SimplifyCFG: Delay check for "no-jump-tables" attribute

Allow the replacement of switches with anything other than a lookup
table, even if the "no-jump-table" attribute is set.

Author: OutOfCache

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6929,9 +6929,6 @@ static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
 
   BasicBlock *BB = SI->getParent();
   Function *Fn = BB->getParent();
-  // Only build lookup table when the attribute is not set.
-  if (Fn->getFnAttribute("no-jump-tables").getValueAsBool())
-    return false;
 
   // FIXME: If the switch is too sparse for a lookup table, perhaps we could
   // split off a dense part and build a lookup table for that.
@@ -7101,11 +7098,13 @@ static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
   //        This is a problem if the switch expression itself can be restricted
   //        by inlining or CVP.
   //     2. The target does not support lookup tables.
+  //     3. The "no-jump-tables" function attribute is set.
   // However, these objections do not apply to other switch replacements, like
   // the bitmap, so we only stop here if any of these conditions are met and we
   // want to create a LUT. Otherwise, continue with the switch replacement.
   if (AnyLookupTables &&
-      (!ConvertSwitchToLookupTable || !TTI.shouldBuildLookupTables()))
+      (!ConvertSwitchToLookupTable || !TTI.shouldBuildLookupTables() ||
+       Fn->getFnAttribute("no-jump-tables").getValueAsBool()))
     return false;
 
   Builder.SetInsertPoint(SI);

llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll

@@ -340,34 +340,16 @@ end:
 define i32 @single_value_no_jump_tables(i32 %x) "no-jump-tables"="true" {
 ; OPTNOLUT-LABEL: define i32 @single_value_no_jump_tables(
 ; OPTNOLUT-SAME: i32 [[X:%.*]]) #[[ATTR0:[0-9]+]] {
-; OPTNOLUT-NEXT:  [[ENTRY:.*]]:
-; OPTNOLUT-NEXT:    switch i32 [[X]], label %[[DEFAULT:.*]] [
-; OPTNOLUT-NEXT:      i32 0, label %[[END:.*]]
-; OPTNOLUT-NEXT:      i32 1, label %[[END]]
-; OPTNOLUT-NEXT:      i32 2, label %[[END]]
-; OPTNOLUT-NEXT:      i32 3, label %[[END]]
-; OPTNOLUT-NEXT:      i32 4, label %[[END]]
-; OPTNOLUT-NEXT:    ]
-; OPTNOLUT:       [[DEFAULT]]:
-; OPTNOLUT-NEXT:    br label %[[END]]
-; OPTNOLUT:       [[END]]:
-; OPTNOLUT-NEXT:    [[IDX:%.*]] = phi i32 [ 3, %[[DEFAULT]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ]
+; OPTNOLUT-NEXT:  [[ENTRY:.*:]]
+; OPTNOLUT-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[X]], 5
+; OPTNOLUT-NEXT:    [[IDX:%.*]] = select i1 [[TMP0]], i32 2, i32 3
 ; OPTNOLUT-NEXT:    ret i32 [[IDX]]
 ;
 ; TTINOLUT-LABEL: define i32 @single_value_no_jump_tables(
 ; TTINOLUT-SAME: i32 [[X:%.*]]) #[[ATTR0:[0-9]+]] {
-; TTINOLUT-NEXT:  [[ENTRY:.*]]:
-; TTINOLUT-NEXT:    switch i32 [[X]], label %[[DEFAULT:.*]] [
-; TTINOLUT-NEXT:      i32 0, label %[[END:.*]]
-; TTINOLUT-NEXT:      i32 1, label %[[END]]
-; TTINOLUT-NEXT:      i32 2, label %[[END]]
-; TTINOLUT-NEXT:      i32 3, label %[[END]]
-; TTINOLUT-NEXT:      i32 4, label %[[END]]
-; TTINOLUT-NEXT:    ]
-; TTINOLUT:       [[DEFAULT]]:
-; TTINOLUT-NEXT:    br label %[[END]]
-; TTINOLUT:       [[END]]:
-; TTINOLUT-NEXT:    [[IDX:%.*]] = phi i32 [ 3, %[[DEFAULT]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ]
+; TTINOLUT-NEXT:  [[ENTRY:.*:]]
+; TTINOLUT-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[X]], 5
+; TTINOLUT-NEXT:    [[IDX:%.*]] = select i1 [[TMP0]], i32 2, i32 3
 ; TTINOLUT-NEXT:    ret i32 [[IDX]]
 ;
 entry: