Mesa (master): broadcom/vc4: Use a single-entry cached last_hindex value.

[Eric Anholt]

Module: Mesa
Branch: master
Commit: 0ed952c7e9b811bc11dec64bd4bebcdf4222cc85
URL:    
http://cgit.freedesktop.org/mesa/mesa/commit/?id=0ed952c7e9b811bc11dec64bd4bebcdf4222cc85

Author: Eric Anholt <e...@anholt.net>
Date:   Tue Nov 28 16:17:16 2017 -0800

broadcom/vc4: Use a single-entry cached last_hindex value.

Since almost all BOs will be in one CL at a time, this cache will almost
always hit except for the first usage of the BO in each CL.

This didn't show up as statistically significant on the minetest trace
(n=340), but if I lop off the throttled lobe of the bimodal distribution,
it very clearly does (0.74731% +/- 0.162093%, n=269).

---

 src/gallium/drivers/vc4/vc4_bufmgr.h |  8 ++++++++
 src/gallium/drivers/vc4/vc4_cl.c     | 14 ++++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/src/gallium/drivers/vc4/vc4_bufmgr.h 
b/src/gallium/drivers/vc4/vc4_bufmgr.h
index 4e7b23e086..e0f6bbcfd8 100644
--- a/src/gallium/drivers/vc4/vc4_bufmgr.h
+++ b/src/gallium/drivers/vc4/vc4_bufmgr.h
@@ -39,6 +39,14 @@ struct vc4_bo {
         uint32_t handle;
         uint32_t size;
 
+        /* This will be read/written by multiple threads without a lock -- you
+         * should take a snapshot and use it to see if you happen to be in the
+         * CL's handles at this position, to make most lookups O(1).  It's
+         * volatile to make sure that the compiler doesn't emit multiple loads
+         * from the address, which would make the lookup racy.
+         */
+        volatile uint32_t last_hindex;
+
         /** Entry in the linked list of buffers freed, by age. */
         struct list_head time_list;
         /** Entry in the per-page-count linked list of buffers freed (by age). 
*/
diff --git a/src/gallium/drivers/vc4/vc4_cl.c b/src/gallium/drivers/vc4/vc4_cl.c
index 508281a27b..7ae092ebce 100644
--- a/src/gallium/drivers/vc4/vc4_cl.c
+++ b/src/gallium/drivers/vc4/vc4_cl.c
@@ -61,10 +61,19 @@ vc4_gem_hindex(struct vc4_job *job, struct vc4_bo *bo)
 {
         uint32_t hindex;
         uint32_t *current_handles = job->bo_handles.base;
+        uint32_t cl_hindex_count = cl_offset(&job->bo_handles) / 4;
+        uint32_t last_hindex = bo->last_hindex; /* volatile read! */
 
-        for (hindex = 0; hindex < cl_offset(&job->bo_handles) / 4; hindex++) {
-                if (current_handles[hindex] == bo->handle)
+        if (last_hindex < cl_hindex_count &&
+            current_handles[last_hindex] == bo->handle) {
+                return last_hindex;
+        }
+
+        for (hindex = 0; hindex < cl_hindex_count; hindex++) {
+                if (current_handles[hindex] == bo->handle) {
+                        bo->last_hindex = hindex;
                         return hindex;
+                }
         }
 
         struct vc4_cl_out *out;
@@ -79,5 +88,6 @@ vc4_gem_hindex(struct vc4_job *job, struct vc4_bo *bo)
 
         job->bo_space += bo->size;
 
+        bo->last_hindex = hindex;
         return hindex;
 }

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