float> values = Dispatch();
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_NO_ERROR));
EXPECT_FLOAT_EQ(values[0], 5.0f) << "scalar double";
EXPECT_FLOAT_EQ(values[1], 11.0f) << "dvec3 .x";
EXPECT_FLOAT_EQ(values[2], 12.0f) << "dvec3 .y";
EXPECT_FLOAT_EQ(values[3], 13.0f) << "dvec3 .z";
EXPECT_FLOAT_EQ(values[4], 100.0f) << "dmat4 [0][0]";
EXPECT_FLOAT_EQ(values[5], 103.0f) << "dmat4 [0][3] - within the first column";
EXPECT_FLOAT_EQ(values[6], 112.0f) << "dmat4 [3][0] - column stride";
EXPECT_FLOAT_EQ(values[7], 115.0f) << "dmat4 [3][3]";
EXPECT_FLOAT_EQ(values[8], 71.0f) << "double array element 0";
EXPECT_FLOAT_EQ(values[9], 72.0f) << "double array element 1 - element stride";
EXPECT_FLOAT_EQ(values[10], 73.0f) << "double array element 2";
}
TEST_F(DoublePrecisionScenario, TheTransposeFlagStillTransposes) {
if (!Ready()) return;
glUseProgram(m_program);
const GLint matrix = glGetUniformLocation(m_program, "uMatrix");
ASSERT_GE(matrix, 0);
GLdouble matrixValue[16] = {};
for (int i = 0; i < 16; ++i) matrixValue[i] = 100.0 + i;
glUniformMatrix4dv(matrix, 1, GL_TRUE, matrixValue);
glUseProgram(0);
const std::vector<float> values = Dispatch();
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_NO_ERROR));
// Transposed, so [column][row] now reads the source's [row][column].
EXPECT_FLOAT_EQ(values[4], 100.0f) << "dmat4 [0][0] is on the diagonal either way";
EXPECT_FLOAT_EQ(values[5], 112.0f) << "dmat4 [0][3] after transpose";
EXPECT_FLOAT_EQ(values[6], 103.0f) << "dmat4 [3][0] after transpose";
EXPECT_FLOAT_EQ(values[7], 115.0f) << "dmat4 [3][3] is on the diagonal either way";
}
TEST_F(DoublePrecisionScenario, TheUniformIsStillReportedAsADouble) {
if (!Ready()) return;
// The demotion is an implementation detail of how the value is STORED. What the
// shader source declared is what the application asked about, so the reflection
// keeps answering GL_DOUBLE* - an application that switches on the type and calls
// glUniform*d has to keep working, and it is the glUniform*d path that is correct
// for these uniforms.
struct Expectation {
const char* name;
GLenum type;
GLint size;
};
const Expectation expectations[] = {
{"uScalar", GL_DOUBLE, 1},
{"uVector", GL_DOUBLE_VEC3, 1},
{"uMatrix", GL_DOUBLE_MAT4, 1},
{"uArray[0]", GL_DOUBLE, 3},
};
GLint activeUniforms = 0;
glGetProgramiv(m_program, GL_ACTIVE_UNIFORMS, &activeUniforms);
ASSERT_GT(activeUniforms, 0);
for (const Expectation& expectation : expectations) {
bool found = false;
for (GLint index = 0; index < activeUniforms; ++index) {
char name[128] = {};
GLsizei length = 0;
GLint size = 0;
GLenum type = 0;
glGetActiveUniform(m_program, static_cast<GLuint>(index), sizeof(name) - 1, &length, &size,
&type, name);
if (std::string(name, static_cast<size_t>(length)) != expectation.name) continue;
found = true;
EXPECT_EQ(type, expectation.type) << expectation.name;
EXPECT_EQ(size, expectation.size) << expectation.name;
break;
}
EXPECT_TRUE(found) << "glGetActiveUniform never reported " << expectation.name;
}
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_NO_ERROR));
}
TEST_F(DoublePrecisionScenario, GetUniformdvReadsBackWhatWasStored) {
if (!Ready()) return;
glUseProgram(m_program);
const GLint scalar = glGetUniformLocation(m_program, "uScalar");
const GLint vector = glGetUniformLocation(m_program, "uVector");
const GLint matrix = glGetUniformLocation(m_program, "uMatrix");
ASSERT_GE(scalar, 0);
ASSERT_GE(vector, 0);
ASSERT_GE(matrix, 0);
glUniform1d(scalar, 0.1);
const GLdouble vectorValue[3] = {11.5, 12.5, 13.5};
glUniform3dv(vector, 1, vectorValue);
GLdouble matrixValue[16] = {};
for (int i = 0; i < 16; ++i) matrixValue[i] = 100.0 + i;
glUniformMatrix4dv(matrix, 1, GL_FALSE, matrixValue);
glUseProgram(0);
// The readback has to undo exactly what the write did - the same std140 column
// padding, the same 4-byte components - or a dmat4 comes back with its columns
// shifted and nothing else in the API would say so.
