[llvm] [SimplifyCFG] Delete the unnecessary range check for small mask operation (PR #65835)

[Nikita Popov via llvm-commits]

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@@ -6552,6 +6552,20 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
     // Note: We call removeProdecessor later since we need to be able to get the
     // PHI value for the default case in case we're using a bit mask.
   } else {
+    const APInt *MaskC;
+    if (UseSwitchConditionAsTableIndex &&
+        match(TableIndex, m_And(m_Value(), m_APInt(MaskC)))) {
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nikic wrote:

This seems unfortunately specific. For example if we replace `x & 15` with `x % 42` or so, it will no longer work, even though the basic idea is the same. We could use something like computeConstantRange() to generalize this.

https://github.com/llvm/llvm-project/pull/65835