[SimplifyCFG] add nuw/nsw on BuildLookuptable BitMap shiftwidth calculation

[khei4]

Summary

When converting the switch to BitMap lookup table, its shift amount calculation is guaranteed to never overflow when deciding to use BitMap, on WouldFitInRegister.

This is part of https://reviews.llvm.org/D146903.

[khei4]

update tests

[nikic]

LGTM

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

6139

Does this comply with clang-format?

[khei4]

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

6139

Actually seems subtle, inserted newlines look optional, but I will remove them.
as

ShiftAmt = Builder.CreateMul(
    ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
    "switch.shiftamt", /*HasNUW =*/true, /*HasNSW =*/true);

Thanks!

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

case BitMapKind: {
     // Type of the bitmap (e.g. i59).
     IntegerType *MapTy = BitMap->getType();
 
     // Cast Index to the same type as the bitmap.
     // Note: The Index is <= the number of elements in the table, so
     // truncating it to the width of the bitmask is safe.
     Value *ShiftAmt = Builder.CreateZExtOrTrunc(Index, MapTy, "switch.cast");
 
-    // Multiply the shift amount by the element width.
+    // Multiply the shift amount by the element width. NUW/NSW can always be
+    // set, because WouldFitInRegister guarantees Index * ShiftAmt is in
+    // BitMap's bit width.
     ShiftAmt = Builder.CreateMul(
         ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
-        "switch.shiftamt");
+        "switch.shiftamt",
+        /*HasNUW =*/true,
+        /*HasNSW =*/true);
 
     // Shift down.
     Value *DownShifted =
         Builder.CreateLShr(BitMap, ShiftAmt, "switch.downshift");
     // Mask off.
     return Builder.CreateTrunc(DownShifted, BitMapElementTy, "switch.masked");
   }
   case ArrayKind: {