Don't make _gp absolute

[• rafael]

Summary

_gp points to a position in the file, so it is not really absolute. It is also simpler to not force it to be absolute, so if there is no strong ABI requirement we should not do it.

[atanasyan]

Is it still possible to assign _gp an arbitrary value using a linker script? For example, point into the middle of .sdata section?

[atanasyan]

Is it still possible to assign _gp an arbitrary value using a linker script? For example, point into the middle of .sdata section?

test/ELF/mips-gp-ext.s tests that, no?

You are right. The patch is LGTM.

[atanasyan]

Closed by commit rL313333.

ELF/Writer.cpp

private:
 
   std::vector<PhdrEntry *> createPhdrs();
   void removeEmptyPTLoad();
   void addPtArmExid(std::vector<PhdrEntry *> &Phdrs);
   void assignFileOffsets();
   void assignFileOffsetsBinary();
   void setPhdrs();
   void fixSectionAlignments();
-  void makeMipsGpAbs();
   void openFile();
   void writeTrapInstr();
   void writeHeader();
   void writeSections();
   void writeSectionsBinary();
   void writeBuildId();
 
   std::unique_ptr<FileOutputBuffer> Buffer;
template <class ELFT> void Writer<ELFT>::run() {
   else
     assignFileOffsetsBinary();
 
   setPhdrs();
 
   if (Config->Relocatable) {
     for (OutputSection *Sec : OutputSections)
       Sec->Addr = 0;
-  } else {
-    makeMipsGpAbs();
   }
 
   // It does not make sense try to open the file if we have error already.
   if (ErrorCount)
     return;
   // Write the result down to a file.
   openFile();
   if (ErrorCount)

 static uint16_t getELFType() {
   if (Config->Pic)
     return ET_DYN;
   if (Config->Relocatable)
     return ET_REL;
   return ET_EXEC;
 }
 
-// For some reason we have to make the mips gp symbol absolute.
-template <class ELFT> void Writer<ELFT>::makeMipsGpAbs() {
-  if (ElfSym::MipsGp && ElfSym::MipsGp->Section) {
-    ElfSym::MipsGp->Value += cast<OutputSection>(ElfSym::MipsGp->Section)->Addr;
-    ElfSym::MipsGp->Section = nullptr;
-  }
-}
-
 template <class ELFT> void Writer<ELFT>::writeHeader() {
   uint8_t *Buf = Buffer->getBufferStart();
   memcpy(Buf, "\177ELF", 4);
 
   // Write the ELF header.
   auto *EHdr = reinterpret_cast<Elf_Ehdr *>(Buf);
   EHdr->e_ident[EI_CLASS] = Config->Is64 ? ELFCLASS64 : ELFCLASS32;
   EHdr->e_ident[EI_DATA] = Config->IsLE ? ELFDATA2LSB : ELFDATA2MSB;

test/ELF/basic-mips.s

 # CHECK-NEXT:     Name: _gp
 # CHECK-NEXT:     Value: 0x37FF0
 # CHECK-NEXT:     Size: 0
 # CHECK-NEXT:     Binding: Local
 # CHECK-NEXT:     Type: None (0x0)
 # CHECK-NEXT:     Other [ (0x2)
 # CHECK-NEXT:       STV_HIDDEN (0x2)
 # CHECK-NEXT:     ]
-# CHECK-NEXT:     Section: Absolute
+# CHECK-NEXT:     Section: .got
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Symbol {
 # CHECK-NEXT:     Name: __start
 # CHECK-NEXT:     Value: 0x20000
 # CHECK-NEXT:     Size: 0
 # CHECK-NEXT:     Binding: Global (0x1)
 # CHECK-NEXT:     Type: None (0x0)
 # CHECK-NEXT:     Other: 0

test/ELF/mips-64-gprel-so.s

 # Check setup of GP relative offsets in a function's prologue.
 
 # RUN: llvm-mc -filetype=obj -triple=mips64-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o -shared -o %t.so
 # RUN: llvm-objdump -d -t %t.so | FileCheck %s
 
