[AMDGPU] Re-enable closed-world assumption as an opt-in feature (#115371)

Although the ABI (if one exists) doesn’t explicitly prohibit
cross-code-object function calls—particularly since our loader can
handle them—such calls are not actually allowed in any of the officially
supported programming models. However, this limitation has some nuances.
For instance, the loader can handle cross-code-object global variables,
which complicates the situation further.

Given this complexity, assuming a closed-world model at link time isn’t
always safe. To address this, this PR introduces an option that enables
this assumption, providing end users the flexibility to enable it for
improved compiler optimizations. However, it is the user’s
responsibility to ensure they do not violate this assumption.

Author: shiltian

llvm/lib/Target/AMDGPU/AMDGPUAttributor.cpp

@@ -1286,6 +1286,10 @@ static bool runImpl(Module &M, AnalysisGetter &AG, TargetMachine &TM,
 
   Attributor A(Functions, InfoCache, AC);
 
+  LLVM_DEBUG(dbgs() << "[AMDGPUAttributor] Module " << M.getName() << " is "
+                    << (AC.IsClosedWorldModule ? "" : "not ")
+                    << "assumed to be a closed world.\n");
+
   for (auto *F : Functions) {
     A.getOrCreateAAFor<AAAMDAttributes>(IRPosition::function(*F));
     A.getOrCreateAAFor<AAUniformWorkGroupSize>(IRPosition::function(*F));

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

@@ -454,6 +454,11 @@ static cl::opt<bool> NewRegBankSelect(
              "regbankselect"),
     cl::init(false), cl::Hidden);
 
+static cl::opt<bool> HasClosedWorldAssumption(
+    "amdgpu-link-time-closed-world",
+    cl::desc("Whether has closed-world assumption at link time"),
+    cl::init(false), cl::Hidden);
+
 extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeAMDGPUTarget() {
   // Register the target
   RegisterTargetMachine<R600TargetMachine> X(getTheR600Target());
@@ -859,8 +864,12 @@ void AMDGPUTargetMachine::registerPassBuilderCallbacks(PassBuilder &PB) {
             PM.addPass(InternalizePass(mustPreserveGV));
             PM.addPass(GlobalDCEPass());
           }
-          if (EnableAMDGPUAttributor)
-            PM.addPass(AMDGPUAttributorPass(*this));
+          if (EnableAMDGPUAttributor) {
+            AMDGPUAttributorOptions Opt;
+            if (HasClosedWorldAssumption)
+              Opt.IsClosedWorld = true;
+            PM.addPass(AMDGPUAttributorPass(*this, Opt));
+          }
         }
       });
 

llvm/test/LTO/AMDGPU/closed-world-assumption.ll

@@ -0,0 +1,12 @@
+; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -O3 -debug-only=amdgpu-attributor -o - %s 2>&1 | FileCheck %s --check-prefix=NO-CW
+; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -passes="lto<O3>" -debug-only=amdgpu-attributor -o - %s 2>&1 | FileCheck %s --check-prefix=NO-CW
+; RUN: opt -S -mtriple=amdgcn-amd-amdhsa -passes="lto<O3>" -debug-only=amdgpu-attributor -amdgpu-link-time-closed-world=1 -o - %s 2>&1 | FileCheck %s --check-prefix=CW
+
+; REQUIRES: amdgpu-registered-target
+; REQUIRES: asserts
+
+; NO-CW: Module {{.*}} is not assumed to be a closed world.
+; CW: Module {{.*}} is assumed to be a closed world.
+define hidden noundef i32 @_Z3foov() {
+  ret i32 1
+}