Don't remove llvm.compiler.used at sycl post link

This is removed at lowering for NVPTX/AMDGCN.

TODO: check that this is working for SPIR-V.

Author: hdelan

llvm/test/CodeGen/AMDGPU/remove-compiler-used.ll

@@ -0,0 +1,19 @@
+;; This test verifies llc on AMDGCN will delete the llvm.compiler.used symbol
+;; while keeping the symbol in the outputted ASM.
+
+; RUN: llc < %s -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 | FileCheck %s
+; RUN: llc < %s -mtriple=amdgcn-amd-amdhsa -mcpu=gfx906 | FileCheck %s
+; RUN: llc < %s -mtriple=amdgcn-amd-amdhsa -mcpu=gfx90a | FileCheck %s
+
+@keep_this = internal global i32 2, align 4
+@llvm.compiler.used = appending global [1 x ptr] [ptr @keep_this], section "llvm.metadata"
+
+; CHECK-NOT: llvm.metadata
+; CHECK-NOT: llvm{{.*}}used
+; CHECK-NOT: llvm{{.*}}compiler{{.*}}used
+
+; CHECK: .type keep_this,@object ;
+
+; CHECK-NOT: llvm.metadata
+; CHECK-NOT: llvm{{.*}}used
+; CHECK-NOT: llvm{{.*}}compiler{{.*}}used

llvm/test/CodeGen/NVPTX/remove-compiler-used.ll

@@ -0,0 +1,17 @@
+;; This test verifies llc on NVPTX will delete the llvm.compiler.used symbol
+;; while keeping the symbol in the outputted ASM.
+
+; RUN: llc < %s -march=nvptx64 | FileCheck %s
+
+@keep_this = internal global i32 2, align 4
+@llvm.compiler.used = appending global [1 x ptr] [ptr @keep_this], section "llvm.metadata"
+
+; CHECK-NOT: llvm.metadata
+; CHECK-NOT: llvm{{.*}}used
+; CHECK-NOT: llvm{{.*}}compiler{{.*}}used
+
+; CHECK: .global .align 4 .u32 keep_this
+
+; CHECK-NOT: llvm.metadata
+; CHECK-NOT: llvm{{.*}}used
+; CHECK-NOT: llvm{{.*}}compiler{{.*}}used

llvm/tools/sycl-post-link/sycl-post-link.cpp

@@ -572,70 +572,6 @@ static bool removeSYCLKernelsConstRefArray(Module &M) {
   return true;
 }
 
-// Removes all device_global variables from the llvm.compiler.used global
-// variable. A device_global with internal linkage will be in llvm.compiler.used
-// to avoid the compiler wrongfully removing it during optimizations. However,
-// as an effect the device_global variables will also be distributed across
-// binaries, even if llvm.compiler.used has served its purpose. To avoid
-// polluting other binaries with unused device_global variables, we remove them
-// from llvm.compiler.used and erase them if they have no further uses.
-static bool removeDeviceGlobalFromCompilerUsed(Module &M) {
-  GlobalVariable *GV = M.getGlobalVariable("llvm.compiler.used");
-  if (!GV)
-    return false;
-
-  // Erase the old llvm.compiler.used. A new one will be created at the end if
-  // there are other values in it (other than device_global).
-  assert(GV->user_empty() && "Unexpected llvm.compiler.used users");
-  Constant *Initializer = GV->getInitializer();
-  const auto *VAT = cast<ArrayType>(GV->getValueType());
-  GV->setInitializer(nullptr);
-  GV->eraseFromParent();
-
-  // Destroy the initializer. Keep the operands so we keep the ones we need.
-  SmallVector<Constant *, 8> IOperands;
-  for (auto It = Initializer->op_begin(); It != Initializer->op_end(); It++)
-    IOperands.push_back(cast<Constant>(*It));
-  assert(llvm::isSafeToDestroyConstant(Initializer) &&
-         "Cannot remove initializer of llvm.compiler.used global");
-  Initializer->destroyConstant();
-
-  // Iterate through all operands. If they are device_global then we drop them
-  // and erase them if they have no uses afterwards. All other values are kept.
-  SmallVector<Constant *, 8> NewOperands;
-  for (auto It = IOperands.begin(); It != IOperands.end(); It++) {
-    Constant *Op = *It;
-    auto *DG = dyn_cast<GlobalVariable>(Op->stripPointerCasts());
-
-    // If it is not a device_global we keep it.
-    if (!DG || !isDeviceGlobalVariable(*DG)) {
-      NewOperands.push_back(Op);
-      continue;
-    }
-
-    // Destroy the device_global operand.
-    if (llvm::isSafeToDestroyConstant(Op))
-      Op->destroyConstant();
-
-    // Remove device_global if it no longer has any uses.
-    if (!DG->isConstantUsed())
-      DG->eraseFromParent();
-  }
-
-  // If we have any operands left from the original llvm.compiler.used we create
-  // a new one with the new size.
-  if (!NewOperands.empty()) {
-    ArrayType *ATy = ArrayType::get(VAT->getElementType(), NewOperands.size());
-    GlobalVariable *NGV =
-        new GlobalVariable(M, ATy, false, GlobalValue::AppendingLinkage,
-                           ConstantArray::get(ATy, NewOperands), "");
-    NGV->setName("llvm.compiler.used");
-    NGV->setSection("llvm.metadata");
-  }
-
-  return true;
-}
-
 SmallVector<module_split::ModuleDesc, 2>
 handleESIMD(module_split::ModuleDesc &&MDesc, bool &Modified,
             bool &SplitOccurred) {
@@ -783,16 +719,6 @@ processInputModule(std::unique_ptr<Module> M) {
   // actions.
   Modified |= removeSYCLKernelsConstRefArray(*M.get());
 
-  // There may be device_global variables kept alive in "llvm.compiler.used"
-  // to keep the optimizer from wrongfully removing them. Since it has served
-  // its purpose, these device_global variables can be removed. If they are not
-  // used inside the device code after they have been removed from
-  // "llvm.compiler.used" they can be erased safely.
-  if (auto Triple = M->getTargetTriple();
-      Triple.find("nvptx") != std::string::npos &&
-      Triple.find("amdgcn") != std::string::npos)
-    Modified |= removeDeviceGlobalFromCompilerUsed(*M.get());
-
   // Instrument each image scope device globals if the module has been
   // instrumented by sanitizer pass.
   if (isModuleUsingAsan(*M))