[llvm] [AMDGPU] Add IR LiveReg type-based optimization (PR #66838)

[Matt Arsenault via llvm-commits]

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@@ -102,14 +169,245 @@ bool AMDGPULateCodeGenPrepare::runOnFunction(Function &F) {
   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
   UA = &getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
 
+  // "Optimize" the virtual regs that cross basic block boundaries. When
+  // building the SelectionDAG, vectors of illegal types that cross basic blocks
+  // will be scalarized and widened, with each scalar living in its
+  // own register. To work around this, this optimization converts the
+  // vectors to equivalent vectors of legal type (which are converted back
+  // before uses in subsequent blocks), to pack the bits into fewer physical
+  // registers (used in CopyToReg/CopyFromReg pairs).
+  LiveRegOptimizer LRO(Mod, &ST);
+
   bool Changed = false;
+
   for (auto &BB : F)
-    for (Instruction &I : llvm::make_early_inc_range(BB))
+    for (Instruction &I : make_early_inc_range(BB)) {
       Changed |= visit(I);
+      Changed |= LRO.optimizeLiveType(&I);
+    }
 
+  LRO.removeDeadInstrs();
   return Changed;
 }
 
+Type *LiveRegOptimizer::calculateConvertType(Type *OriginalType) {
+  assert(OriginalType->getScalarSizeInBits() <=
+         ConvertToScalar->getScalarSizeInBits());
+
+  FixedVectorType *VTy = cast<FixedVectorType>(OriginalType);
+
+  TypeSize OriginalSize = DL->getTypeSizeInBits(VTy);
+  TypeSize ConvertScalarSize = DL->getTypeSizeInBits(ConvertToScalar);
+  unsigned ConvertEltCount =
+      (OriginalSize + ConvertScalarSize - 1) / ConvertScalarSize;
+
+  if (OriginalSize <= ConvertScalarSize)
+    return IntegerType::get(Mod->getContext(), ConvertScalarSize);
+
+  return VectorType::get(Type::getIntNTy(Mod->getContext(), ConvertScalarSize),
+                         ConvertEltCount, false);
+}
+
+Value *LiveRegOptimizer::convertToOptType(Instruction *V,
+                                          BasicBlock::iterator &InsertPt) {
+  FixedVectorType *VTy = cast<FixedVectorType>(V->getType());
+  Type *NewTy = calculateConvertType(V->getType());
+
+  TypeSize OriginalSize = DL->getTypeSizeInBits(VTy);
+  TypeSize NewSize = DL->getTypeSizeInBits(NewTy);
+
+  IRBuilder<> Builder(V->getParent(), InsertPt);
+  // If there is a bitsize match, we can fit the old vector into a new vector of
+  // desired type.
+  if (OriginalSize == NewSize)
+    return cast<Instruction>(
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arsenm wrote:

this cast is pointless, the function returns a Value * 

https://github.com/llvm/llvm-project/pull/66838