[PATCH] D91928: [nvptx] Skip alloca for read-only byval arguments.

[Michael Liao via Phabricator via llvm-commits]

hliao created this revision.
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- Once a byval argument is attributed with read-only, there's no store into that argument and it's safe to skip generating alloca to match read-only input parameter space property. Cast that generic pointer to the parameter space and back so that the address space inference pass could infer the correct parameter space.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D91928

Files:
  llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
  llvm/test/CodeGen/NVPTX/lower-args.ll


Index: llvm/test/CodeGen/NVPTX/lower-args.ll
===================================================================
--- llvm/test/CodeGen/NVPTX/lower-args.ll
+++ llvm/test/CodeGen/NVPTX/lower-args.ll
@@ -10,6 +10,27 @@
 ; Check that nvptx-lower-args preserves arg alignment
 define void @load_alignment(%class.outer* nocapture readonly byval(%class.outer) align 8 %arg) {
 entry:
+; IR-LABEL: @load_alignment
+; IR: addrspacecast %class.outer* %arg to %class.outer addrspace(101)*
+; IR-NEXT: addrspacecast %class.outer addrspace(101)* %arg1 to %class.outer*
+; PTX: ld.param.u64
+; PTX-NOT: ld.param.u8
+  %arg.idx = getelementptr %class.outer, %class.outer* %arg, i64 0, i32 0, i32 0
+  %arg.idx.val = load i32*, i32** %arg.idx, align 8
+  %arg.idx1 = getelementptr %class.outer, %class.outer* %arg, i64 0, i32 0, i32 1
+  %arg.idx1.val = load i32*, i32** %arg.idx1, align 8
+  %arg.idx2 = getelementptr %class.outer, %class.outer* %arg, i64 0, i32 1
+  %arg.idx2.val = load i32, i32* %arg.idx2, align 8
+  %arg.idx.val.val = load i32, i32* %arg.idx.val, align 4
+  %add.i = add nsw i32 %arg.idx.val.val, %arg.idx2.val
+  store i32 %add.i, i32* %arg.idx1.val, align 4
+  ret void
+}
+
+; Check that nvptx-lower-args preserves arg alignment
+define void @load_alignment_without_readonly(%class.outer* nocapture byval(%class.outer) align 8 %arg) {
+entry:
+; IR-LABEL: @load_alignment_without_readonly
 ; IR: load %class.outer, %class.outer addrspace(101)*
 ; IR-SAME: align 8
 ; PTX: ld.param.u64
Index: llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
===================================================================
--- llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
+++ llvm/lib/Target/NVPTX/NVPTXLowerArgs.cpp
@@ -159,6 +159,25 @@
   assert(PType && "Expecting pointer type in handleByValParam");
 
   Type *StructType = PType->getElementType();
+
+  if (Arg->onlyReadsMemory()) {
+    // Once there's no store to that byval argument, there's no need to
+    // generate an `alloca`. Cast it into the parameter space and cast it back
+    // to the generic space so that the address space inference could infer the
+    // correct address space.
+    Value *ArgInParam = new AddrSpaceCastInst(
+        Arg, PointerType::get(StructType, ADDRESS_SPACE_PARAM), Arg->getName(),
+        FirstInst);
+    Value *ArgInGeneric = new AddrSpaceCastInst(
+        ArgInParam, PType, Arg->getName() + ".addrspacecast", FirstInst);
+    for (auto &U : Arg->uses()) {
+      if (U.getUser() == ArgInParam)
+        continue;
+      U.getUser()->setOperand(U.getOperandNo(), ArgInGeneric);
+    }
+    return;
+  }
+
   const DataLayout &DL = Func->getParent()->getDataLayout();
   unsigned AS = DL.getAllocaAddrSpace();
   AllocaInst *AllocA = new AllocaInst(StructType, AS, Arg->getName(), FirstInst);


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