[compiler-rt] [llvm] [AIX] PGO codegen changes for function-sections. (PR #139761)

[Amy Kwan via llvm-commits]

================
@@ -2810,6 +2826,57 @@ void PPCAIXAsmPrinter::emitGlobalVariableHelper(const GlobalVariable *GV) {
   MCSectionXCOFF *Csect = cast<MCSectionXCOFF>(
       getObjFileLowering().SectionForGlobal(GV, GVKind, TM));
 
+  // When compiling with function sections enabled, we need some special
+  // codegen to rename the CSECTs. For each profiling data symbol find its
+  // associated profiling counters.
+  if (TM.getFunctionSections() &&
+      Csect->getName().starts_with("__llvm_prf_data.")) {
+    // Unraveling the initializer to find the related counters variable. The
+    // initializer is a structure whose third member is a subtract expression
+    // between the counters label and the label for the start of this structure.
+    // Use the subtract expression to get the GlobalValue for the counters
+    // global.
+    assert(GV->hasInitializer() &&
+           "profiling data symbol must have an initializer");
+    assert(isa<ConstantStruct>(GV->getInitializer()) &&
+           "expect the initializer for a profiling data symbol to be a struct");
+    const ConstantStruct *Initializer =
+        cast<ConstantStruct>(GV->getInitializer());
+
+    // The initializer structure is: { i64, i64, i32, ptr, ptr, i32, [4 x i16] }
+    // and the reference to the global variable for the counters is in the
+    // first i32 member.
+    const Constant *Member = Initializer->getAggregateElement(2);
+    assert(Member && "profiling data symbol has more then 3 elements");
+
+    // Want to decompose a constant expression of the form:
+    // sub (ptrtoint (ptr @__profc_sym), ptrtoint (ptr @__profd_sym))
+    // to get the GlobalVariable for the '@__profc_sym` symbol.
+    assert(isa<ConstantExpr>(Member) &&
+           "expected member initializer is a constant expression.");
+    const ConstantExpr *CExpr = cast<ConstantExpr>(Member);
+    assert(CExpr->getOpcode() == Instruction::Sub &&
+           "expected member intializer is a sub expression.");
----------------
amy-kwan wrote:

```suggestion
           "expected member initializer is a sub expression.");
```

https://github.com/llvm/llvm-project/pull/139761