Add missing file

Also add finish_timestamps call, which is needed for DX12 (there are
other issues but this is an easy fix for that one).

piet-gpu-hal/examples/collatz.rs

@@ -21,6 +21,7 @@ fn main() {
         cmd_buf.write_timestamp(&query_pool, 0);
         cmd_buf.dispatch(&pipeline, &descriptor_set, (256, 1, 1), (1, 1, 1));
         cmd_buf.write_timestamp(&query_pool, 1);
+        cmd_buf.finish_timestamps(&query_pool);
         cmd_buf.host_barrier();
         cmd_buf.finish();
         let submitted = session.run_cmd_buf(cmd_buf, &[], &[]).unwrap();

tests/shader/prefix_scan.comp

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT OR Unlicense
+
+// A scan for a tree reduction prefix scan (either root or not, by ifdef).
+
+#version 450
+
+#define N_ROWS 8
+#define LG_WG_SIZE 9
+#define WG_SIZE (1 << LG_WG_SIZE)
+#define PARTITION_SIZE (WG_SIZE * N_ROWS)
+
+layout(local_size_x = WG_SIZE, local_size_y = 1) in;
+
+struct Monoid {
+    uint element;
+};
+
+layout(set = 0, binding = 0) buffer DataBuf {
+    Monoid[] data;
+};
+
+#ifndef ROOT
+layout(set = 0, binding = 1) buffer ParentBuf {
+    Monoid[] parent;
+};
+#endif
+
+shared Monoid sh_scratch[WG_SIZE];
+
+Monoid combine_monoid(Monoid a, Monoid b) {
+    return Monoid(a.element + b.element);
+}
+
+void main() {
+    Monoid local[N_ROWS];
+
+    uint ix = gl_GlobalInvocationID.x * N_ROWS;
+
+    // TODO: gate buffer read
+    local[0] = data[ix];
+    for (uint i = 1; i < N_ROWS; i++) {
+        local[i] = combine_monoid(local[i - 1], data[ix + i]);
+    }
+    Monoid agg = local[N_ROWS - 1];
+    sh_scratch[gl_LocalInvocationID.x] = agg;
+    for (uint i = 0; i < LG_WG_SIZE; i++) {
+        barrier();
+        if (gl_LocalInvocationID.x >= (1 << i)) {
+            Monoid other = sh_scratch[gl_LocalInvocationID.x - (1 << i)];
+            agg = combine_monoid(other, agg);
+        }
+        barrier();
+        sh_scratch[gl_LocalInvocationID.x] = agg;
+    }
+    
+    barrier();
+    // This could be a semigroup instead of a monoid if we reworked the
+    // conditional logic, but that might impact performance.
+    Monoid row = Monoid(0);
+#ifdef ROOT
+    if (gl_LocalInvocationID.x > 0) {
+        row = sh_scratch[gl_LocalInvocationID.x - 1];
+    }
+#else
+    if (gl_WorkGroupID.x > 0) {
+        row = parent[gl_WorkGroupID.x - 1];
+    }
+    if (gl_LocalInvocationID.x > 0) {
+        row = combine_monoid(row, sh_scratch[gl_LocalInvocationID.x - 1]);
+    }
+#endif
+    for (uint i = 0; i < N_ROWS; i++) {
+        Monoid m = combine_monoid(row, local[i]);
+        // TODO: gate buffer write
+        data[ix + i] = m;
+    }
+}

tests/src/runner.rs

@@ -76,6 +76,7 @@ impl Runner {
     pub unsafe fn submit(&mut self, commands: Commands) -> f64 {
         let mut cmd_buf = commands.cmd_buf;
         let query_pool = commands.query_pool;
+        cmd_buf.finish_timestamps(&query_pool);
         cmd_buf.host_barrier();
         cmd_buf.finish();
         let submitted = self.session.run_cmd_buf(cmd_buf, &[], &[]).unwrap();