[llvm] [InstCombine] Add fold combining adjacent extracted bit fields to restore optimization for separate truncations (PR #219598)

[Andreas Jonson via llvm-commits]

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@@ -4319,6 +4319,29 @@ Instruction *InstCombinerImpl::visitOr(BinaryOperator &I) {
           return BinaryOperator::CreateAnd(Or, C01);
         }
       }
+
+      // (trunc (lshr X, S) & C0) | (lshr (trunc X), S & C1)
+      // --> (trunc (lshr X, S) & (C0 | C1) (and similar cases) 
+      // A = trunc (lshr X, S) B = lshr (trunc X), S
+      const APInt *ShiftAmt;
+      if (match(A, m_OneUse(m_Trunc(m_LShr(m_Value(X), m_APInt(ShiftAmt))))) &&
+          match(B, m_LShr(m_Trunc(m_Specific(X)),
+                          m_SpecificInt(ShiftAmt->getZExtValue())))) {
+        APInt CombinedMask = *C0 | *C1;
+        return BinaryOperator::CreateAnd(
+            A, ConstantInt::get(I.getType(), CombinedMask));
+      }
+      // A = lshr (trunc X), S
+      // B = trunc (lshr X, S)
+      const APInt *ReverseShiftAmt;
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andjo403 wrote:

no need for an other APInt reuse the other one only adds extra noise

https://github.com/llvm/llvm-project/pull/219598