agx: implement live range splits of phis

this requires some special handling but closes the last soundness gap (I hope)
in our RA. with later patches in this series, we actually hit this (50+ tests on
the CTS even) so I can be sure this actually works ^^

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28483>

src/asahi/compiler/agx_register_allocate.c

@@ -305,12 +305,6 @@ assign_regs_by_copying(struct ra_ctx *rctx, unsigned npot_count, unsigned align,
 {
    assert(cls == RA_GPR);
 
-   /* XXX: This needs some special handling but so far it has been prohibitively
-    * difficult to hit the case
-    */
-   if (I->op == AGX_OPCODE_PHI)
-      unreachable("TODO");
-
    /* Expand the destination to the next power-of-two size. This simplifies
     * splitting and is accounted for by the demand calculation, so is legal.
     */
@@ -469,6 +463,8 @@ insert_copies_for_clobbered_killed(struct ra_ctx *rctx, unsigned reg,
    if (nr_vars == 0)
       return;
 
+   assert(I->op != AGX_OPCODE_PHI && "kill bit not set for phis");
+
    /* Sort by descending alignment so they are packed with natural alignment */
    util_qsort_r(vars, nr_vars, sizeof(vars[0]), sort_by_size, rctx->sizes);
 
@@ -512,6 +508,49 @@ insert_copies_for_clobbered_killed(struct ra_ctx *rctx, unsigned reg,
    assert(base <= reg + count && "no overflow");
 }
 
+/*
+ * When shuffling registers to assign a phi destination, we can't simply insert
+ * the required moves before the phi, since phis happen in parallel along the
+ * edge. Instead, there are two cases:
+ *
+ * 1. The source of the copy is the destination of a phi. Since we are
+ *    emitting shuffle code, there will be no more reads of that destination
+ *    with the old register. Since the phis all happen in parallel and writes
+ *    precede reads, there was no previous read of that destination either. So
+ *    the old destination is dead. Just replace the phi's destination with the
+ *    moves's destination instead.
+ *
+ * 2. Otherwise, the source of the copy is a live-in value, since it's
+ *    live when assigning phis at the start of a block but it is not a phi.
+ *    If we move in parallel with the phi, the phi will still read the correct
+ *    old register regardless and the destinations can't alias. So, insert a phi
+ *    to do the copy in parallel along the incoming edges.
+ */
+static void
+agx_emit_move_before_phi(agx_context *ctx, agx_block *block,
+                         struct agx_copy *copy)
+{
+   assert(!copy->dest_mem && !copy->src.memory && "no memory shuffles");
+
+   /* Look for the phi writing the destination */
+   agx_foreach_phi_in_block(block, phi) {
+      if (agx_is_equiv(phi->dest[0], copy->src) && !phi->dest[0].memory) {
+         phi->dest[0].value = copy->dest;
+         return;
+      }
+   }
+
+   /* There wasn't such a phi, so it's live-in. Insert a phi instead. */
+   agx_builder b = agx_init_builder(ctx, agx_before_block(block));
+
+   agx_instr *phi = agx_phi_to(&b, agx_register_like(copy->dest, copy->src),
+                               agx_num_predecessors(block));
+
+   agx_foreach_src(phi, s) {
+      phi->src[s] = copy->src;
+   }
+}
+
 static unsigned
 find_regs(struct ra_ctx *rctx, agx_instr *I, unsigned dest_idx, unsigned count,
           unsigned align)
@@ -549,10 +588,19 @@ find_regs(struct ra_ctx *rctx, agx_instr *I, unsigned dest_idx, unsigned count,
       insert_copies_for_clobbered_killed(rctx, reg, count, I, &copies,
                                          clobbered);
 
-      /* Insert the necessary copies */
-      agx_builder b = agx_init_builder(rctx->shader, agx_before_instr(I));
-      agx_emit_parallel_copies(
-         &b, copies.data, util_dynarray_num_elements(&copies, struct agx_copy));
+      /* Insert the necessary copies. Phis need special handling since we can't
+       * insert instructions before the phi.
+       */
+      if (I->op == AGX_OPCODE_PHI) {
+         util_dynarray_foreach(&copies, struct agx_copy, copy) {
+            agx_emit_move_before_phi(rctx->shader, rctx->block, copy);
+         }
+      } else {
+         agx_builder b = agx_init_builder(rctx->shader, agx_before_instr(I));
+         agx_emit_parallel_copies(
+            &b, copies.data,
+            util_dynarray_num_elements(&copies, struct agx_copy));
+      }
 
       /* assign_regs asserts this is cleared, so clear to be reassigned */
       BITSET_CLEAR_RANGE(rctx->used_regs[cls], reg, reg + count - 1);