 # REQUIRES: mips
 
 # CHECK:      Disassembly of section .text:
 # CHECK-NEXT: foo:
 # CHECK-NEXT:    10000:    3c 1c 00 01    lui     $gp, 1
 # CHECK-NEXT:    10004:    03 99 e0 2d    daddu   $gp, $gp, $25
 # CHECK-NEXT:    10008:    67 9c 7f f0    daddiu  $gp, $gp, 32752
 
-# CHECK: 0000000000027ff0   *ABS*   00000000 .hidden _gp
+# CHECK: 0000000000027ff0   .got    00000000 .hidden _gp
 # CHECK: 0000000000010000   .text   00000000 foo
 
   .text
   .global foo
 foo:
   lui     $gp,%hi(%neg(%gp_rel(foo)))
   daddu   $gp,$gp,$t9
   daddiu  $gp,$gp,%lo(%neg(%gp_rel(foo)))

test/ELF/mips-64-rels.s

 #                                                    ^-- 0 - 0xfffffffffffe8010
 #                                                    ^-- %lo(0x17ff0)
 
 # CHECK:      Contents of section .rodata:
 # CHECK-NEXT:  10158 ffffffff fffe8014
 #                    ^-- 0x20004 - 0x37ff0 = 0xfffffffffffe8014
 
 # CHECK: 0000000000020004   .text   00000000 loc
-# CHECK: 0000000000037ff0   *ABS*   00000000 .hidden _gp
+# CHECK: 0000000000037ff0   .got    00000000 .hidden _gp
 # CHECK: 0000000000020000   .text   00000000 __start
 
 # REL:      Relocations [
 # REL-NEXT: ]
 
   .text
   .global  __start
 __start:

test/ELF/mips-got-relocs.s

__start:
   .type  v1,@object
   .size  v1,4
 v1:
   .word 0
 
 # EXE_SYM: Sections:
 # EXE_SYM: .got 0000000c 0000000000030010 DATA
 # EXE_SYM: SYMBOL TABLE:
-# EXE_SYM: 00038000         *ABS*    00000000 .hidden _gp
+# EXE_SYM: 00038000         .got    00000000 .hidden _gp
 #          ^-- .got + GP offset (0x7ff0)
 # EXE_SYM: 00030000 g       .data    00000004 v1
 
 
 # EXE_GOT_BE: Contents of section .got:
 # EXE_GOT_BE:  30010 00000000 80000000 00030000
 #                    ^        ^        ^-- v1 (0x30000)
 #                    |        +-- Module pointer (0x80000000)
 #                    +-- Lazy resolver (0x0)
 
 # EXE_GOT_EL: Contents of section .got:
 # EXE_GOT_EL:  30010 00000000 00000080 00000300
 #                    ^        ^        ^-- v1 (0x30000)
 #                    |        +-- Module pointer (0x80000000)
 #                    +-- Lazy resolver (0x0)
 
 # v1GotAddr (0x3000c) - _gp (0x37ff4) = -0x7fe8 => 0x8018 = 32792
 # EXE_DIS_BE:  20000:  3c 02 80 18  lui $2, 32792
 # EXE_DIS_EL:  20000:  18 80 02 3c  lui $2, 32792
 
 # DSO_SYM: Sections:
 # DSO_SYM: .got 0000000c 0000000000020010 DATA
 # DSO_SYM: SYMBOL TABLE:
-# DSO_SYM: 00028000         *ABS*    00000000 .hidden _gp
+# DSO_SYM: 00028000         .got    00000000 .hidden _gp
 #          ^-- .got + GP offset (0x7ff0)
 # DSO_SYM: 00020000 g       .data    00000004 v1
 
 # DSO_GOT_BE: Contents of section .got:
 # DSO_GOT_BE:  20010 00000000 80000000 00020000
 #                    ^        ^        ^-- v1 (0x20000)
 #                    |        +-- Module pointer (0x80000000)
 #                    +-- Lazy resolver (0x0)

test/ELF/mips-gp-disp.s

 # EXT-SO:      Name: _gp_disp
 # EXT-SO-NEXT: Value: 0x20000
 
 # DIS:      Disassembly of section .text:
 # DIS-NEXT: __start:
 # DIS-NEXT:    10000:  3c 08 00 01  lui   $8, 1
 # DIS-NEXT:    10004:  21 08 7f f0  addi  $8, $8, 32752
 #                                                 ^-- 0x37ff0 & 0xffff
-# DIS: 00027ff0  *ABS*  00000000 .hidden _gp
+# DIS: 00027ff0  .got  00000000 .hidden _gp
 
