nir: Let nir_eval_const_opcode() return a poison mask in case of UB.

This is unused by any callers currently, but will be useful for nir
algebraic pattern testing, and as a way to turn our comments in
nir_opcodes.py into actual C code.  For now, always returns false.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39076>

src/amd/vulkan/nir/radv_nir_opt_tid_function.c

@@ -181,10 +181,10 @@ constant_fold_scalar(nir_scalar s, unsigned invocation_id, nir_shader *shader, n
          srcs[i] = sources[i];
       nir_const_value dests[NIR_MAX_VEC_COMPONENTS];
       if (op_info->output_size == 0) {
-         nir_eval_const_opcode(alu->op, dests, 1, bit_size, srcs, exec_mode);
+         nir_eval_const_opcode(alu->op, dests, NULL, 1, bit_size, srcs, exec_mode);
          *dest = dests[0];
       } else {
-         nir_eval_const_opcode(alu->op, dests, s.def->num_components, bit_size, srcs, exec_mode);
+         nir_eval_const_opcode(alu->op, dests, NULL, s.def->num_components, bit_size, srcs, exec_mode);
          *dest = dests[s.comp];
       }
       return true;

src/compiler/nir/nir_constant_expressions.h

@@ -35,7 +35,14 @@
 extern "C" {
 #endif
 
-void nir_eval_const_opcode(nir_op op, nir_const_value *dest,
+/**
+ * Evaluates the NIR opcode for the given source constant values.
+ *
+ * If @poison is non-NULL, it will containe the nir_component_mask of output
+ * channels that invoked undefined behavior (define not used here, to avoid
+ * pulling in all of nir.h).
+ */
+void nir_eval_const_opcode(nir_op op, nir_const_value *dest, uint16_t *poison,
                            unsigned num_components, unsigned bit_size,
                            nir_const_value **src,
                            unsigned float_controls_execution_mode);

src/compiler/nir/nir_constant_expressions.py

@@ -475,6 +475,7 @@ struct ${type}${width}_vec {
       ## components and apply the constant expression one component
       ## at a time.
       for (unsigned _i = 0; _i < num_components; _i++) {
+         bool poison = false;
          ## For each per-component input, create a variable srcN that
          ## contains the value of the current (_i'th) component.
          % for j in range(op.num_inputs):
@@ -538,6 +539,9 @@ struct ${type}${width}_vec {
                }
             %endif
          % endif
+
+         if (poison)
+            poison_mask |= (1 << _i);
       }
    % else:
       ## In the non-per-component case, create a struct dst with
@@ -590,13 +594,15 @@ struct ${type}${width}_vec {
 </%def>
 
 % for name, op in sorted(opcodes.items()):
-static void
+static nir_component_mask_t
 evaluate_${name}(nir_const_value *_dst_val,
                  UNUSED unsigned num_components,
                  ${"UNUSED" if op_bit_sizes(op) is None else ""} unsigned bit_size,
                  UNUSED nir_const_value **_src,
                  UNUSED unsigned execution_mode)
 {
+   nir_component_mask_t poison_mask = 0;
+
    % if op_bit_sizes(op) is not None:
       switch (bit_size) {
       % for bit_size in op_bit_sizes(op):
@@ -612,24 +618,31 @@ evaluate_${name}(nir_const_value *_dst_val,
    % else:
       ${evaluate_op(op, 0, execution_mode)}
    % endif
+
+   return poison_mask;
 }
 % endfor
 
 void
-nir_eval_const_opcode(nir_op op, nir_const_value *dest,
+nir_eval_const_opcode(nir_op op, nir_const_value *dest, nir_component_mask_t *out_poison,
                       unsigned num_components, unsigned bit_width,
                       nir_const_value **src,
                       unsigned float_controls_execution_mode)
 {
+   nir_component_mask_t poison;
+
    switch (op) {
 % for name in sorted(opcodes.keys()):
    case nir_op_${name}:
-      evaluate_${name}(dest, num_components, bit_width, src, float_controls_execution_mode);
+      poison = evaluate_${name}(dest, num_components, bit_width, src, float_controls_execution_mode);
-      return;
+      break;
 % endfor
    default:
       UNREACHABLE("shouldn't get here");
    }
+
+   if (out_poison)
+      *out_poison = poison;
 }"""
 
