From 4bd106b089aeb17c075d5305c00384c11c129005 Mon Sep 17 00:00:00 2001
From: alan-baker <33432579+alan-baker@users.noreply.github.com>
Date: Wed, 3 Apr 2019 10:30:12 -0400
Subject: [PATCH] Handle dead infinite loops in DCE (#2471)

Fixes #2456

* When eliminating a structured construct that has an unreachable merge,
replace that unreachable terminator with an appropriate return
* New tests
---
 source/opt/aggressive_dead_code_elim_pass.cpp |  20 +++
 test/opt/aggressive_dead_code_elim_test.cpp   | 128 ++++++++++++++++++
 2 files changed, 148 insertions(+)

diff --git a/source/opt/aggressive_dead_code_elim_pass.cpp b/source/opt/aggressive_dead_code_elim_pass.cpp
index 82d749905..b5ce4cc7d 100644
--- a/source/opt/aggressive_dead_code_elim_pass.cpp
+++ b/source/opt/aggressive_dead_code_elim_pass.cpp
@@ -513,6 +513,26 @@ bool AggressiveDCEPass::AggressiveDCE(Function* func) {
       AddBranch(mergeBlockId, *bi);
       for (++bi; (*bi)->id() != mergeBlockId; ++bi) {
       }
+
+      auto merge_terminator = (*bi)->terminator();
+      if (merge_terminator->opcode() == SpvOpUnreachable) {
+        // The merge was unreachable. This is undefined behaviour so just
+        // return (or return an undef). Then mark the new return as live.
+        auto func_ret_type_inst = get_def_use_mgr()->GetDef(func->type_id());
+        if (func_ret_type_inst->opcode() == SpvOpTypeVoid) {
+          merge_terminator->SetOpcode(SpvOpReturn);
+        } else {
+          // Find an undef for the return value and make sure it gets kept by
+          // the pass.
+          auto undef_id = Type2Undef(func->type_id());
+          auto undef = get_def_use_mgr()->GetDef(undef_id);
+          live_insts_.Set(undef->unique_id());
+          merge_terminator->SetOpcode(SpvOpReturnValue);
+          merge_terminator->SetInOperands({{SPV_OPERAND_TYPE_ID, {undef_id}}});
+          get_def_use_mgr()->AnalyzeInstUse(merge_terminator);
+        }
+        live_insts_.Set(merge_terminator->unique_id());
+      }
     } else {
       ++bi;
     }
diff --git a/test/opt/aggressive_dead_code_elim_test.cpp b/test/opt/aggressive_dead_code_elim_test.cpp
index 8b9b40a04..a883fb2b3 100644
--- a/test/opt/aggressive_dead_code_elim_test.cpp
+++ b/test/opt/aggressive_dead_code_elim_test.cpp
@@ -6210,6 +6210,134 @@ TEST_F(AggressiveDCETest, Dead) {
   SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
   SinglePassRunAndMatch<AggressiveDCEPass>(test, true);
 }
+
+TEST_F(AggressiveDCETest, DeadInfiniteLoop) {
+  const std::string test = R"(
+; CHECK: OpSwitch {{%\w+}} {{%\w+}} {{\w+}} {{%\w+}} {{\w+}} [[block:%\w+]]
+; CHECK: [[block]] = OpLabel
+; CHECK-NEXT: OpBranch [[block:%\w+]]
+; CHECK: [[block]] = OpLabel
+; CHECK-NEXT: OpBranch [[block:%\w+]]
+; CHECK: [[block]] = OpLabel
+; CHECK-NEXT: OpReturn
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main"
+               OpExecutionMode %2 OriginUpperLeft
+          %6 = OpTypeVoid
+          %7 = OpTypeFunction %6
+          %8 = OpTypeFloat 32
+          %9 = OpTypeVector %8 3
+         %10 = OpTypeFunction %9
+         %11 = OpConstant %8 1
+         %12 = OpConstantComposite %9 %11 %11 %11
+         %13 = OpTypeInt 32 1
+         %32 = OpUndef %13
+          %2 = OpFunction %6 None %7
