[llvm] [AMDGPU] Optimize block count calculations to the new ABI (PR #174112)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -323,6 +326,50 @@ static bool processUse(CallInst *CI, bool IsV5OrAbove) {
     }
   }
 
+  // Upgrade the old method of calculating the block size using the grid size.
+  // We pattern match any case where the implicit argument group size is the
+  // divisor to a dispatch packet grid size read of the same dimension.
+  if (IsV5OrAbove) {
+    for (int I = 0; I < 3; I++) {
+      Value *GroupSize = GroupSizes[I];
+      if (!GroupSize)
+        continue;
+
+      for (User *U : GroupSize->users()) {
+        Instruction *Inst = cast<Instruction>(U);
+        if (isa<ZExtInst>(Inst) && !Inst->use_empty())
+          Inst = dyn_cast<Instruction>(*Inst->user_begin());
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jmmartinez wrote:

Maybe use `cast<Instruction>(...)` to hit the assertion early if for whatever strange reason `*Inst->user_begin()` is not an instruction.

https://github.com/llvm/llvm-project/pull/174112