[llvm] [GISel] Fold bitreverse(shl/srl(bitreverse(x),y)) -> srl/shl(x,y) (PR #91355)

[Matt Arsenault via llvm-commits]

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@@ -2016,6 +2016,32 @@ void CombinerHelper::applyCombineShlOfExtend(MachineInstr &MI,
   MI.eraseFromParent();
 }
 
+bool CombinerHelper::matchBitreverseShift(MachineInstr &MI,
+                                          BuildFnTy &MatchInfo) {
+  assert(MI.getOpcode() == TargetOpcode::G_BITREVERSE && "Expected BITREVERSE");
+  Register Dst = MI.getOperand(0).getReg();
+  Register Src = MI.getOperand(1).getReg();
+  Register Val, Amt;
+
+  // fold (bitreverse (shl (bitreverse x), y)) -> (lshr x, y)
+  if (mi_match(Src, MRI, m_GShl(m_GBitreverse(m_Reg(Val)), m_Reg(Amt))) &&
+      isLegalOrBeforeLegalizer(
+          {TargetOpcode::G_LSHR, {MRI.getType(Val), MRI.getType(Amt)}})) {
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildLShr(Dst, Val, Amt); };
+    return true;
+  }
+
+  // fold (bitreverse (lshr (bitreverse x), y)) -> (shl x, y)
+  if (mi_match(Src, MRI, m_GLShr(m_GBitreverse(m_Reg(Val)), m_Reg(Amt))) &&
+      isLegalOrBeforeLegalizer(
+          {TargetOpcode::G_SHL, {MRI.getType(Val), MRI.getType(Amt)}})) {
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildShl(Dst, Val, Amt); };
+    return true;
+  }
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arsenm wrote:

It's unfortunate you have to rematch the same pattern in here. Should split these into separate apply functions, the 2 entry points just hit the 2 different cases here 

https://github.com/llvm/llvm-project/pull/91355