[ELF] Make shouldAddProvideSym return values consistent when demoted to Undefined

Case: `PROVIDE(f1 = bar);` when both `f1` and `bar` are in separate
sections that would be discarded by GC.

Due to `demoteDefined`, `shouldAddProvideSym(f1)` may initially return
false (when Defined) and then return true (been demoted to Undefined).

```
addScriptReferencedSymbolsToSymTable
  shouldAddProvideSym(f1): false
  // the RHS (bar) is not added to `referencedSymbols` and may be GCed
declareSymbols
  shouldAddProvideSym(f1): false
markLive
demoteSymbolsAndComputeIsPreemptible
  // demoted f1 to Undefined
processSymbolAssignments
  addSymbol
    shouldAddProvideSym(f1): true
```

The inconsistency can cause `cmd->expression()` in `addSymbol` to be
evaluated, leading to `symbol not found: bar` errors (since `bar` in the
RHS is not in `referencedSymbols` and is GCed) (#111478).

Fix this by adding a `sym->isUsedInRegularObj` condition, making
`shouldAddProvideSym(f1)` values consistent. In addition, we need a
`sym->exportDynamic` condition to keep provide-shared.s working.

Fixes: ebb326a

Pull Request: #111945

(cherry picked from commit 1c6688a)

Author: MaskRay

lld/ELF/LinkerScript.cpp

@@ -1718,6 +1718,13 @@ void LinkerScript::addScriptReferencedSymbolsToSymTable() {
 }
 
 bool LinkerScript::shouldAddProvideSym(StringRef symName) {
+  // This function is called before and after garbage collection. To prevent
+  // undefined references from the RHS, the result of this function for a
+  // symbol must be the same for each call. We use isUsedInRegularObj to not
+  // change the return value of a demoted symbol. The exportDynamic condition,
+  // while not so accurate, allows PROVIDE to define a symbol referenced by a
+  // DSO.
   Symbol *sym = symtab.find(symName);
-  return sym && !sym->isDefined() && !sym->isCommon();
+  return sym && !sym->isDefined() && !sym->isCommon() &&
+         (sym->isUsedInRegularObj || sym->exportDynamic);
 }

lld/test/ELF/linkerscript/provide-defined.s

@@ -0,0 +1,36 @@
+# 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: # referenced by another relocatable file
+.section .text.f3,"ax"; f3: # live
+.section .text.bar,"ax"; bar:
+
+.comm comm,4,4
+
+#--- b.s
+  call f2
+
+#--- a.t
+SECTIONS {
+  . = . + SIZEOF_HEADERS;
+  PROVIDE(f1 = bar+1);
+  PROVIDE(f2 = bar+2);
+  PROVIDE(f3 = bar+3);
+  PROVIDE(f4 = comm+4);
+}