[ELF][MIPS] Add handling for __gnu_local_gp symbol

This symbol is a "fake" symbol like "_gp_disp" and denotes
the GOT + 0x7FF0 value.

llvm-svn: 259781

lld/ELF/Config.h

@@ -37,6 +37,7 @@ enum ELFKind {
 struct Configuration {
   SymbolBody *EntrySym = nullptr;
   SymbolBody *MipsGpDisp = nullptr;
+  SymbolBody *MipsLocalGp = nullptr;
   InputFile *FirstElf = nullptr;
   llvm::StringRef DynamicLinker;
   llvm::StringRef Entry;

lld/ELF/Driver.cpp

@@ -328,6 +328,7 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &Args) {
     // start of function and gp pointer into GOT. Use 'strong' variant of
     // the addIgnored to prevent '_gp_disp' substitution.
     Config->MipsGpDisp = Symtab.addIgnored("_gp_disp");
+    Config->MipsLocalGp = Symtab.addIgnored("__gnu_local_gp");
 
     // Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
     // so that it points to an absolute address which is relative to GOT.

lld/ELF/InputSection.cpp

@@ -234,6 +234,8 @@ void InputSectionBase<ELFT>::relocate(uint8_t *Buf, uint8_t *BufEnd,
         SymVA = getMipsGpAddr<ELFT>() - AddrLoc;
       else if (Type == R_MIPS_LO16 && Body == Config->MipsGpDisp)
         SymVA = getMipsGpAddr<ELFT>() - AddrLoc + 4;
+      else if (Body == Config->MipsLocalGp)
+        SymVA = getMipsGpAddr<ELFT>();
     }
     uintX_t Size = Body->getSize<ELFT>();
     Target->relocateOne(BufLoc, BufEnd, Type, AddrLoc, SymVA + A, Size + A,

lld/ELF/Writer.cpp

@@ -388,7 +388,7 @@ void Writer<ELFT>::scanRelocs(
         // relocation too because that case is possible for executable file
         // linking only.
         continue;
-      if (Body == Config->MipsGpDisp)
+      if (Body == Config->MipsGpDisp || Body == Config->MipsLocalGp)
         // MIPS _gp_disp designates offset between start of function and gp
         // pointer into GOT therefore any relocations against it do not require
         // dynamic relocation.

lld/test/ELF/mips-gp-local.s

@@ -0,0 +1,20 @@
+# Check handling of relocations against __gnu_local_gp symbol.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
+# RUN: ld.lld -o %t.exe %t.o
+# RUN: llvm-objdump -d -t %t.exe | FileCheck %s
+
+# REQUIRES: mips
+
+# CHECK:      Disassembly of section .text:
+# CHECK-NEXT: __start:
+# CHECK-NEXT:    20000:  3c 08 00 00  lui   $8, 0
+# CHECK-NEXT:    20004:  21 08 00 00  addi  $8, $8, 0
+
+# CHECK: 00000000  *ABS*  00000000 _gp
+
+  .text
+  .globl  __start
+__start:
+  lui    $t0,%hi(__gnu_local_gp)
+  addi   $t0,$t0,%lo(__gnu_local_gp)