[ARM][LLD] Synthetic symbol definitions for ARM static linking

[peter.smith]

Summary

When static linking in ARM (like Mips) __tls_get_addr is defined by the library so we should not define it as a synthetic.

We also need to add exidx_start and exidx_end for the .ARM.exidx section as the static libc library startup code is expecting them to be defined by the default linker script for static linking on ARM when GNU ld is used.

Extract from arm-linux-gnueabihf-ld --static --verbose
PROVIDE_HIDDEN (__exidx_start = .);
.ARM.exidx : { *(.ARM.exidx* .gnu.linkonce.armexidx.*) }
PROVIDE_HIDDEN (__exidx_end = .);

There is still more work to do for static linking to work on ARM, there are problems with ifunc, TLS and unresolved weak references. I have patches ready for most of these.

[ruiu]

LGTM

ELF/Writer.cpp

961 ↗

(On Diff #75843)

Does ".ARM.exidx" always exist? If not, you want to do something like this

if (OutputSectionBase<ELFT> *Sec = findSection(".ARM.exidx")
  Define("__exidx_start", "__exidx_end", Sec);

[peter.smith]

Thanks for the suggestion, I've incorporated it and committed in r285279

lld/trunk/ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::addReservedSymbols() {
   // Given that the symbol is effectively unused, we just create a dummy
   // hidden one to avoid the undefined symbol error.
   if (!Config->Relocatable)
     Symtab<ELFT>::X->addIgnored("_GLOBAL_OFFSET_TABLE_");
 
   // __tls_get_addr is defined by the dynamic linker for dynamic ELFs. For
   // static linking the linker is required to optimize away any references to
   // __tls_get_addr, so it's not defined anywhere. Create a hidden definition
-  // to avoid the undefined symbol error. As usual as special case is MIPS -
-  // MIPS libc defines __tls_get_addr itself because there are no TLS
-  // optimizations for this target.
-  if (!Out<ELFT>::DynSymTab && Config->EMachine != EM_MIPS)
+  // to avoid the undefined symbol error. As usual special cases are ARM and
+  // MIPS - the libc for these targets defines __tls_get_addr itself because
+  // there are no TLS optimizations for these targets.
+  if (!Out<ELFT>::DynSymTab &&
+      (Config->EMachine != EM_MIPS && Config->EMachine != EM_ARM))
     Symtab<ELFT>::X->addIgnored("__tls_get_addr");
 
   // If linker script do layout we do not need to create any standart symbols.
   if (ScriptConfig->HasSections)
     return;
 
   ElfSym<ELFT>::EhdrStart = Symtab<ELFT>::X->addIgnored("__ehdr_start");
 
auto Define = [&](StringRef Start, StringRef End,
     addOptionalSynthetic(Start, OS, 0);
     addOptionalSynthetic(End, OS, OS ? DefinedSynthetic<ELFT>::SectionEnd : 0);
   };
 
   Define("__preinit_array_start", "__preinit_array_end",
          Out<ELFT>::PreinitArray);
   Define("__init_array_start", "__init_array_end", Out<ELFT>::InitArray);
   Define("__fini_array_start", "__fini_array_end", Out<ELFT>::FiniArray);
+
+  if (OutputSectionBase<ELFT> *Sec = findSection(".ARM.exidx"))
+    Define("__exidx_start", "__exidx_end", Sec);
 }
 
 // If a section name is valid as a C identifier (which is rare because of
 // the leading '.'), linkers are expected to define __start_<secname> and
 // __stop_<secname> symbols. They are at beginning and end of the section,
 // respectively. This is not requested by the ELF standard, but GNU ld and
 // gold provide the feature, and used by many programs.
 template <class ELFT>

lld/trunk/test/ELF/arm-static-defines.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: ld.lld %t --static -o %t2 2>&1
+// RUN: llvm-readobj --symbols %t2 | FileCheck %s
+// REQUIRES: arm
+
+// Check that on ARM we don't get a multiply defined symbol for __tls_get_addr
+// and undefined symbols for references to __exidx_start and __exidx_end
+ .syntax unified
+.section .text
+ .global __tls_get_addr
+__tls_get_addr:
+ bx lr
+
+ .global _start
+ .global __exidx_start
+ .global __exidx_end
+_start:
+ .fnstart
+ bx lr
+ .word __exidx_start
+ .word __exidx_end
+ .cantunwind
+ .fnend
+
+// CHECK:          Name: __exidx_end
+// CHECK-NEXT:     Value: 0x100DC
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Local
+// CHECK-NEXT:     Type: None
+// CHECK-NEXT:     Other [
+// CHECK-NEXT:       STV_HIDDEN
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Section: .ARM.exidx
+// CHECK:          Name: __exidx_start
+// CHECK-NEXT:     Value: 0x100D4
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Local
+// CHECK-NEXT:     Type: None
+// CHECK-NEXT:     Other [
+// CHECK-NEXT:       STV_HIDDEN
+// CHECK-NEXT:     ]
+// CHECK-NEXT:   Section: .ARM.exidx
+// CHECK:          Symbol {
+// CHECK-NEXT:     Name: __tls_get_addr