[SimplifyCFG] Replace unreachable switch lookup table holes with poison

Author: DaMatrix

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6281,8 +6281,26 @@ SwitchLookupTable::SwitchLookupTable(
     uint64_t Idx = (CaseVal->getValue() - Offset->getValue()).getLimitedValue();
     TableContents[Idx] = CaseRes;
 
-    if (CaseRes != SingleValue)
-      SingleValue = nullptr;
+    if (SingleValue && CaseRes != SingleValue) {
+      if (isa<UndefValue>(SingleValue)) {
+        // All of the switch cases until now have returned undef/poison.
+        // If this case returns a defined value, ignore the previous
+        // undefs/poisons and use this case's result as the single constant
+        // value.
+        bool IgnorePreviousUndef = !isa<UndefValue>(CaseRes);
+
+        // If this case returns undef, but all of the previous cases have
+        // returned poison, promote the single constant value from poison to
+        // undef.
+        bool PromotePoisonToUndef = isa<UndefValue>(CaseRes) &&
+                                    !isa<PoisonValue>(CaseRes) &&
+                                    isa<PoisonValue>(SingleValue);
+
+        if (IgnorePreviousUndef || PromotePoisonToUndef)
+          SingleValue = CaseRes;
+      } else if (!isa<UndefValue>(CaseRes))
+        SingleValue = nullptr;
+    }
   }
 
   // Fill in any holes in the table with the default result.
@@ -6295,7 +6313,10 @@ SwitchLookupTable::SwitchLookupTable(
         TableContents[I] = DefaultValue;
     }
 
-    if (DefaultValue != SingleValue)
+    // If the default value is undef, all the holes are undef.
+    bool DefaultValueIsUndef = isa<UndefValue>(DefaultValue);
+
+    if (DefaultValue != SingleValue && !DefaultValueIsUndef)
       SingleValue = nullptr;
   }
 
@@ -6319,6 +6340,16 @@ SwitchLookupTable::SwitchLookupTable(
     // Check if there is the same distance between two consecutive values.
     for (uint64_t I = 0; I < TableSize; ++I) {
       ConstantInt *ConstVal = dyn_cast<ConstantInt>(TableContents[I]);
+
+      if (!ConstVal && TableContents[I] == DefaultValue) {
+        // This is an undef, but is also (probably) a lookup table hole. To
+        // prevent any regressions from before we switched to using poison as
+        // the default value, holes will fall back to using the first value.
+        // This can be removed once we add proper handling for undefs in lookup
+        // tables.
+        ConstVal = dyn_cast<ConstantInt>(Values[0].second);
+      }
+
       if (!ConstVal) {
         // This is an undef. We could deal with it, but undefs in lookup tables
         // are very seldom. It's probably not worth the additional complexity.
@@ -6752,16 +6783,16 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
 
   // If the table has holes but the default destination doesn't produce any
   // constant results, the lookup table entries corresponding to the holes will
-  // contain undefined values.
-  bool AllHolesAreUndefined = TableHasHoles && !HasDefaultResults;
+  // contain poison.
+  bool AllHolesArePoison = TableHasHoles && !HasDefaultResults;
 
   // If the default destination doesn't produce a constant result but is still
   // reachable, and the lookup table has holes, we need to use a mask to
   // determine if the current index should load from the lookup table or jump
   // to the default case.
   // The mask is unnecessary if the table has holes but the default destination
   // is unreachable, as in that case the holes must also be unreachable.
-  bool NeedMask = AllHolesAreUndefined && DefaultIsReachable;
+  bool NeedMask = AllHolesArePoison && DefaultIsReachable;
   if (NeedMask) {
     // As an extra penalty for the validity test we require more cases.
     if (SI->getNumCases() < 4) // FIXME: Find best threshold value (benchmark).
@@ -6906,9 +6937,11 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
   for (PHINode *PHI : PHIs) {
     const ResultListTy &ResultList = ResultLists[PHI];
 
+    Type *ResultType = ResultList.begin()->second->getType();
+
     // Use any value to fill the lookup table holes.
     Constant *DV =
-        AllHolesAreUndefined ? ResultLists[PHI][0].second : DefaultResults[PHI];
+        AllHolesArePoison ? PoisonValue::get(ResultType) : DefaultResults[PHI];
     StringRef FuncName = Fn->getName();
     SwitchLookupTable Table(Mod, TableSize, TableIndexOffset, ResultList, DV,
                             DL, FuncName);

llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll

@@ -34,18 +34,12 @@ target triple = "x86_64-unknown-linux-gnu"
 ; CHECK: @switch.table.unreachable_case = private unnamed_addr constant [9 x i32] [i32 0, i32 0, i32 0, i32 2, i32 -1, i32 1, i32 1, i32 1, i32 1], align 4
 ; CHECK: @switch.table.unreachable_default = private unnamed_addr constant [4 x i32] [i32 42, i32 52, i32 1, i32 2], align 4
 ; CHECK: @switch.table.nodefaultnoholes = private unnamed_addr constant [4 x i32] [i32 55, i32 123, i32 0, i32 -1], align 4
-; CHECK: @switch.table.nodefaultwithholes = private unnamed_addr constant [6 x i32] [i32 55, i32 123, i32 0, i32 -1, i32 55, i32 -1], align 4
+; CHECK: @switch.table.nodefaultwithholes = private unnamed_addr constant [6 x i32] [i32 55, i32 123, i32 0, i32 -1, i32 poison, i32 -1], align 4
 ; CHECK: @switch.table.threecases = private unnamed_addr constant [3 x i32] [i32 10, i32 7, i32 5], align 4
-; CHECK: @switch.table.covered_switch_with_bit_tests = private unnamed_addr constant [8 x i32] [i32 2, i32 2, i32 2, i32 2, i32 2, i32 2, i32 1, i32 1], align 4
+; CHECK: @switch.table.covered_switch_with_bit_tests = private unnamed_addr constant [8 x i32] [i32 2, i32 2, i32 poison, i32 poison, i32 poison, i32 poison, i32 1, i32 1], align 4
 ; CHECK: @switch.table.signed_overflow1 = private unnamed_addr constant [4 x i32] [i32 3333, i32 4444, i32 1111, i32 2222], align 4
-; CHECK: @switch.table.signed_overflow2 = private unnamed_addr constant [4 x i32] [i32 3333, i32 4444, i32 2222, i32 2222], align 4
-; CHECK: @switch.table.constant_hole_unreachable_default_firstundef = private unnamed_addr constant [5 x i32] [i32 undef, i32 undef, i32 1, i32 1, i32 1], align 4
-; CHECK: @switch.table.constant_hole_unreachable_default_lastundef = private unnamed_addr constant [5 x i32] [i32 1, i32 1, i32 1, i32 1, i32 undef], align 4
-; CHECK: @switch.table.constant_hole_unreachable_default_firstpoison = private unnamed_addr constant [5 x i32] [i32 poison, i32 poison, i32 1, i32 1, i32 1], align 4
-; CHECK: @switch.table.constant_hole_unreachable_default_lastpoison = private unnamed_addr constant [5 x i32] [i32 1, i32 1, i32 1, i32 1, i32 poison], align 4
-; CHECK: @switch.table.constant_hole_unreachable_default_undef_poison = private unnamed_addr constant [5 x i32] [i32 undef, i32 undef, i32 poison, i32 poison, i32 poison], align 4
-; CHECK: @switch.table.constant_hole_unreachable_default_poison_undef = private unnamed_addr constant [5 x i32] [i32 poison, i32 poison, i32 poison, i32 poison, i32 undef], align 4
-; CHECK: @switch.table.linearmap_hole_unreachable_default = private unnamed_addr constant [5 x i32] [i32 1, i32 1, i32 5, i32 7, i32 9], align 4
+; CHECK: @switch.table.signed_overflow2 = private unnamed_addr constant [4 x i32] [i32 3333, i32 4444, i32 poison, i32 2222], align 4
+; CHECK: @switch.table.linearmap_hole_unreachable_default = private unnamed_addr constant [5 x i32] [i32 1, i32 poison, i32 5, i32 7, i32 9], align 4
 ;.
 define i32 @f(i32 %c) {
 ; CHECK-LABEL: @f(
@@ -2187,9 +2181,7 @@ return:
 define i32 @constant_hole_unreachable_default_firstundef(i32 %x) {
 ; CHECK-LABEL: @constant_hole_unreachable_default_firstundef(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [5 x i32], ptr @switch.table.constant_hole_unreachable_default_firstundef, i32 0, i32 [[X:%.*]]
-; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
-; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   switch i32 %x, label %sw.default [
@@ -2213,9 +2205,7 @@ return:
 define i32 @constant_hole_unreachable_default_lastundef(i32 %x) {
 ; CHECK-LABEL: @constant_hole_unreachable_default_lastundef(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [5 x i32], ptr @switch.table.constant_hole_unreachable_default_lastundef, i32 0, i32 [[X:%.*]]
-; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
-; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   switch i32 %x, label %sw.default [
@@ -2239,9 +2229,7 @@ return:
 define i32 @constant_hole_unreachable_default_firstpoison(i32 %x) {
 ; CHECK-LABEL: @constant_hole_unreachable_default_firstpoison(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [5 x i32], ptr @switch.table.constant_hole_unreachable_default_firstpoison, i32 0, i32 [[X:%.*]]
-; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
-; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   switch i32 %x, label %sw.default [
@@ -2265,9 +2253,7 @@ return:
 define i32 @constant_hole_unreachable_default_lastpoison(i32 %x) {
 ; CHECK-LABEL: @constant_hole_unreachable_default_lastpoison(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [5 x i32], ptr @switch.table.constant_hole_unreachable_default_lastpoison, i32 0, i32 [[X:%.*]]
-; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
-; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   switch i32 %x, label %sw.default [
@@ -2289,9 +2275,7 @@ return:
 define i32 @constant_hole_unreachable_default_undef_poison(i32 %x) {
 ; CHECK-LABEL: @constant_hole_unreachable_default_undef_poison(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [5 x i32], ptr @switch.table.constant_hole_unreachable_default_undef_poison, i32 0, i32 [[X:%.*]]
-; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
-; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
+; CHECK-NEXT:    ret i32 undef
 ;
 entry:
   switch i32 %x, label %sw.default [
@@ -2313,9 +2297,7 @@ return:
 define i32 @constant_hole_unreachable_default_poison_undef(i32 %x) {
 ; CHECK-LABEL: @constant_hole_unreachable_default_poison_undef(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [5 x i32], ptr @switch.table.constant_hole_unreachable_default_poison_undef, i32 0, i32 [[X:%.*]]
-; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i32, ptr [[SWITCH_GEP]], align 4
-; CHECK-NEXT:    ret i32 [[SWITCH_LOAD]]
+; CHECK-NEXT:    ret i32 undef
 ;
 entry:
   switch i32 %x, label %sw.default [

llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table_big.ll

@@ -7,11 +7,11 @@ target triple = "i386-pc-linux-gnu"
 ;.
 ; CHECK: @switch.table.reachable_default_dense_0to31 = private unnamed_addr constant [32 x i32] [i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1], align 4
 ; CHECK: @switch.table.unreachable_default_dense_0to31 = private unnamed_addr constant [32 x i32] [i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1], align 4
-; CHECK: @switch.table.reachable_default_holes_0to31 = private unnamed_addr constant [32 x i32] [i32 0, i32 7, i32 6, i32 0, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 0, i32 2, i32 1, i32 0, i32 7, i32 0, i32 5, i32 4, i32 3, i32 2, i32 0, i32 0, i32 7, i32 6, i32 5, i32 0, i32 3, i32 2, i32 1], align 4
-; CHECK: @switch.table.unreachable_default_holes_0to31 = private unnamed_addr constant [32 x i32] [i32 0, i32 7, i32 6, i32 0, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 0, i32 2, i32 1, i32 0, i32 7, i32 0, i32 5, i32 4, i32 3, i32 2, i32 0, i32 0, i32 7, i32 6, i32 5, i32 0, i32 3, i32 2, i32 1], align 4
+; CHECK: @switch.table.reachable_default_holes_0to31 = private unnamed_addr constant [32 x i32] [i32 0, i32 7, i32 6, i32 poison, i32 4, i32 3, i32 2, i32 1, i32 poison, i32 7, i32 6, i32 5, i32 4, i32 poison, i32 2, i32 1, i32 0, i32 7, i32 poison, i32 5, i32 4, i32 3, i32 2, i32 poison, i32 0, i32 7, i32 6, i32 5, i32 poison, i32 3, i32 2, i32 1], align 4
+; CHECK: @switch.table.unreachable_default_holes_0to31 = private unnamed_addr constant [32 x i32] [i32 0, i32 7, i32 6, i32 poison, i32 4, i32 3, i32 2, i32 1, i32 poison, i32 7, i32 6, i32 5, i32 4, i32 poison, i32 2, i32 1, i32 0, i32 7, i32 poison, i32 5, i32 4, i32 3, i32 2, i32 poison, i32 0, i32 7, i32 6, i32 5, i32 poison, i32 3, i32 2, i32 1], align 4
 ; CHECK: @switch.table.reachable_default_dense_0to32 = private unnamed_addr constant [33 x i32] [i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0], align 4
 ; CHECK: @switch.table.unreachable_default_dense_0to32 = private unnamed_addr constant [33 x i32] [i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0], align 4
-; CHECK: @switch.table.unreachable_default_holes_0to32 = private unnamed_addr constant [33 x i32] [i32 0, i32 7, i32 6, i32 0, i32 4, i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 0, i32 2, i32 1, i32 0, i32 7, i32 0, i32 5, i32 4, i32 3, i32 2, i32 0, i32 0, i32 7, i32 6, i32 5, i32 0, i32 3, i32 2, i32 1, i32 0], align 4
+; CHECK: @switch.table.unreachable_default_holes_0to32 = private unnamed_addr constant [33 x i32] [i32 0, i32 7, i32 6, i32 poison, i32 4, i32 3, i32 2, i32 1, i32 poison, i32 7, i32 6, i32 5, i32 4, i32 poison, i32 2, i32 1, i32 0, i32 7, i32 poison, i32 5, i32 4, i32 3, i32 2, i32 poison, i32 0, i32 7, i32 6, i32 5, i32 poison, i32 3, i32 2, i32 1, i32 0], align 4
 ;.
 define i32 @reachable_default_dense_0to31(i32 %x, i32 %y) {
 ; CHECK-LABEL: @reachable_default_dense_0to31(