nir: Add a writemask to store intrinsics.

Tessellation control shaders need to be careful when writing outputs.
Because multiple threads can concurrently write the same output
variables, we need to only write the exact components we were told.

Traditionally, for sub-vector writes, we've read the whole vector,
updated the temporary, and written the whole vector back.  This breaks
down with concurrent access.

This patch prepares the way for a solution by adding a writemask field
to store_var intrinsics, as well as the other store intrinsics.  It then
updates all produces to emit a writemask of "all channels enabled".  It
updates nir_lower_io to copy the writemask to output store intrinsics.

Finally, it updates nir_lower_vars_to_ssa to handle partial writemasks
by doing a read-modify-write cycle (which is safe, because local
variables are specific to a single thread).

This should have no functional change, since no one actually emits
partial writemasks yet.

v2: Make nir_validate momentarily assert that writemasks cover the
    complete value - we shouldn't have partial writemasks yet
    (requested by Jason Ekstrand).

v3: Fix accidental SSBO change that arose from merge conflicts.

v4: Don't try to handle writemasks in ir3_compiler_nir - my code
    for indirects was likely wrong, and TTN doesn't generate partial
    writemasks today anyway.  Change them to asserts as requested by
    Rob Clark.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com> [v3]

9 files changed, 54 insertions, 19 deletions

diff --git a/src/glsl/nir/glsl_to_nir.cpp b/src/glsl/nir/glsl_to_nir.cpp
index df8a18a..c0de7c4 100644
--- a/src/glsl/nir/glsl_to_nir.cpp
+++ b/src/glsl/nir/glsl_to_nir.cpp
@@ -1067,6 +1067,7 @@ nir_visitor::visit(ir_call *ir)
          nir_intrinsic_instr *store_instr =
             nir_intrinsic_instr_create(shader, nir_intrinsic_store_var);
          store_instr->num_components = ir->return_deref->type->vector_elements;
+         store_instr->const_index[0] = (1 << store_instr->num_components) - 1;
 
          store_instr->variables[0] =
             evaluate_deref(&store_instr->instr, ir->return_deref);
@@ -1165,6 +1166,7 @@ nir_visitor::visit(ir_assignment *ir)
    nir_intrinsic_instr *store =
       nir_intrinsic_instr_create(this->shader, nir_intrinsic_store_var);
    store->num_components = ir->lhs->type->vector_elements;
+   store->const_index[0] = (1 << store->num_components) - 1;
    nir_deref *store_deref = nir_copy_deref(store, &lhs_deref->deref);
    store->variables[0] = nir_deref_as_var(store_deref);
    store->src[0] = nir_src_for_ssa(src);
diff --git a/src/glsl/nir/nir_builder.h b/src/glsl/nir/nir_builder.h
index 332bb02..5883d86 100644
--- a/src/glsl/nir/nir_builder.h
+++ b/src/glsl/nir/nir_builder.h
@@ -310,13 +310,15 @@ nir_load_var(nir_builder *build, nir_variable *var)
 }
 
 static inline void
-nir_store_var(nir_builder *build, nir_variable *var, nir_ssa_def *value)
+nir_store_var(nir_builder *build, nir_variable *var, nir_ssa_def *value,
+              unsigned writemask)
 {
    const unsigned num_components = glsl_get_vector_elements(var->type);
 
    nir_intrinsic_instr *store =
       nir_intrinsic_instr_create(build->shader, nir_intrinsic_store_var);
    store->num_components = num_components;
+   store->const_index[0] = writemask;
    store->variables[0] = nir_deref_var_create(store, var);
    store->src[0] = nir_src_for_ssa(value);
    nir_builder_instr_insert(build, &store->instr);
diff --git a/src/glsl/nir/nir_intrinsics.h b/src/glsl/nir/nir_intrinsics.h
index ec9e845..5815dbe 100644
--- a/src/glsl/nir/nir_intrinsics.h
+++ b/src/glsl/nir/nir_intrinsics.h
@@ -43,7 +43,7 @@
 
 
 INTRINSIC(load_var, 0, ARR(), true, 0, 1, 0, NIR_INTRINSIC_CAN_ELIMINATE)
-INTRINSIC(store_var, 1, ARR(0), false, 0, 1, 0, 0)
+INTRINSIC(store_var, 1, ARR(0), false, 0, 1, 1, 0)
 INTRINSIC(copy_var, 0, ARR(), false, 0, 2, 0, 0)
 
