[lld] [ELF] Make shouldAddProvideSym return values consistent when demoted to Undefined (PR #111945)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay updated https://github.com/llvm/llvm-project/pull/111945

>From 6ca49904c11182318fffee95307ba45ce1febc7b Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Thu, 10 Oct 2024 20:58:02 -0700
Subject: [PATCH 1/2] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20in?=
 =?UTF-8?q?itial=20version?=
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Created using spr 1.3.5-bogner
---
 lld/ELF/LinkerScript.cpp                    |  7 ++++-
 lld/test/ELF/linkerscript/provide-defined.s | 33 +++++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 lld/test/ELF/linkerscript/provide-defined.s

diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index f3f95ec589bd82..2247d592c8bca3 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -1811,6 +1811,11 @@ void LinkerScript::addScriptReferencedSymbolsToSymTable() {
 }
 
 bool LinkerScript::shouldAddProvideSym(StringRef symName) {
+  // A Defined may be demoted to Undefined but the return value must stay the
+  // same. Use isUsedInRegularObj to ensure this. The exportDynamic condition,
+  // while not so accurate, allows PROVIDE to define a symbol referenced by a
+  // DSO.
   Symbol *sym = elf::ctx.symtab->find(symName);
-  return sym && !sym->isDefined() && !sym->isCommon();
+  return sym && !sym->isDefined() && !sym->isCommon() &&
+         (sym->isUsedInRegularObj || sym->exportDynamic);
 }
diff --git a/lld/test/ELF/linkerscript/provide-defined.s b/lld/test/ELF/linkerscript/provide-defined.s
new file mode 100644
index 00000000000000..6ebfd706c16e1b
--- /dev/null
+++ b/lld/test/ELF/linkerscript/provide-defined.s
@@ -0,0 +1,33 @@
+# REQUIRES: x86
+## Test the GC behavior when the PROVIDE symbol is defined by a relocatable file.
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 b.s -o b.o
+# RUN: ld.lld -T a.t --gc-sections a.o b.o -o a
+# RUN: llvm-readelf -s a | FileCheck %s
+
+# CHECK:     1: {{.*}}               0 NOTYPE  GLOBAL DEFAULT     1 _start
+# CHECK-NEXT:2: {{.*}}               0 NOTYPE  GLOBAL DEFAULT     2 f3
+# CHECK-NOT: {{.}}
+
+#--- a.s
+.global _start, f1, f2, f3, bar
+_start:
+  call f3
+
+.section .text.f1,"ax"; f1:
+.section .text.f2,"ax"; f2:
+.section .text.f3,"ax"; f3:
+.section .text.bar,"ax"; bar:
+
+#--- b.s
+  call f2
+
+#--- a.t
+SECTIONS {
+  . = . + SIZEOF_HEADERS;
+  PROVIDE(f1 = bar+1);
+  PROVIDE(f2 = bar+2);
+  PROVIDE(f3 = bar+3);
+}

>From 665ccc46734eeb8b33930211e0499f9a4b478d1f Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Fri, 11 Oct 2024 08:45:21 -0700
Subject: [PATCH 2/2] add common symbol test

Created using spr 1.3.5-bogner
---
 lld/test/ELF/linkerscript/provide-defined.s | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/lld/test/ELF/linkerscript/provide-defined.s b/lld/test/ELF/linkerscript/provide-defined.s
index 6ebfd706c16e1b..1d44bef3d4068d 100644
--- a/lld/test/ELF/linkerscript/provide-defined.s
+++ b/lld/test/ELF/linkerscript/provide-defined.s
@@ -17,10 +17,12 @@ _start:
   call f3
 
 .section .text.f1,"ax"; f1:
-.section .text.f2,"ax"; f2:
-.section .text.f3,"ax"; f3:
+.section .text.f2,"ax"; f2: # referenced by another relocatable file
+.section .text.f3,"ax"; f3: # live
 .section .text.bar,"ax"; bar:
 
+.comm comm,4,4
+
 #--- b.s
   call f2
 
@@ -30,4 +32,5 @@ SECTIONS {
   PROVIDE(f1 = bar+1);
   PROVIDE(f2 = bar+2);
   PROVIDE(f3 = bar+3);
+  PROVIDE(f4 = comm+4);
 }