microsoft/spirv_to_dxil: Fix discard semantics

Unlike in nir, discard is not a super return in DXIL. Therefore, we
will lower discard and terminate to demote + return.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17179>

src/microsoft/ci/spirv2dxil_reference.txt

@@ -645,14 +645,7 @@ Compilation failed
 
 Test:SpvParserCFGTest_EmitBody_IfSelection_TrueBranch_LoopBreak.spvasm:main|Fragment: Pass
 Test:SpvParserCFGTest_EmitBody_Kill_InsideIf.spvasm:main|Fragment: Pass
-Test:SpvParserCFGTest_EmitBody_Kill_InsideLoop.spvasm:main|Fragment: Fail
-DXIL: Function: main: error: Loop must have break. Use /Zi for source location.
-Validation failed.
-
-
-Failed to validate DXIL
-
-
+Test:SpvParserCFGTest_EmitBody_Kill_InsideLoop.spvasm:main|Fragment: Pass
 Test:SpvParserCFGTest_EmitBody_Kill_TopLevel.spvasm:main|Fragment: Pass
 Test:SpvParserCFGTest_EmitBody_Loop_BodyAlwaysBreaks.spvasm:main|Fragment: Pass
 Test:SpvParserCFGTest_EmitBody_Loop_BodyConditionallyBreaks_FromFalse.spvasm:main|Fragment: Pass

src/microsoft/compiler/dxil_nir.c

@@ -2032,3 +2032,44 @@ dxil_nir_fix_io_uint_type(nir_shader *s, uint64_t in_mask, uint64_t out_mask)
 
    return progress;
 }
+
+static bool
+lower_kill(struct nir_builder *builder, nir_instr *instr, void *_cb_data)
+{
+   if (instr->type != nir_instr_type_intrinsic)
+      return false;
+
+   nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
+
+   if (intr->intrinsic != nir_intrinsic_discard &&
+       intr->intrinsic != nir_intrinsic_terminate &&
+       intr->intrinsic != nir_intrinsic_discard_if &&
+       intr->intrinsic != nir_intrinsic_terminate_if)
+      return false;
+
+   builder->cursor = nir_instr_remove(instr);
+   if (intr->intrinsic == nir_intrinsic_discard ||
+       intr->intrinsic == nir_intrinsic_terminate) {
+      nir_demote(builder);
+   } else {
+      assert(intr->src[0].is_ssa);
+      nir_demote_if(builder, intr->src[0].ssa);
+   }
+
+   nir_jump(builder, nir_jump_return);
+
+   return true;
+}
+
+bool
+dxil_nir_lower_discard_and_terminate(nir_shader *s)
+{
+   if (s->info.stage != MESA_SHADER_FRAGMENT)
+      return false;
+
+   // This pass only works if all functions have been inlined
+   assert(exec_list_length(&s->functions) == 1);
+
+   return nir_shader_instructions_pass(s, lower_kill, nir_metadata_none,
+                                       NULL);
+}

src/microsoft/compiler/dxil_nir.h

@@ -75,6 +75,7 @@ bool dxil_nir_fixup_tess_level_for_domain(nir_shader *nir);
 bool dxil_nir_set_tcs_patches_in(nir_shader *nir, unsigned num_control_points);
 bool dxil_nir_lower_ubo_array_one_to_static(nir_shader *s);
 bool dxil_nir_fix_io_uint_type(nir_shader *s, uint64_t in_mask, uint64_t out_mask);
+bool dxil_nir_lower_discard_and_terminate(nir_shader* s);
 
 #ifdef __cplusplus
 }

src/microsoft/compiler/nir_to_dxil.c

@@ -4182,8 +4182,10 @@ emit_intrinsic(struct ntd_context *ctx, nir_intrinsic_instr *intr)
    case nir_intrinsic_load_scratch_dxil:
       return emit_load_scratch(ctx, intr);
    case nir_intrinsic_discard_if:
+   case nir_intrinsic_demote_if:
       return emit_discard_if(ctx, intr);
    case nir_intrinsic_discard:
+   case nir_intrinsic_demote:
       return emit_discard(ctx);
    case nir_intrinsic_emit_vertex:
       return emit_emit_vertex(ctx, intr);

src/microsoft/spirv_to_dxil/dxil_spirv_nir.c

@@ -681,6 +681,10 @@ dxil_spirv_nir_passes(nir_shader *nir,
                      .use_fragcoord_sysval = false,
                      .use_layer_id_sysval = true,
                  });
+
+      NIR_PASS_V(nir, dxil_nir_lower_discard_and_terminate);
+      NIR_PASS_V(nir, nir_lower_returns);
+
    }
 
    NIR_PASS_V(nir, nir_opt_deref);