 /*
@@ -302,10 +302,10 @@ LOAD(shared, 1, 1, NIR_INTRINSIC_CAN_ELIMINATE)
 #define STORE(name, srcs, indices, flags) \
    INTRINSIC(store_##name, srcs, ARR(0, 1, 1, 1), false, 0, 0, indices, flags)
 
-/* src[] = { value, offset }. const_index[] = { base } */
-STORE(output, 2, 1, 0)
-/* src[] = { value, vertex, offset }. const_index[] = { base } */
-STORE(per_vertex_output, 3, 1, 0)
+/* src[] = { value, offset }. const_index[] = { base, write_mask } */
+STORE(output, 2, 2, 0)
+/* src[] = { value, vertex, offset }. const_index[] = { base, write_mask } */
+STORE(per_vertex_output, 3, 2, 0)
 /* src[] = { value, block_index, offset }. const_index[] = { write_mask } */
 STORE(ssbo, 3, 1, 0)
 /* src[] = { value, offset }. const_index[] = { base, write_mask } */
diff --git a/src/glsl/nir/nir_lower_gs_intrinsics.c b/src/glsl/nir/nir_lower_gs_intrinsics.c
index e0d0678..1325459 100644
--- a/src/glsl/nir/nir_lower_gs_intrinsics.c
+++ b/src/glsl/nir/nir_lower_gs_intrinsics.c
@@ -99,7 +99,8 @@ rewrite_emit_vertex(nir_intrinsic_instr *intrin, struct state *state)
 
    /* Increment the vertex count by 1 */
    nir_store_var(b, state->vertex_count_var,
-                 nir_iadd(b, count, nir_imm_int(b, 1)));
+                 nir_iadd(b, count, nir_imm_int(b, 1)),
+                 0x1); /* .x */
 
    nir_instr_remove(&intrin->instr);
 
diff --git a/src/glsl/nir/nir_lower_io.c b/src/glsl/nir/nir_lower_io.c
index 3d646eb..a3565cc 100644
--- a/src/glsl/nir/nir_lower_io.c
+++ b/src/glsl/nir/nir_lower_io.c
@@ -261,6 +261,9 @@ nir_lower_io_block(nir_block *block, void *void_state)
          store->const_index[0] =
             intrin->variables[0]->var->data.driver_location;
 
+         /* Copy the writemask */
+         store->const_index[1] = intrin->const_index[0];
+
          if (per_vertex)
             store->src[1] = nir_src_for_ssa(vertex_index);
 
diff --git a/src/glsl/nir/nir_lower_locals_to_regs.c b/src/glsl/nir/nir_lower_locals_to_regs.c
index 17b53ca..3e21ac0 100644
--- a/src/glsl/nir/nir_lower_locals_to_regs.c
+++ b/src/glsl/nir/nir_lower_locals_to_regs.c
@@ -243,7 +243,7 @@ lower_locals_to_regs_block(nir_block *block, void *void_state)
 
          nir_alu_instr *mov = nir_alu_instr_create(state->shader, nir_op_imov);
          nir_src_copy(&mov->src[0].src, &intrin->src[0], mov);
-         mov->dest.write_mask = (1 << intrin->num_components) - 1;
+         mov->dest.write_mask = intrin->const_index[0];
          mov->dest.dest.is_ssa = false;
          mov->dest.dest.reg.reg = reg_src.reg.reg;
          mov->dest.dest.reg.base_offset = reg_src.reg.base_offset;
diff --git a/src/glsl/nir/nir_lower_var_copies.c b/src/glsl/nir/nir_lower_var_copies.c
index 98c107a..a9017de 100644
--- a/src/glsl/nir/nir_lower_var_copies.c
+++ b/src/glsl/nir/nir_lower_var_copies.c
@@ -128,6 +128,7 @@ emit_copy_load_store(nir_intrinsic_instr *copy_instr,
       nir_intrinsic_instr *store =
          nir_intrinsic_instr_create(mem_ctx, nir_intrinsic_store_var);
       store->num_components = num_components;
+      store->const_index[0] = (1 << num_components) - 1;
       store->variables[0] = nir_deref_as_var(nir_copy_deref(store, &dest_head->deref));
 
       store->src[0].is_ssa = true;
diff --git a/src/glsl/nir/nir_lower_vars_to_ssa.c b/src/glsl/nir/nir_lower_vars_to_ssa.c
index e670dbd..3ec0e1d 100644
--- a/src/glsl/nir/nir_lower_vars_to_ssa.c
+++ b/src/glsl/nir/nir_lower_vars_to_ssa.c
@@ -26,6 +26,7 @@
  */
 
