Mesa (main): ac/llvm: rework how negative W affects culling to not call accept_func twice

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Module: Mesa
Branch: main
Commit: d9173b3b46d72d2fabc24f6eb8d75cf306fd751c
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=d9173b3b46d72d2fabc24f6eb8d75cf306fd751c

Author: Marek Olšák <marek.olsak at amd.com>
Date:   Tue Jul  6 23:01:01 2021 -0400

ac/llvm: rework how negative W affects culling to not call accept_func twice

Always execute the bbox code regardless of negative W, and then simply
use || to discard the result if any W is negative. This is expected to be
rare. (it only happens when a primitive intersects the near plane)

This allows us to eliminate the else statement, which is no longer
executed for accepted primitives with negative W, which are the only
primitives that needed the else branch.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer at amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11754>

---

 src/amd/llvm/ac_llvm_cull.c | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/src/amd/llvm/ac_llvm_cull.c b/src/amd/llvm/ac_llvm_cull.c
index e853d921aae..3ef1dab8311 100644
--- a/src/amd/llvm/ac_llvm_cull.c
+++ b/src/amd/llvm/ac_llvm_cull.c
@@ -47,7 +47,7 @@ struct ac_position_w_info {
     */
    LLVMValueRef w_accepted;
 
-   LLVMValueRef all_w_positive;
+   /* The bounding box culling doesn't work and should be skipped when this is true. */
    LLVMValueRef any_w_negative;
 };
 
@@ -69,7 +69,6 @@ static void ac_analyze_position_w(struct ac_llvm_context *ctx, LLVMValueRef pos[
       w->any_w_negative = LLVMBuildOr(builder, w->any_w_negative, neg_w, "");
       all_w_negative = LLVMBuildAnd(builder, all_w_negative, neg_w, "");
    }
-   w->all_w_positive = LLVMBuildNot(builder, w->any_w_negative, "");
    w->w_accepted = LLVMBuildNot(builder, all_w_negative, "");
 }
 
@@ -131,16 +130,10 @@ static void cull_bbox(struct ac_llvm_context *ctx, LLVMValueRef pos[3][4],
       return;
    }
 
-   /* Skip the culling if the primitive has already been rejected or
-    * if any W is negative. The bounding box culling doesn't work when
-    * W is negative.
-    */
-   LLVMValueRef cond = LLVMBuildAnd(builder, initially_accepted, w->all_w_positive, "");
-
-   ac_build_ifcc(ctx, cond, 10000000 /* does this matter? */);
+   ac_build_ifcc(ctx, initially_accepted, 10000000);
    {
       LLVMValueRef bbox_min[3], bbox_max[3];
-      LLVMValueRef accepted = initially_accepted;
+      LLVMValueRef accepted = ctx->i1true;
 
       /* Compute the primitive bounding box for easy culling. */
       for (unsigned chan = 0; chan < (cull_view_near_z || cull_view_far_z ? 3 : 2); chan++) {
@@ -203,14 +196,14 @@ static void cull_bbox(struct ac_llvm_context *ctx, LLVMValueRef pos[3][4],
          accepted = LLVMBuildAnd(builder, accepted, visible, "");
       }
 
+      /* Disregard the bounding box culling if any W is negative because the code
+       * doesn't work with that.
+       */
+      accepted = LLVMBuildOr(builder, accepted, w->any_w_negative, "");
+
       if (accept_func)
          accept_func(ctx, accepted, userdata);
    }
-   if (accept_func) {
-      /* If the caller provided a accept_func, call it in the else branch */
-      ac_build_else(ctx, 10000000);
-      accept_func(ctx, initially_accepted, userdata);
-   }
    ac_build_endif(ctx, 10000000);
 }