[Piglit] [PATCH] time-elapsed: Spend time in the FS rather than do a lot of draws.

[Paul Berry]

The time-elapsed test needs to consume a lot of GPU time so that it
can validate GPU time queries against elapsed wall-clock time.
Previously we did this by painting the window thousands of times.
That sometimes caused false failures due to the CPU time consumed in
preparing the draw calls.

This patch changes the test so that it consumes time by using a slow
fragment shader rather than by painting the window over and over.
---
 tests/spec/ext_timer_query/time-elapsed.c | 57 ++++++++++++++++++++++++++++---
 1 file changed, 53 insertions(+), 4 deletions(-)

diff --git a/tests/spec/ext_timer_query/time-elapsed.c b/tests/spec/ext_timer_query/time-elapsed.c
index 8b93d89..942d737 100644
--- a/tests/spec/ext_timer_query/time-elapsed.c
+++ b/tests/spec/ext_timer_query/time-elapsed.c
@@ -41,11 +41,51 @@ PIGLIT_GL_TEST_CONFIG_BEGIN
 
 PIGLIT_GL_TEST_CONFIG_END
 
+static char *vs_text =
+	"#version 110\n"
+	"void main()\n"
+	"{\n"
+	"  gl_Position = gl_Vertex;\n"
+	"}\n";
+
+/**
+ * Time-wasting fragment shader.  This fragment shader computes:
+ *
+ *   x = (sum(i=0..(iters-1): 2*i) % iters) / iters
+ *
+ * This value should always work out to 0.0, but it's highly unlikely
+ * that an optimizer will figure this out.  Hence we can use this
+ * shader to waste an arbitrary amount of time (by suitable choice of
+ * the value of iters).
+ *
+ * The shader outputs a color of (x, 1.0, 0.0, 0.0).
+ */
+static char *fs_text =
+	"#version 110\n"
+	"uniform int iters;\n"
+	"void main()\n"
+	"{\n"
+	"  int cumulative_sum = 0;\n"
+	"  for (int i = 0; i < iters; ++i) {\n"
+	"    cumulative_sum += i;\n"
+	"    if (cumulative_sum >= iters)\n"
+	"      cumulative_sum -= iters;\n"
+	"    cumulative_sum += i;\n"
+	"    if (cumulative_sum >= iters)\n"
+	"      cumulative_sum -= iters;\n"
+	"  }\n"
+	"  float x = float(cumulative_sum) / float(iters);\n"
+	"  gl_FragColor = vec4(x, 1.0, 0.0, 0.0);\n"
+	"}\n";
+
 enum {
 	TIME_ELAPSED,
 	TIMESTAMP
 } test = TIME_ELAPSED;
 
+GLuint prog;
+GLint iters_loc;
+
 static float
 get_time(void)
 {
@@ -72,7 +112,9 @@ static float
 draw(GLuint *q, int iters)
 {
 	float start_time, end_time;
-	int i;
+
+	glUseProgram(prog);
+	glUniform1i(iters_loc, iters);
 
 	start_time = get_time();
 
@@ -81,9 +123,7 @@ draw(GLuint *q, int iters)
 	} else {
 		glBeginQuery(GL_TIME_ELAPSED, q[0]);
 	}
-	for (i = 0; i < iters; i++) {
-		piglit_draw_rect(-1, -1, 2, 2);
-	}
+	piglit_draw_rect(-1, -1, 2, 2);
 	if (test == TIMESTAMP) {
 		glQueryCounter(q[1], GL_TIMESTAMP);
 	} else {
@@ -270,8 +310,17 @@ retry:
 void
 piglit_init(int argc, char **argv)
 {
+	GLint vs, fs;
+
 	piglit_require_gl_version(20);
 
+	vs = piglit_compile_shader_text(GL_VERTEX_SHADER, vs_text);
+	fs = piglit_compile_shader_text(GL_FRAGMENT_SHADER, fs_text);
+	prog = piglit_link_simple_program(vs, fs);
+	glDeleteShader(vs);
+	glDeleteShader(fs);
+	iters_loc = glGetUniformLocation(prog, "iters");
+
 	piglit_require_extension("GL_EXT_timer_query");
 
 	if (argc == 2 && strcmp(argv[1], "timestamp") == 0) {
-- 
1.8.0