 #include "nir.h"
+#include "nir_builder.h"
 #include "nir_vla.h"
 
 
@@ -590,6 +591,9 @@ add_phi_sources(nir_block *block, nir_block *pred,
 static bool
 rename_variables_block(nir_block *block, struct lower_variables_state *state)
 {
+   nir_builder b;
+   nir_builder_init(&b, state->impl);
+
    nir_foreach_instr_safe(block, instr) {
       if (instr->type == nir_instr_type_phi) {
          nir_phi_instr *phi = nir_instr_as_phi(instr);
@@ -675,20 +679,40 @@ rename_variables_block(nir_block *block, struct lower_variables_state *state)
 
             assert(intrin->src[0].is_ssa);
 
-            nir_alu_instr *mov = nir_alu_instr_create(state->shader,
-                                                      nir_op_imov);
-            mov->src[0].src.is_ssa = true;
-            mov->src[0].src.ssa = intrin->src[0].ssa;
-            for (unsigned i = intrin->num_components; i < 4; i++)
-               mov->src[0].swizzle[i] = 0;
+            nir_ssa_def *new_def;
+            b.cursor = nir_before_instr(&intrin->instr);
 
-            mov->dest.write_mask = (1 << intrin->num_components) - 1;
-            nir_ssa_dest_init(&mov->instr, &mov->dest.dest,
-                              intrin->num_components, NULL);
+            if (intrin->const_index[0] == (1 << intrin->num_components) - 1) {
+               /* Whole variable store - just copy the source.  Note that
+                * intrin->num_components and intrin->src[0].ssa->num_components
+                * may differ.
+                */
+               unsigned swiz[4];
+               for (unsigned i = 0; i < 4; i++)
+                  swiz[i] = i < intrin->num_components ? i : 0;
+
+               new_def = nir_swizzle(&b, intrin->src[0].ssa, swiz,
+                                     intrin->num_components, false);
+            } else {
+               nir_ssa_def *old_def = get_ssa_def_for_block(node, block, state);
+               /* For writemasked store_var intrinsics, we combine the newly
+                * written values with the existing contents of unwritten
+                * channels, creating a new SSA value for the whole vector.
+                */
+               nir_ssa_def *srcs[4];
+               for (unsigned i = 0; i < intrin->num_components; i++) {
+                  if (intrin->const_index[0] & (1 << i)) {
+                     srcs[i] = nir_channel(&b, intrin->src[0].ssa, i);
+                  } else {
+                     srcs[i] = nir_channel(&b, old_def, i);
+                  }
+               }
+               new_def = nir_vec(&b, srcs, intrin->num_components);
+            }
 
-            nir_instr_insert_before(&intrin->instr, &mov->instr);
+            assert(new_def->num_components == intrin->num_components);
 
-            def_stack_push(node, &mov->dest.dest.ssa, state);
+            def_stack_push(node, new_def, state);
 
             /* We'll wait to remove the instruction until the next pass
              * where we pop the node we just pushed back off the stack.
diff --git a/src/glsl/nir/nir_validate.c b/src/glsl/nir/nir_validate.c
index 06879d6..89cf0b8 100644
--- a/src/glsl/nir/nir_validate.c
+++ b/src/glsl/nir/nir_validate.c
@@ -417,6 +417,8 @@ validate_intrinsic_instr(nir_intrinsic_instr *instr, validate_state *state)
       assert(instr->variables[0]->var->data.mode != nir_var_shader_in &&
              instr->variables[0]->var->data.mode != nir_var_uniform &&
              instr->variables[0]->var->data.mode != nir_var_shader_storage);
+      /* Currently, writemasks must cover the entire value */
+      assert(instr->const_index[0] == (1 << instr->num_components) - 1);
       break;
    }
    case nir_intrinsic_copy_var: