[ELF] Fix the semantic of PROVIDE in Linker scripts

[davide]

Summary

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/assignments.html says that a symbol needs to be defined only if it is referenced and is not defined by any object included in the link. We created the symbol in the symbol table no matter what.

[ruiu]

LGTM

ELF/LinkerScript.cpp

355–356

Move "to it." to the end of previous line.

ELF/LinkerScript.cpp

 
 template <class ELFT> void LinkerScript<ELFT>::addScriptedSymbols() {
   for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands) {
     auto *Cmd = dyn_cast<SymbolAssignment>(Base.get());
     if (!Cmd || Cmd->Name == ".")
       continue;
 
     SymbolBody *B = Symtab<ELFT>::X->find(Cmd->Name);
-    if (!B || B->isUndefined())
+    // The semantic of PROVIDE is that of introducing a symbol
+    // only if it's not defined and there's at least a reference
+    // to it.
+    if ((!B && !Cmd->Provide) || (B && B->isUndefined()))
       Symtab<ELFT>::X->addAbsolute(Cmd->Name,
                                    Cmd->Hidden ? STV_HIDDEN : STV_DEFAULT);
     else
       // Symbol already exists in symbol table. If it is provided
       // then we can't override its value.
       Cmd->Ignore = Cmd->Provide;
   }
 }

test/ELF/linkerscript/linkerscript-symbolreferenced.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# Provide new symbol. The value should be 1, like set in PROVIDE()
+# RUN: echo "SECTIONS { PROVIDE(newsym = 1);}" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -t %t1 | FileCheck --check-prefix=PROVIDE1 %s
+# PROVIDE1: 0000000000000001         *ABS*    00000000 newsym
+
+# Provide new symbol (hidden). The value should be 1
+# RUN: echo "SECTIONS { PROVIDE_HIDDEN(newsym = 1);}" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -t %t1 | FileCheck --check-prefix=HIDDEN1 %s
+# HIDDEN1: 0000000000000001         *ABS*    00000000 .hidden newsym
+
+.global _start
+_start:
+ nop
+
+.globl patatino
+patatino:
+  movl newsym, %eax

test/ELF/linkerscript/linkerscript-symbols.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 
 # Simple symbol assignment. Should raise conflict in case we 
 # have duplicates in any input section, but currently simply
 # replaces the value.
 # RUN: echo "SECTIONS {.text : {*(.text.*)} text_end = .;}" > %t.script
 # RUN: ld.lld -o %t1 --script %t.script %t
 # RUN: llvm-objdump -t %t1 | FileCheck --check-prefix=SIMPLE %s
 # SIMPLE: 0000000000000121         *ABS*    00000000 text_end
 
-# Provide new symbol. The value should be 1, like set in PROVIDE()
+# The symbol is not referenced. Don't provide it.
 # RUN: echo "SECTIONS { PROVIDE(newsym = 1);}" > %t.script
 # RUN: ld.lld -o %t1 --script %t.script %t
 # RUN: llvm-objdump -t %t1 | FileCheck --check-prefix=PROVIDE1 %s
-# PROVIDE1: 0000000000000001         *ABS*    00000000 newsym
+# PROVIDE1-NOT: 0000000000000001         *ABS*    00000000 newsym
 
-# Provide new symbol (hidden). The value should be 1
+# The symbol is not referenced. Don't provide it.
 # RUN: echo "SECTIONS { PROVIDE_HIDDEN(newsym = 1);}" > %t.script
 # RUN: ld.lld -o %t1 --script %t.script %t
 # RUN: llvm-objdump -t %t1 | FileCheck --check-prefix=HIDDEN1 %s
-# HIDDEN1: 0000000000000001         *ABS*    00000000 .hidden newsym
+# HIDDEN1-NOT: 0000000000000001         *ABS*    00000000 .hidden newsym
 
 # Provide existing symbol. The value should be 0, even though we 
 # have value of 1 in PROVIDE()
 # RUN: echo "SECTIONS { PROVIDE(somesym = 1);}" > %t.script
 # RUN: ld.lld -o %t1 --script %t.script %t
 # RUN: llvm-objdump -t %t1 | FileCheck --check-prefix=PROVIDE2 %s
 # PROVIDE2: 0000000000000000         *ABS*    00000000 somesym