 from mako.template import Template

src/compiler/nir/nir_loop_analyze.c

@@ -583,7 +583,7 @@ eval_const_unop(nir_op op, unsigned bit_size, nir_const_value src0,
    assert(nir_op_infos[op].num_inputs == 1);
    nir_const_value dest;
    nir_const_value *src[1] = { &src0 };
-   nir_eval_const_opcode(op, &dest, 1, bit_size, src, execution_mode);
+   nir_eval_const_opcode(op, &dest, NULL, 1, bit_size, src, execution_mode);
    return dest;
 }
 
@@ -595,7 +595,7 @@ eval_const_binop(nir_op op, unsigned bit_size,
    assert(nir_op_infos[op].num_inputs == 2);
    nir_const_value dest;
    nir_const_value *src[2] = { &src0, &src1 };
-   nir_eval_const_opcode(op, &dest, 1, bit_size, src, execution_mode);
+   nir_eval_const_opcode(op, &dest, NULL, 1, bit_size, src, execution_mode);
    return dest;
 }
 
@@ -687,7 +687,7 @@ try_eval_const_alu(nir_const_value *dest, nir_scalar alu_s, const nir_scalar *or
       }
    }
 
-   nir_eval_const_opcode(alu->op, dest, 1, bit_size, src_ptrs, execution_mode);
+   nir_eval_const_opcode(alu->op, dest, NULL, 1, bit_size, src_ptrs, execution_mode);
 
    return true;
 }
@@ -897,7 +897,7 @@ test_iterations(int32_t iter_int, nir_const_value step,
 
    /* Evaluate the loop exit condition */
    nir_const_value result;
-   nir_eval_const_opcode(cond_op, &result, 1, bit_size, src, execution_mode);
+   nir_eval_const_opcode(cond_op, &result, NULL, 1, bit_size, src, execution_mode);
 
    return invert_cond ? !result.b : result.b;
 }

src/compiler/nir/nir_opcodes.py

@@ -67,6 +67,13 @@ class Opcode(object):
       and the result will be equivalent to "dst = <expression>" for
       per-component instructions and "dst.x = dst.y = ... = <expression>"
       for non-per-component instructions.
+
+      The expression may set a poison = true flag to indicate that the
+      calculation invoked deferred undefined behavior (see
+      https://llvm.org/docs/UndefinedBehavior.html, which is similar to the
+      SPIRV 2.2.6 "Validity and Defined Behavior" definition.).  For
+      non-per-component opcodes, poison_mask must be set to the undefined
+      components, instead.
       """
       assert isinstance(name, str)
       assert isinstance(output_size, int)

src/compiler/nir/nir_opt_constant_folding.c

@@ -77,7 +77,7 @@ nir_try_constant_fold_alu(nir_builder *b, nir_alu_instr *alu)
    memset(dest, 0, sizeof(dest));
    for (unsigned i = 0; i < nir_op_infos[alu->op].num_inputs; ++i)
       srcs[i] = src[i];
-   nir_eval_const_opcode(alu->op, dest, alu->def.num_components,
+   nir_eval_const_opcode(alu->op, dest, NULL, alu->def.num_components,
                          bit_size, srcs,
                          b->shader->info.float_controls_execution_mode);
 

src/compiler/spirv/spirv_to_nir.c

@@ -2953,7 +2953,7 @@ vtn_handle_constant(struct vtn_builder *b, SpvOp opcode,
          nir_const_value *srcs[3] = {
             src[0], src[1], src[2],
          };
-         nir_eval_const_opcode(op, val->constant->values,
+         nir_eval_const_opcode(op, val->constant->values, NULL,
                                num_components, bit_size, srcs,
                                b->shader->info.float_controls_execution_mode);