[llvm] [TTI] Introduce getInstructionUniformity API for flexible uniformity analysis (PR #137639)

[Jay Foad via llvm-commits]

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@@ -1574,3 +1575,55 @@ unsigned GCNTTIImpl::getNumberOfParts(Type *Tp) const {
   }
   return BaseT::getNumberOfParts(Tp);
 }
+
+// New API that wraps the old isSourceOfDivergence and isAlwaysUniform APIs
+// with additional support for new uniformity classifications
+InstructionUniformity
+GCNTTIImpl::getInstructionUniformity(const Value *V) const {
+  // Check for special cases requiring custom uniformity analysis
+  if (const IntrinsicInst *Intrinsic = dyn_cast<IntrinsicInst>(V)) {
+    switch (Intrinsic->getIntrinsicID()) {
+    case Intrinsic::amdgcn_permlane16:
+    case Intrinsic::amdgcn_permlanex16:
+      return InstructionUniformity::Custom;
+    default:
+      break;
+    }
+  }
+
+  // Delegate to old APIs for backward compatibility
+  if (isAlwaysUniform(V))
+    return InstructionUniformity::AlwaysUniform;
+
+  // Check if source of divergence
+  if (isSourceOfDivergence(V))
+    return InstructionUniformity::NeverUniform;
+
+  // Default behavior
+  return InstructionUniformity::Default;
+}
+
+bool GCNTTIImpl::isUniform(const Instruction *I,
+                           const SmallBitVector &UniformArgs) const {
+  // Custom uniformity check for permlane16/permlanex16
+  if (const IntrinsicInst *Intrinsic = dyn_cast<IntrinsicInst>(I)) {
+    switch (Intrinsic->getIntrinsicID()) {
+    case Intrinsic::amdgcn_permlane16:
+    case Intrinsic::amdgcn_permlanex16:
+      // For permlane16/permlanex16:
+      // Operand 0: old value (ignored for uniformity)
+      // Operand 1: src0 (source value to permute)
+      // Operand 2: src1 (lane select within 16-lane group)
+      // Operand 3: src2 (which 16-lane group)
+      // Result is uniform if either src0 (op 1) or src1 (op 2) is uniform
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jayfoad wrote:

As Nicolai pointed out this is completely wrong and permlane16 is the wrong example to use to demonstrate the new functionality.

https://github.com/llvm/llvm-project/pull/137639