[llvm] AMDGPU: Add range attribute to mbcnt intrinsic callsites (PR #189191)

[Matt Arsenault via llvm-commits]

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@@ -1422,11 +1422,31 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
 
     break;
   }
-  case Intrinsic::amdgcn_mbcnt_hi: {
+  case Intrinsic::amdgcn_mbcnt_hi:
     // exec_hi is all 0, so this is just a copy.
     if (ST->isWave32())
       return IC.replaceInstUsesWith(II, II.getArgOperand(1));
-    break;
+    [[fallthrough]];
+  case Intrinsic::amdgcn_mbcnt_lo: {
+    ConstantRange AccRange = computeConstantRange(II.getArgOperand(1),
+                                                  /*ForSigned=*/false);
+    if (AccRange.isFullSet())
+      return nullptr;
+
+    // TODO: Can raise upper bound by inspecting first argument.
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arsenm wrote:

Also I think this is more of an API failure. computeKnownConstantRange should unconditionally fall back on computeKnownBits instead of having an arbitrarily different entry point 

https://github.com/llvm/llvm-project/pull/189191