[llvm] [AMDGPU] Hoist permlane64/readlane/readfirstlane through unary/binary operands (PR #129037)

[Matt Arsenault via llvm-commits]

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@@ -481,6 +482,81 @@ bool GCNTTIImpl::simplifyDemandedLaneMaskArg(InstCombiner &IC,
   return false;
 }
 
+Instruction *
+GCNTTIImpl::hoistLaneIntrinsicThroughOperand(InstCombiner &IC,
+                                             IntrinsicInst &II) const {
+  const auto IID = II.getIntrinsicID();
+  assert(IID == Intrinsic::amdgcn_readlane ||
+         IID == Intrinsic::amdgcn_readfirstlane ||
+         IID == Intrinsic::amdgcn_permlane64);
+
+  Instruction *Op = dyn_cast<Instruction>(II.getOperand(0));
+
+  // Only do this if both instructions are in the same block
+  // (so the exec mask won't change) and the readlane is the only user of its
+  // operand.
+  if (!Op || !Op->hasOneUser() || Op->getParent() != II.getParent())
+    return nullptr;
+
+  const bool IsReadLane = (IID == Intrinsic::amdgcn_readlane);
+
+  // If this is a readlane, check that the second operand is a constant, or is
+  // defined before Op so we know it's safe to move this intrinsic higher.
+  Value *LaneID = nullptr;
+  if (IsReadLane) {
+    LaneID = II.getOperand(1);
+    // Check LaneID is available at Op, otherwise we can't move the readlane
+    // higher.
+    if (!IC.getDominatorTree().dominates(LaneID, Op))
+      return nullptr;
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arsenm wrote:

Use isSafeToMoveBefore? I'm somewhat surprised you can do a dominates check on a constant 

https://github.com/llvm/llvm-project/pull/129037