[SimplifyCFG] Avoid truncation in linear map overflow check

This is supposed to test multiplication of the linear multiplifier
with the largest value it can be multiplied with. However, if
we truncate TableSize-1 here, it might not actually be the largest
value. I think in practice this still works out, because in cases
where we'd truncate the value here we'd also fail the NonMonotonic
check. But to match the intent of the code, we should treat the
truncating case as overflowing.

Author: nikic

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6528,9 +6528,10 @@ SwitchLookupTable::SwitchLookupTable(
     if (LinearMappingPossible) {
       LinearOffset = cast<ConstantInt>(TableContents[0]);
       LinearMultiplier = ConstantInt::get(M.getContext(), DistToPrev);
-      bool MayWrap = false;
       APInt M = LinearMultiplier->getValue();
-      (void)M.smul_ov(APInt(M.getBitWidth(), TableSize - 1), MayWrap);
+      bool MayWrap = true;
+      if (isIntN(M.getBitWidth(), TableSize - 1))
+        (void)M.smul_ov(APInt(M.getBitWidth(), TableSize - 1), MayWrap);
       LinearMapValWrapped = NonMonotonic || MayWrap;
       Kind = LinearMapKind;
       ++NumLinearMaps;

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

@@ -2118,6 +2118,31 @@ cond.end:                                         ; preds = %entry, %cond.false
   ret i8 %conv
 }
 
+define i1 @linearmap_trunc_smaller_table_size(i8 %arg) {
+; CHECK-LABEL: @linearmap_trunc_smaller_table_size(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i8 [[ARG:%.*]], 10
+; CHECK-NEXT:    [[SWITCH_IDX_CAST:%.*]] = trunc i8 [[ARG]] to i1
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[TMP0]], i1 [[SWITCH_IDX_CAST]], i1 false
+; CHECK-NEXT:    ret i1 [[SPEC_SELECT]]
+;
+entry:
+  switch i8 %arg, label %exit [
+  i8 1, label %sw
+  i8 3, label %sw
+  i8 5, label %sw
+  i8 7, label %sw
+  i8 9, label %sw
+  ]
+
+sw:
+  br label %exit
+
+exit:
+  %phi = phi i1 [ true, %sw ], [ false, %entry ]
+  ret i1 %phi
+}
+
 ; Don't create a table with an unknown type
 define { i8, i8 } @test_unknown_result_type(i8 %n) {
 ; CHECK-LABEL: @test_unknown_result_type(