nir: Delete the nir_function_impl::start_block field.

It's simply the first nir_cf_node in the nir_function_impl::body list,
which is easy enough to access - we don't to store a pointer to it
explicitly.  Removing it means we don't need to maintain the pointer
when, say, splitting the start block when modifying control flow.

Thanks to Connor Abbott for suggesting this.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/nir/nir.c

@@ -262,7 +262,6 @@ nir_function_impl_create(nir_function_overload *overload)
    nir_block *end_block = nir_block_create(mem_ctx);
    start_block->cf_node.parent = &impl->cf_node;
    end_block->cf_node.parent = &impl->cf_node;
-   impl->start_block = start_block;
    impl->end_block = end_block;
 
    exec_list_push_tail(&impl->body, &start_block->cf_node.node);

src/glsl/nir/nir.h

@@ -1309,7 +1309,7 @@ typedef struct {
 
    struct exec_list body; /** < list of nir_cf_node */
 
-   nir_block *start_block, *end_block;
+   nir_block *end_block;
 
    /** list for all local variables in the function */
    struct exec_list locals;
@@ -1336,6 +1336,12 @@ typedef struct {
    nir_metadata valid_metadata;
 } nir_function_impl;
 
+static inline nir_block *
+nir_start_block(nir_function_impl *impl)
+{
+   return (nir_block *) exec_list_get_head(&impl->body);
+}
+
 static inline nir_cf_node *
 nir_cf_node_next(nir_cf_node *node)
 {

src/glsl/nir/nir_dominance.c

@@ -42,7 +42,7 @@ static bool
 init_block_cb(nir_block *block, void *_state)
 {
    dom_state *state = (dom_state *) _state;
-   if (block == state->impl->start_block)
+   if (block == nir_start_block(state->impl))
       block->imm_dom = block;
    else
       block->imm_dom = NULL;
@@ -78,7 +78,7 @@ static bool
 calc_dominance_cb(nir_block *block, void *_state)
 {
    dom_state *state = (dom_state *) _state;
-   if (block == state->impl->start_block)
+   if (block == nir_start_block(state->impl))
       return true;
 
    nir_block *new_idom = NULL;
@@ -209,12 +209,13 @@ nir_calc_dominance_impl(nir_function_impl *impl)
 
    nir_foreach_block(impl, calc_dom_frontier_cb, &state);
 
-   impl->start_block->imm_dom = NULL;
+   nir_block *start_block = nir_start_block(impl);
+   start_block->imm_dom = NULL;
 
    calc_dom_children(impl);
 
    unsigned dfs_index = 0;
-   calc_dfs_indicies(impl->start_block, &dfs_index);
+   calc_dfs_indicies(start_block, &dfs_index);
 }
 
 void

src/glsl/nir/nir_lower_vars_to_ssa.c

@@ -935,7 +935,7 @@ nir_lower_vars_to_ssa_impl(nir_function_impl *impl)
    nir_foreach_block(impl, register_variable_uses_block, &state);
 
    insert_phi_nodes(&state);
-   rename_variables_block(impl->start_block, &state);
+   rename_variables_block(nir_start_block(impl), &state);
 
    nir_metadata_preserve(impl, nir_metadata_block_index |
                                nir_metadata_dominance);

src/glsl/nir/nir_opt_gcm.c

@@ -256,7 +256,7 @@ gcm_schedule_early_instr(nir_instr *instr, struct gcm_state *state)
    /* Start with the instruction at the top.  As we iterate over the
     * sources, it will get moved down as needed.
     */
-   instr->block = state->impl->start_block;
+   instr->block = nir_start_block(state->impl);
    state->instr = instr;
 
    nir_foreach_src(instr, gcm_schedule_early_src, state);

src/glsl/nir/nir_to_ssa.c

@@ -516,7 +516,7 @@ nir_convert_to_ssa_impl(nir_function_impl *impl)
    rewrite_state state;
    init_rewrite_state(impl, &state);
 
-   rewrite_block(impl->start_block, &state);
+   rewrite_block(nir_start_block(impl), &state);
 
    remove_unused_regs(impl, &state);