[Pseudo Probe] Do not place functions in nodeduplicate COMDATs

[MaskRay]

Summary

For a function not in an IR COMDAT, currently we place it into a nodeduplicate IR
COMDAT so that its text section and its associated .pseudo_probe section will be
in the same section group, which can be retained or discarded by the linker as a
unit. However, the section group wastes space.

After D153189 uses SHF_LINK_ORDER to ensure a .pseudo_probe section will be
discarded when its associated text section is discarded, we can remove the
nodeduplicate IR change.

In the following example, the .pseudo_probe associated with .text.f is discarded as expected.

clang -c -ffunction-sections -fpseudo-probe-for-profiling -xc =(printf 'void _start(){} void f(){}') -o a.o
ld.lld --gc-sections --print-gc-sections a.o

[hoy]

Thanks for the cleanup!

llvm/lib/Transforms/IPO/SampleProfileProbe.cpp

void SampleProfileProber::instrumentOneFunc(Function &F, TargetMachine *TM) {
   // - FunctionGUID
   // - FunctionHash.
   // - FunctionName
   auto Hash = getFunctionHash();
   auto *MD = MDB.createPseudoProbeDesc(Guid, Hash, FName);
   auto *NMD = M->getNamedMetadata(PseudoProbeDescMetadataName);
   assert(NMD && "llvm.pseudo_probe_desc should be pre-created");
   NMD->addOperand(MD);
-
-  // Preserve a comdat group to hold all probes materialized later. This
-  // allows that when the function is considered dead and removed, the
-  // materialized probes are disposed too.
-  // Imported functions are defined in another module. They do not need
-  // the following handling since same care will be taken for them in their
-  // original module. The pseudo probes inserted into an imported functions
-  // above will naturally not be emitted since the imported function is free
-  // from object emission. However they will be emitted together with the
-  // inliner functions that the imported function is inlined into. We are not
-  // creating a comdat group for an import function since it's useless anyway.
-  if (!F.isDeclarationForLinker()) {
-    if (TM) {
-      auto Triple = TM->getTargetTriple();
-      if (Triple.supportsCOMDAT() && TM->getFunctionSections())
-        getOrCreateFunctionComdat(F, Triple);
-    }
-  }
 }
 
 PreservedAnalyses SampleProfileProbePass::run(Module &M,
                                               ModuleAnalysisManager &AM) {
   auto ModuleId = getUniqueModuleId(&M);
   // Create the pseudo probe desc metadata beforehand.
   // Note that modules with only data but no functions will require this to
   // be set up so that they will be known as probed later.

llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll

 
 ;; Check the generation of pseudoprobe intrinsic call.
 
 @a = dso_local global i32 0, align 4
 
 define void @foo(i32 %x) !dbg !3 {
 bb0:
   %cmp = icmp eq i32 %x, 0
+; CHECK-IL-LABEL: void @foo(i32 %x) !dbg ![[#]] {
 ; CHECK-IL: call void @llvm.pseudoprobe(i64 [[#GUID:]], i64 1, i32 0, i64 -1), !dbg ![[#FAKELINE:]]
 ; CHECK-MIR: PSEUDO_PROBE [[#GUID:]], 1, 0, 0
 ; CHECK-ASM: .pseudoprobe [[#GUID:]] 1 0 0 foo
   br i1 %cmp, label %bb1, label %bb2
 
 bb1:
 ; CHECK-IL: call void @llvm.pseudoprobe(i64 [[#GUID:]], i64 2, i32 0, i64 -1), !dbg ![[#FAKELINE]]
 ; CHECK-MIR: PSEUDO_PROBE [[#GUID]], 3, 0, 0

 ; CHECK-IL: call void @llvm.pseudoprobe(i64 [[#GUID]], i64 4, i32 0, i64 -1), !dbg ![[#REALLINE:]]
   ret void, !dbg !12
 }
 
 declare void @bar(i32 %x)
 
 define internal void @foo2(ptr %f) !dbg !4 {
 entry:
+; CHECK-IL-LABEL: void @foo2(ptr %f) !dbg ![[#]] {
 ; CHECK-IL: call void @llvm.pseudoprobe(i64 [[#GUID2:]], i64 1, i32 0, i64 -1)
 ; CHECK-MIR: PSEUDO_PROBE [[#GUID2:]], 1, 0, 0
 ; CHECK-ASM: .pseudoprobe [[#GUID2:]] 1 0 0 foo2
 ; Check pseudo_probe metadata attached to the indirect call instruction.
 ; CHECK-IL: call void %f(i32 1), !dbg ![[#PROBE0:]]
 ; CHECK-MIR: PSEUDO_PROBE [[#GUID2]], 2, 1, 0
 ; CHECK-ASM: .pseudoprobe [[#GUID2]] 2 1 0 foo2
   call void %f(i32 1), !dbg !13