[llvm] [NFC] SimplifyCFG: Detect switch replacement earlier in `switchToLookup` (PR #155602)

[Nikita Popov via llvm-commits]

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@@ -7022,9 +7031,76 @@ static bool switchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
       return false;
   }
 
+  // Compute the table index value.
+  Value *TableIndex;
+  if (UseSwitchConditionAsTableIndex) {
+    TableIndex = SI->getCondition();
+    if (HasDefaultResults) {
+      // Grow the table to cover all possible index values to avoid the range
+      // check. It will use the default result to fill in the table hole later,
+      // so make sure it exist.
+      ConstantRange CR =
+          computeConstantRange(TableIndex, /* ForSigned */ false);
+      // Grow the table shouldn't have any size impact by checking
+      // wouldFitInRegister.
+      // TODO: Consider growing the table also when it doesn't fit in a register
+      // if no optsize is specified.
+      const uint64_t UpperBound = CR.getUpper().getLimitedValue();
+      if (!CR.isUpperWrapped() && all_of(ResultTypes, [&](const auto &KV) {
+            return SwitchReplacement::wouldFitInRegister(
+                DL, UpperBound, KV.second /* ResultType */);
+          })) {
+        // There may be some case index larger than the UpperBound (unreachable
+        // case), so make sure the table size does not get smaller.
+        TableSize = std::max(UpperBound, TableSize);
+        // The default branch is unreachable after we enlarge the lookup table.
+        // Adjust DefaultIsReachable to reuse code path.
+        DefaultIsReachable = false;
+      }
+    }
+  }
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nikic wrote:

Why did this code get moved above shouldBuildLookupTable? Note that TableSize is passed to that function, so I'm not sure this is really NFC.

https://github.com/llvm/llvm-project/pull/155602