[InstrProfiling] Use self-link for counter metadata

[phosek]

Summary

We cannot link to the function because if the function gets discarded,
we'll end up with and invalid link which leads to a linker error:

sh_link points to discarded section

[MaskRay]

We cannot link to the function because if the function gets discarded, we'll end up with and invalid link which leads to a linker error:

If the text section .text.foo gets discarded, the __profc_foo section should get discarded, too.

[phosek]

In D95930#2539874, @MaskRay wrote:

We cannot link to the function because if the function gets discarded, we'll end up with and invalid link which leads to a linker error:

If the text section .text.foo gets discarded, the __profc_foo section should get discarded, too.

Could it be because we discard COMDAT sections early on when reading input files, so later the __profc_foo section doesn't get collected by GC because it's a C identifier and considered a GC root?

[MaskRay]

COMDAT and GC are orthogonal. COMDAT selection runs before GC. Every section flag except SHF_GROUP should be retained. Do you have an object file to demonstrate the GC issue?

[MaskRay]

I'll assume that this is now abandoned? :)

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

InstrProfiling::getOrCreateRegionCounters(InstrProfIncrementInst *Inc) {
   CounterPtr->setAlignment(Align(8));
   MaybeSetComdat(CounterPtr);
   CounterPtr->setLinkage(Linkage);
   // We need a self-link for the counter variable because the ELF section name
   // (that is __llvm_prf_cnts) is a C identifier and considered a GC root in the
   // absence of the SHF_LINK_ORDER flag.
   if (isCounterLinkOrderEnabled())
     CounterPtr->setMetadata(LLVMContext::MD_associated,
-                            MDNode::get(Ctx, ValueAsMetadata::get(Fn)));
+                            MDNode::get(Ctx, ValueAsMetadata::get(CounterPtr)));
 
   auto *Int8PtrTy = Type::getInt8PtrTy(Ctx);
   // Allocate statically the array of pointers to value profile nodes for
   // the current function.
   Constant *ValuesPtrExpr = ConstantPointerNull::get(Int8PtrTy);
   if (ValueProfileStaticAlloc && !needsRuntimeRegistrationOfSectionRange(TT)) {
     uint64_t NS = 0;
     for (uint32_t Kind = IPVK_First; Kind <= IPVK_Last; ++Kind)

llvm/test/Instrumentation/InstrProfiling/icall.ll

 ; STATIC-EXT: call void @__llvm_profile_instrument_target(i64 %3, i8* bitcast ({ i64, i64, i64*, i8*, i8*, i32, [2 x i16] }* @__profd_foo to i8*), i32 zeroext 0)
 ; STATIC-SEXT: call void @__llvm_profile_instrument_target(i64 %3, i8* bitcast ({ i64, i64, i64*, i8*, i8*, i32, [2 x i16] }* @__profd_foo to i8*), i32 signext 0)
 
 ; STATIC: declare void @__llvm_profile_instrument_target(i64, i8*, i32)
 ; STATIC-EXT: declare void @__llvm_profile_instrument_target(i64, i8*, i32 zeroext)
 ; STATIC-SEXT: declare void @__llvm_profile_instrument_target(i64, i8*, i32 signext)
 
 ; METADATA: @__profc_foo = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", align 8, !associated !0
-; METADATA: @__profvp_foo = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", align 8, !associated !1
-; METADATA: @__profc_bar = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", comdat($__profd_bar), align 8, !associated !2
-; METADATA: @__profvp_bar = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", comdat($__profd_bar), align 8, !associated !3
-; METADATA: !0 = !{i32 (i32 ()*)* @foo}
-; METADATA: !1 = !{[1 x i64]* @__profc_foo}
-; METADATA: !2 = !{i32 (i32 ()*)* @bar}
-; METADATA: !3 = !{[1 x i64]* @__profc_bar}
+; METADATA: @__profvp_foo = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", align 8, !associated !0
+; METADATA: @__profc_bar = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", comdat($__profd_bar), align 8, !associated !1
+; METADATA: @__profvp_bar = private global [1 x i64] zeroinitializer, section "{{[^"]+}}", comdat($__profd_bar), align 8, !associated !1
+; METADATA: !0 = !{[1 x i64]* @__profc_foo}
+; METADATA: !1 = !{[1 x i64]* @__profc_bar}

llvm/test/Instrumentation/InstrProfiling/linkage.ll

 @__profn_foo_weak = weak hidden constant [8 x i8] c"foo_weak"
 @"__profn_linkage.ll:foo_internal" = internal constant [23 x i8] c"linkage.ll:foo_internal"
 @__profn_foo_inline = linkonce_odr hidden constant [10 x i8] c"foo_inline"
 @__profn_foo_extern = linkonce_odr hidden constant [10 x i8] c"foo_extern"
 
