vulkan : argsort barriers must be under uniform control flow (#951)

a return before a barrier (that happens only in some threads in
a workgroup) leads to UB.
While the old code actually works on some devices,
it fails on some others (i.e. "smaller" GPUs).

BTW, I think it would be better to set specialization constants
when the graph is built, in that way the local workgroup
could be sized appropriately.
But it would take a lot of work.

Signed-off-by: Salvatore Mesoraca <[email protected]>

src/vulkan-shaders/argsort.comp

@@ -29,20 +29,18 @@ void main() {
     const int col = int(gl_LocalInvocationID.x);
     const uint row = gl_WorkGroupID.y;
 
-    if (col >= p.ncols_pad) {
-        return;
-    }
-
     const uint row_offset = row * p.ncols;
 
     // initialize indices
-    dst_row[col] = col;
+    if (col < p.ncols_pad) {
+        dst_row[col] = col;
+    }
     barrier();
 
     for (uint k = 2; k <= p.ncols_pad; k *= 2) {
         for (uint j = k / 2; j > 0; j /= 2) {
             const uint ixj = col ^ j;
-            if (ixj > col) {
+            if (col < p.ncols_pad && ixj > col) {
                 if ((col & k) == 0) {
                     if (dst_row[col] >= p.ncols ||
                         (dst_row[ixj] < p.ncols && (p.order == ASC ?