[Bf-blender-cvs] [cde858ae98a] blender-v2.83-release: Particles: Fixed thread work size calculation.

[Stefan Werner]

Commit: cde858ae98a5bcc214cabae0890a09b5b41e9909
Author: Stefan Werner
Date:   Wed Jan 20 11:17:32 2021 +0100
Branches: blender-v2.83-release
https://developer.blender.org/rBcde858ae98a5bcc214cabae0890a09b5b41e9909

Particles: Fixed thread work size calculation.

Dividing the workload by number of tasks in float is imprecise and
lead in some cases to particles not being calculated at all
(example: 20000 particles, 144 tasks).

Switching this calculation to integer makes sure we don't lose count.

Differential Revision: https://developer.blender.org/D10157

===================================================================

M	source/blender/blenkernel/intern/particle_system.c

===================================================================

diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index b3472bfe716..615c4350fe4 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -476,20 +476,24 @@ void psys_tasks_create(ParticleThreadContext *ctx,
 {
   ParticleTask *tasks;
   int numtasks = min_ii(BLI_system_thread_count() * 4, endpart - startpart);
-  float particles_per_task = (float)(endpart - startpart) / (float)numtasks, p, pnext;
-  int i;
+  int particles_per_task = numtasks > 0 ? (endpart - startpart) / numtasks : 0;
+  int remainder = numtasks > 0 ? (endpart - startpart) - particles_per_task * numtasks : 0;
 
   tasks = MEM_callocN(sizeof(ParticleTask) * numtasks, "ParticleThread");
   *r_numtasks = numtasks;
   *r_tasks = tasks;
 
-  p = (float)startpart;
-  for (i = 0; i < numtasks; i++, p = pnext) {
-    pnext = p + particles_per_task;
-
+  int p = startpart;
+  for (int i = 0; i < numtasks; i++) {
     tasks[i].ctx = ctx;
-    tasks[i].begin = (int)p;
-    tasks[i].end = min_ii((int)pnext, endpart);
+    tasks[i].begin = p;
+    p = p + particles_per_task + (i < remainder ? 1 : 0);
+    tasks[i].end = p;
+  }
+
+  /* Verify that all particles are accounted for. */
+  if (numtasks > 0) {
+    BLI_assert(tasks[numtasks - 1].end == endpart);
   }
 }