+         %33 = OpLabel
+               OpBranch %34
+         %34 = OpLabel
+               OpLoopMerge %35 %36 None
+               OpBranch %37
+         %37 = OpLabel
+         %38 = OpFunctionCall %9 %39
+               OpSelectionMerge %40 None
+               OpSwitch %32 %40 14 %41 58 %42
+         %42 = OpLabel
+               OpBranch %43
+         %43 = OpLabel
+               OpLoopMerge %44 %45 None
+               OpBranch %45
+         %45 = OpLabel
+               OpBranch %43
+         %44 = OpLabel
+               OpUnreachable
+         %41 = OpLabel
+               OpBranch %36
+         %40 = OpLabel
+               OpBranch %36
+         %36 = OpLabel
+               OpBranch %34
+         %35 = OpLabel
+               OpReturn
+               OpFunctionEnd
+         %39 = OpFunction %9 None %10
+         %46 = OpLabel
+               OpReturnValue %12
+               OpFunctionEnd
+)";
+
+  SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
+  SinglePassRunAndMatch<AggressiveDCEPass>(test, true);
+}
+
+TEST_F(AggressiveDCETest, DeadInfiniteLoopReturnValue) {
+  const std::string test = R"(
+; CHECK: [[vec3:%\w+]] = OpTypeVector
+; CHECK: [[undef:%\w+]] = OpUndef [[vec3]]
+; CHECK: OpSwitch {{%\w+}} {{%\w+}} {{\w+}} {{%\w+}} {{\w+}} [[block:%\w+]]
+; CHECK: [[block]] = OpLabel
+; CHECK-NEXT: OpBranch [[block:%\w+]]
+; CHECK: [[block]] = OpLabel
+; CHECK-NEXT: OpBranch [[block:%\w+]]
+; CHECK: [[block]] = OpLabel
+; CHECK-NEXT: OpReturnValue [[undef]]
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main"
+               OpExecutionMode %2 OriginUpperLeft
+          %6 = OpTypeVoid
+          %7 = OpTypeFunction %6
+          %8 = OpTypeFloat 32
+          %9 = OpTypeVector %8 3
+         %10 = OpTypeFunction %9
+         %11 = OpConstant %8 1
+         %12 = OpConstantComposite %9 %11 %11 %11
+         %13 = OpTypeInt 32 1
+         %32 = OpUndef %13
+          %2 = OpFunction %6 None %7
+      %entry = OpLabel
+       %call = OpFunctionCall %9 %func
+               OpReturn
+               OpFunctionEnd
+       %func = OpFunction %9 None %10
+         %33 = OpLabel
+               OpBranch %34
+         %34 = OpLabel
+               OpLoopMerge %35 %36 None
+               OpBranch %37
+         %37 = OpLabel
+         %38 = OpFunctionCall %9 %39
+               OpSelectionMerge %40 None
+               OpSwitch %32 %40 14 %41 58 %42
+         %42 = OpLabel
+               OpBranch %43
+         %43 = OpLabel
+               OpLoopMerge %44 %45 None
+               OpBranch %45
+         %45 = OpLabel
+               OpBranch %43
+         %44 = OpLabel
+               OpUnreachable
+         %41 = OpLabel
+               OpBranch %36
+         %40 = OpLabel
+               OpBranch %36
+         %36 = OpLabel
+               OpBranch %34
+         %35 = OpLabel
+               OpReturnValue %12
+               OpFunctionEnd
+         %39 = OpFunction %9 None %10
+         %46 = OpLabel
+               OpReturnValue %12
+               OpFunctionEnd
+)";
+
+  SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
+  SinglePassRunAndMatch<AggressiveDCEPass>(test, true);
+}
+
 // TODO(greg-lunarg): Add tests to verify handling of these cases:
 //
 //    Check that logical addressing required