[Mesa-dev] [PATCH 2/2] nir/spirv: Use a nop intrinsic for tagging the ends of blocks

[Jason Ekstrand]

Previously, we were saving off the last nir_block in a vtn_block before
moving on so that we could find the nir_block again when it came time to
handle phi sources.  Unfortunately, NIR's control flow modification code is
inconsistent when it comes to how it splits blocks so the block pointer we
saved off may point to a block somewhere else in the shader by the time we
get around to handling phi sources.  In order to get around this, we insert
a nop instruction and use that as the logical end of our block.  Since the
control flow manipulation code respects instructions, the nop will keeps
its place like any other instruction and we can easily find the end of our
block when we need it.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
---
 src/compiler/spirv/vtn_cfg.c     | 6 ++++--
 src/compiler/spirv/vtn_private.h | 4 ++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/compiler/spirv/vtn_cfg.c b/src/compiler/spirv/vtn_cfg.c
index d9096f4..599ed69 100644
--- a/src/compiler/spirv/vtn_cfg.c
+++ b/src/compiler/spirv/vtn_cfg.c
@@ -518,7 +518,7 @@ vtn_handle_phi_second_pass(struct vtn_builder *b, SpvOp opcode,
       struct vtn_block *pred =
          vtn_value(b, w[i + 1], vtn_value_type_block)->block;
 
-      b->nb.cursor = nir_after_block_before_jump(pred->end_block);
+      b->nb.cursor = nir_after_instr(&pred->end_nop->instr);
 
       vtn_local_store(b, src, nir_deref_var_create(b, phi_var));
    }
@@ -576,7 +576,9 @@ vtn_emit_cf_list(struct vtn_builder *b, struct list_head *cf_list,
 
          vtn_foreach_instruction(b, block_start, block_end, handler);
 
-         block->end_block = nir_cursor_current_block(b->nb.cursor);
+         block->end_nop = nir_intrinsic_instr_create(b->nb.shader,
+                                                     nir_intrinsic_nop);
+         nir_builder_instr_insert(&b->nb, &block->end_nop->instr);
 
          if ((*block->branch & SpvOpCodeMask) == SpvOpReturnValue) {
             struct vtn_ssa_value *src = vtn_ssa_value(b, block->branch[1]);
diff --git a/src/compiler/spirv/vtn_private.h b/src/compiler/spirv/vtn_private.h
index 7f5444e..6f34f09 100644
--- a/src/compiler/spirv/vtn_private.h
+++ b/src/compiler/spirv/vtn_private.h
@@ -149,8 +149,8 @@ struct vtn_block {
    /** Points to the switch case started by this block (if any) */
    struct vtn_case *switch_case;
 
-   /** The last block in this SPIR-V block. */
-   nir_block *end_block;
+   /** Every block ends in a nop intrinsic so that we can find it again */
+   nir_intrinsic_instr *end_nop;
 };
 
 struct vtn_function {
-- 
2.5.0.400.gff86faf