GLdouble readScalar = 0.0;
glGetUniformdv(m_program, scalar, &readScalar);
EXPECT_DOUBLE_EQ(readScalar, static_cast<double>(static_cast<float>(0.1)))
<< "the value is what a float can hold, not the double that was passed in";
GLdouble readVector[3] = {};
glGetUniformdv(m_program, vector, readVector);
EXPECT_DOUBLE_EQ(readVector[0], 11.5);
EXPECT_DOUBLE_EQ(readVector[1], 12.5);
EXPECT_DOUBLE_EQ(readVector[2], 13.5);
GLdouble readMatrix[16] = {};
glGetUniformdv(m_program, matrix, readMatrix);
for (int i = 0; i < 16; ++i) {
EXPECT_DOUBLE_EQ(readMatrix[i], 100.0 + i) << "dmat4 component " << i;
}
// The float query sees the same storage through the type it is actually stored as.
GLfloat readFloat = 0.0f;
glGetUniformfv(m_program, scalar, &readFloat);
EXPECT_FLOAT_EQ(readFloat, static_cast<float>(0.1));
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_NO_ERROR));
}
TEST_F(DoublePrecisionScenario, TheFp64ExtensionIsNotAdvertised) {
if (!Ready()) return;
// The shader above compiled, linked and ran without the extension string, which is
// the point: an application does not need GL_ARB_gpu_shader_fp64 advertised to USE
// doubles here. What the string additionally promises is 64-bit precision, and that
// is the one thing the demotion cannot deliver - so it stays off unless
// MOBILEGL_ADVERTISE_FP64 asks for it, and an application that branches on the
// string keeps taking its float path.
GLint extensionCount = 0;
glGetIntegerv(GL_NUM_EXTENSIONS, &extensionCount);
ASSERT_GT(extensionCount, 0);
bool advertised = false;
for (GLint i = 0; i < extensionCount; ++i) {
const char* name = reinterpret_cast<const char*>(glGetStringi(GL_EXTENSIONS, static_cast<GLuint>(i)));
if (name != nullptr && std::string(name) == "GL_ARB_gpu_shader_fp64") advertised = true;
}
EXPECT_FALSE(advertised);
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_NO_ERROR));
}
TEST_F(DoublePrecisionScenario, A64BitVertexFormatIsDeclinedOnEveryBackend) {
if (!Ready()) return;
// The demotion leaves no 64-bit shader input to feed, so there is nothing a 64-bit
// vertex FETCH could be fetched into - on either backend, and no longer only on the
// ones whose device lacks shaderFloat64. Declined loudly rather than accepted and
// drawn as garbage; the matching POST row says the same thing at startup.
GLuint vao = 0;
glGenVertexArrays(1, &vao);
glBindVertexArray(vao);
while (glGetError() != GL_NO_ERROR) {}
glVertexAttribLFormat(0, 3, GL_DOUBLE, 0);
EXPECT_EQ(glGetError(), static_cast<GLenum>(GL_INVALID_OPERATION));
glBindVertexArray(0);
glDeleteVertexArrays(1, &vao);
while (glGetError() != GL_NO_ERROR) {}
}
} // namespace
} // namespace MGITest
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