panfrost/midgard: Check write-before-read in liveness analysis

If we write to an index before reading it, the old copy we're checking
liveness for isn't live in this block, even if it does get read later.
Fixes abnormally high register pressure in shaders with loops.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

diff --git a/src/panfrost/midgard/midgard_liveness.c b/src/panfrost/midgard/midgard_liveness.c
index a18d8b9f8ad45a34b7f00c6e711f4ff54ae2d216..899c8eaa00ffcf6accddf98ee6065bb53b94947c 100644 (file)
--- a/src/panfrost/midgard/midgard_liveness.c
+++ b/src/panfrost/midgard/midgard_liveness.c
@@ -60,11 +60,24 @@ is_live_after_successors(compiler_context *ctx, midgard_block *bl, int src)
 
                 succ->visited = true;
 
+                /* Within this block, check if it's overwritten first */
+                bool block_done = false;
+
                 mir_foreach_instr_in_block(succ, ins) {
                         if (midgard_is_live_in_instr(ins, src))
                                 return true;
+
+                        /* If written-before-use, we're gone */
+
+                        if (ins->ssa_args.dest == src && ins->type == TAG_LOAD_STORE_4 && ins->load_store.op == midgard_op_ld_int4 && ins->load_store.unknown == 0x1EEA) {
+                                block_done = true;
+                                break;
+                        }
                 }
 
+                if (block_done)
+                        continue;
+
                 /* ...and also, check *its* successors */
                 if (is_live_after_successors(ctx, succ, src))
                         return true;