[llvm] [GlobalISel] Fold instructions with fully-known bits to a constant. (PR #224254)

[Matt Arsenault via llvm-commits]

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@@ -3363,6 +3363,35 @@ bool CombinerHelper::matchRedundantAnd(MachineInstr &MI,
   return false;
 }
 
+bool CombinerHelper::matchKnownBitsToConstant(MachineInstr &MI,
+                                              APInt &MatchInfo) const {
+  if (!VT)
+    return false;
+
+  Register Dst = MI.getOperand(0).getReg();
+  LLT Ty = MRI.getType(Dst);
+
+  // Scalars and fixed vectors. getKnownBits intersects all lanes, so a constant
+  // vector result is necessarily a splat-constant.
+  if (!Ty.isScalar() && !Ty.isFixedVector())
+    return false;
+
+  // Don't materialize a def that already has a class/bank constraint into a
+  // constant.
+  if (!MRI.getRegClassOrRegBank(Dst).isNull())
+    return false;
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arsenm wrote:

This would be fine? 

https://github.com/llvm/llvm-project/pull/224254