[ELF] Don't warn on two legitimate cases when reading .llvm.call-graph-profile

[MaskRay]

Summary

Before, superfluous warnings were emitted for the following two cases:

1. When from symbol was in a discarded section. The profile should be thought of as affiliated to the section. It makes sense to ignore the profile if the section is discarded.

2. When to symbol was in a shared object. The object file containing the profile may not know about the to symbol, which can reside in another object file (useful profile) or a shared object (not useful as symbols in the shared object are fixed and unorderable). It makes sense to ignore the profile from the object file.
   
   Note, the warning when to symbol was undefined was suppressed in D53044, which is still useful for --symbol-ordering-file=

This patch silences the warnings. The check is actually more relaxed (no
warnings if either From or To is not Defined) for simplicity and I don't
see a compelling reason to warn on more cases.

[ruiu]

ELF/Driver.cpp

687–690

SymD -> Sym

691–710

SB -> Sec

by convention.

693

This needs a comment.

[MaskRay]

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[MaskRay]

Improve the comment

[ruiu]

ELF/Driver.cpp

693

When you write a multi-line comment, please add a blank line before first line.

696

You generally shouldn't break a line before ")".

[MaskRay]

Fix comment as ruiu@ suggested

[ruiu]

test/ELF/cgprofile-shared-warn.s

7

cgprofile-shared-warn.s is very small. Can you avoid checking in that file by doing this?

echo '.globl B; B: ret' | llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t1.o

[MaskRay]

Inline Inputs/cgprofile-shared-warn.s

[ruiu]

LGTM

ELF/Driver.cpp

for (StringRef L : args::getLines(MB)) {
 
     if (const InputSectionBase *FromSB = FindSection(Fields[0]))
       if (const InputSectionBase *ToSB = FindSection(Fields[1]))
         Config->CallGraphProfile[std::make_pair(FromSB, ToSB)] += Count;
   }
 }
 
 template <class ELFT> static void readCallGraphsFromObjectFiles() {
-  auto FindSection = [&](const Symbol *Sym) -> const InputSectionBase * {
-    warnUnorderableSymbol(Sym);
-    if (const auto *SymD = dyn_cast<Defined>(Sym))
-      return dyn_cast_or_null<InputSectionBase>(SymD->Section);
-    return nullptr;
-  };
-
   for (auto File : ObjectFiles) {
     auto *Obj = cast<ObjFile<ELFT>>(File);
     for (const Elf_CGProfile_Impl<ELFT> &CGPE : Obj->CGProfile) {
-      const InputSectionBase *FromSB =
-          FindSection(&Obj->getSymbol(CGPE.cgp_from));
-      const InputSectionBase *ToSB = FindSection(&Obj->getSymbol(CGPE.cgp_to));
-      if (!FromSB || !ToSB)
+      auto *FromSym = dyn_cast<Defined>(&Obj->getSymbol(CGPE.cgp_from));
+      auto *ToSym = dyn_cast<Defined>(&Obj->getSymbol(CGPE.cgp_to));
+      if (!FromSym || !ToSym)
         continue;
-      Config->CallGraphProfile[{FromSB, ToSB}] += CGPE.cgp_weight;
+      auto *FromSec = dyn_cast_or_null<InputSectionBase>(FromSym->Section);
+      auto *ToSec = dyn_cast_or_null<InputSectionBase>(ToSym->Section);
+
+      // The profile from .llvm.call-graph-profile is conceptually affiliated to
+      // FromSec. Don't warn unorderable symbol if FromSym is not absolute
+      // (FromSec isn't null) and the section is discarded
+      // (!FromSec->Repl->Live).
+      //
+      // We also don't want to warn when ToSym is undefined or is in a shared
+      // object (as symbols in shared objects are fixed and unorderable).
+      //
+      // The check used here is more relaxed (no warning if either FromSym or
+      // ToSym is not Defined) for simplicity and there is no compelling reason
+      // to warn on more cases.
+      if (!FromSec || FromSec->Repl->Live) {
+        warnUnorderableSymbol(FromSym);
+        warnUnorderableSymbol(ToSym);
+      }
+      if (FromSec && ToSec)
+        Config->CallGraphProfile[{FromSec, ToSec}] += CGPE.cgp_weight;
     }
   }
 }
 
 static bool getCompressDebugSections(opt::InputArgList &Args) {
   StringRef S = Args.getLastArgValue(OPT_compress_debug_sections, "none");
   if (S == "none")
     return false;

test/ELF/Inputs/cgprofile-shared-warn.s

+.globl B
+B:
+  ret

test/ELF/cgprofile-obj-warn.s

A:
     .cg_profile A, B, 100
     .cg_profile A, C, 40
     .cg_profile B, C, 30
     .cg_profile adena1, A, 30
     .cg_profile A, adena2, 30
     .cg_profile poppy, A, 30
 
 # CHECK: unable to order absolute symbol: B
-# CHECK: unable to order undefined symbol: adena1
-# CHECK: unable to order undefined symbol: adena2
-# CHECK: unable to order undefined symbol: poppy
 
 # RUN: ld.lld %t -o /dev/null \
 # RUN:   -noinhibit-exec -icf=all --no-warn-symbol-ordering 2>&1 \
 # RUN:   | FileCheck %s --check-prefix=NOWARN
 # NOWARN-NOT: unable to order

test/ELF/cgprofile-shared-warn.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: ld.lld --shared %t.o -o /dev/null 2>&1 | count 0
 # RUN: ld.lld -e A --unresolved-symbols=ignore-all %t.o -o /dev/null 2>&1 | count 0
-    .section    .text.A,"ax",@progbits
-    .globl  A
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/cgprofile-shared-warn.s -o %t1.o
+# RUN: ld.lld --shared %t1.o -o %t1.so
+# RUN: ld.lld -e A %t.o %t1.so -o /dev/null 2>&1 | count 0
+
+# RUN: ld.lld --gc-sections %t.o %t1.so -o /dev/null 2>&1 | count 0
+.globl _start
+_start:
+  ret
+
+.section .text.A,"ax",@progbits
+.globl A
 A:
-    callq B
 
-    .cg_profile A, B, 10