 ; POSIX: @__profc_foo = hidden global
 ; METADATA-SAME: !associated !0
 ; POSIX: @__profd_foo = hidden global
-; METADATA-SAME: !associated !1
+; METADATA-SAME: !associated !0
 ; COFF: @__profc_foo = internal global
 ; COFF-NOT: comdat
 ; COFF: @__profd_foo = internal global
 define void @foo() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @__profn_foo, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
 ; POSIX: @__profc_foo_weak = weak hidden global
-; METADATA: !associated !2
+; METADATA: !associated !1
 ; POSIX: @__profd_foo_weak = weak hidden global
-; METADATA: !associated !3
+; METADATA: !associated !1
 ; COFF: @__profc_foo_weak = internal global
 ; COFF: @__profd_foo_weak = internal global
 define weak void @foo_weak() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @__profn_foo_weak, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
 ; POSIX: @"__profc_linkage.ll:foo_internal" = internal global
-; METADATA-SAME: !associated !4
+; METADATA-SAME: !associated !2
 ; POSIX: @"__profd_linkage.ll:foo_internal" = internal global
-; METADATA-SAME: !associated !5
+; METADATA-SAME: !associated !2
 ; COFF: @"__profc_linkage.ll:foo_internal" = internal global
 ; COFF: @"__profd_linkage.ll:foo_internal" = internal global
 define internal void @foo_internal() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([23 x i8], [23 x i8]* @"__profn_linkage.ll:foo_internal", i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
 ; POSIX: @__profc_foo_inline = linkonce_odr hidden global
-; METADATA-SAME: !associated !6
+; METADATA-SAME: !associated !3
 ; POSIX: @__profd_foo_inline = linkonce_odr hidden global
-; METADATA-SAME: !associated !7
+; METADATA-SAME: !associated !3
 ; COFF: @__profc_foo_inline = internal global{{.*}} section ".lprfc$M", align 8
 ; COFF: @__profd_foo_inline = internal global{{.*}} section ".lprfd$M", align 8
 define linkonce_odr void @foo_inline() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([10 x i8], [10 x i8]* @__profn_foo_inline, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
 ; LINUX: @__profc_foo_extern = linkonce_odr hidden global {{.*}}section "__llvm_prf_cnts", comdat($__profd_foo_extern), align 8
-; METADATA-SAME: !associated !8
+; METADATA-SAME: !associated !4
 ; LINUX: @__profd_foo_extern = linkonce_odr hidden global {{.*}}section "__llvm_prf_data", comdat, align 8
-; METADATA-SAME: !associated !9
+; METADATA-SAME: !associated !4
 ; MACHO: @__profc_foo_extern = linkonce_odr hidden global
 ; MACHO: @__profd_foo_extern = linkonce_odr hidden global
 ; COFF: @__profc_foo_extern = linkonce_odr hidden global {{.*}}section ".lprfc$M", comdat, align 8
 ; COFF: @__profd_foo_extern = linkonce_odr hidden global {{.*}}section ".lprfd$M", comdat, align 8
 define available_externally void @foo_extern() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([10 x i8], [10 x i8]* @__profn_foo_extern, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
 declare void @llvm.instrprof.increment(i8*, i64, i32, i32)
 
 ; MACHO: define linkonce_odr hidden i32 @__llvm_profile_runtime_user() {{.*}} {
 ; MACHO:   %[[REG:.*]] = load i32, i32* @__llvm_profile_runtime
 ; MACHO:   ret i32 %[[REG]]
 ; MACHO: }
 ; COFF: define linkonce_odr hidden i32 @__llvm_profile_runtime_user() {{.*}} comdat {
 ; LINUX-NOT: define linkonce_odr hidden i32 @__llvm_profile_runtime_user() {{.*}} {
 ; LINUX-NOT:   %[[REG:.*]] = load i32, i32* @__llvm_profile_runtime
 
-; METADATA: !0 = !{void ()* @foo}
-; METADATA: !1 = !{[1 x i64]* @__profc_foo}
-; METADATA: !2 = !{void ()* @foo_weak}
-; METADATA: !3 = !{[1 x i64]* @__profc_foo_weak}
-; METADATA: !4 = !{void ()* @foo_internal}
-; METADATA: !5 = !{[1 x i64]* @"__profc_linkage.ll:foo_internal"}
-; METADATA: !6 = !{void ()* @foo_inline}
-; METADATA: !7 = !{[1 x i64]* @__profc_foo_inline}
-; METADATA: !8 = !{void ()* @foo_extern}
-; METADATA: !9 = !{[1 x i64]* @__profc_foo_extern}
+; METADATA: !0 = !{[1 x i64]* @__profc_foo}
+; METADATA: !1 = !{[1 x i64]* @__profc_foo_weak}
+; METADATA: !2 = !{[1 x i64]* @"__profc_linkage.ll:foo_internal"}
+; METADATA: !3 = !{[1 x i64]* @__profc_foo_inline}
+; METADATA: !4 = !{[1 x i64]* @__profc_foo_extern}

llvm/test/Transforms/PGOProfile/associated.ll

 ; RUN: opt < %s -pgo-instr-gen -instrprof -counter-link-order -S | FileCheck %s
 ; RUN: opt < %s -passes=pgo-instr-gen,instrprof -counter-link-order -S | FileCheck %s
 
 ; CHECK: @__profc_foo = private global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", align 8, !associated !0
-; CHECK: @__profd_foo = private global {{.*}}, section "__llvm_prf_data", align 8, !associated !1
+; CHECK: @__profd_foo = private global {{.*}}, section "__llvm_prf_data", align 8, !associated !0
 
 define void @foo() {
   ret void
 }
 
-; CHECK: !0 = !{void ()* @foo}
-; CHECK: !1 = !{[1 x i64]* @__profc_foo}
+; CHECK: !0 = !{[1 x i64]* @__profc_foo}