[llvm] [InstCombine] Lower flag check pattern to use a bitmask-shift (PR #169557)

[Craig Topper via llvm-commits]

================
@@ -3119,6 +3119,127 @@ static Instruction *matchFunnelShift(Instruction &Or, InstCombinerImpl &IC) {
   return nullptr;
 }
 
+static Value *combineOrOfImmCmpToBitExtract(Instruction &Or,
+                                            InstCombiner::BuilderTy &Builder,
+                                            const DataLayout &DL) {
+
+  auto isICmpEqImm = [](Value *N, ConstantInt *&Imm, Value *&X) -> bool {
+    if (X)
+      return match(N, m_OneUse(m_SpecificICmp(ICmpInst::ICMP_EQ, m_Specific(X),
+                                              m_ConstantInt(Imm))));
+
+    return match(N, m_OneUse(m_SpecificICmp(ICmpInst::ICMP_EQ, m_Value(X),
+                                            m_ConstantInt(Imm))));
+  };
+
+  // %srl = lshr %bitmap, %X
+  // %icmp = icmp ult %X, %max_value
+  // %trunc = trunc %srl to i1
+  // %sel = select %icmp, %trunc, false
+  auto CreateBitExtractSeq = [&](APInt BitMap, APInt MaxValue,
+                                 Value *X) -> Value * {
+    LLVMContext &Context = Or.getContext();
+
+    // %srl = lshr %bitmap, %X
+    // It is okay for the shift amount to be truncated because
+    // if information is lost then it is garunteed to fail the bounds
----------------
topperc wrote:

```suggestion
    // if information is lost then it is guaranteed to fail the bounds
```

https://github.com/llvm/llvm-project/pull/169557