 # REL:      Relocations [
 # REL-NEXT: ]
 
   .text
   .globl  __start
 __start:
   lui    $t0,%hi(_gp_disp)
   addi   $t0,$t0,%lo(_gp_disp)
   lw     $v0,%call16(_foo)($gp)

test/ELF/mips-gp-local.s

 # 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 03  lui   $8, 3
 # CHECK-NEXT:    20004:  21 08 7f f0  addi  $8, $8, 32752
 
-# CHECK: 00037ff0  *ABS*  00000000 .hidden _gp
+# CHECK: 00037ff0  .got  00000000 .hidden _gp
 
   .text
   .globl  __start
 __start:
   lui    $t0,%hi(__gnu_local_gp)
   addi   $t0,$t0,%lo(__gnu_local_gp)

test/ELF/mips-gprel32-relocs-gp0.s

 # DUMP: Contents of section .rodata:
 # DUMP:  00f4 ffff0004 ffff0008
 #             ^ 0x10004 + 0x7ff0 - 0x27ff0
 #                      ^ 0x10008 + 0x7ff0 - 0x27ff0
 
 # DUMP: SYMBOL TABLE:
 # DUMP: 00010008         .text          00000000 bar
 # DUMP: 00010004         .text          00000000 foo
-# DUMP: 00027ff0         *ABS*          00000000 .hidden _gp
+# DUMP: 00027ff0         .got           00000000 .hidden _gp
 
 # ERR: error: {{.*}}mips-gp0-non-zero.o: unsupported non-zero ri_gp_value
 
   .text
   .global  __start
 __start:
   lw      $t0,%call16(__start)($gp)
 foo:
   nop
 bar:
   nop
 
   .section .rodata, "a"
 v:
   .gpword foo
   .gpword bar

test/ELF/mips-gprel32-relocs.s

 # CHECK: Contents of section .rodata:
 # CHECK:  00f4 fffe8014 fffe8018
 #              ^ 0x10004 - 0x27ff0
 #                       ^ 0x10008 - 0x27ff0
 
 # CHECK: SYMBOL TABLE:
 # CHECK: 00010008         .text           00000000 bar
 # CHECK: 00010004         .text           00000000 foo
-# CHECK: 00027ff0         *ABS*           00000000 .hidden _gp
+# CHECK: 00027ff0         .got            00000000 .hidden _gp

test/ELF/mips-hilo-gp-disp.s

 # EXE:      bar:
 # EXE-NEXT:  2000c:   3c 08 00 01   lui    $8, 1
 #                                              ^-- %hi(0x38000-0x2000c)
 # EXE-NEXT:  20010:   21 08 7f f4   addi   $8, $8, 32756
 #                                                  ^-- %lo(0x38000-0x20010+4)
 
 # EXE: SYMBOL TABLE:
 # EXE: 0002000c     .text   00000000 bar
-# EXE: 00038000     *ABS*   00000000 .hidden _gp
+# EXE: 00038000     .got    00000000 .hidden _gp
 # EXE: 00020000     .text   00000000 __start
 
 # SO:      Disassembly of section .text:
 # SO-NEXT: __start:
 # SO-NEXT:  10000:   3c 08 00 02   lui    $8, 2
 #                                             ^-- %hi(0x28000-0x10000)
 # SO-NEXT:  10004:   21 08 80 00   addi   $8, $8, -32768
 #                                                 ^-- %lo(0x28000-0x10004+4)
 # SO:       bar:
 # SO-NEXT:   1000c:   3c 08 00 01   lui    $8, 1
 #                                              ^-- %hi(0x28000-0x1000c)
 # SO-NEXT:   10010:   21 08 7f f4   addi   $8, $8, 32756
 #                                                  ^-- %lo(0x28000-0x10010+4)
 
 # SO: SYMBOL TABLE:
 # SO: 0001000c     .text   00000000 bar
-# SO: 00028000     *ABS*   00000000 .hidden _gp
+# SO: 00028000     .got    00000000 .hidden _gp
 # SO: 00010000     .text   00000000 __start

test/ELF/mips-micro-relocs.s

 # FIXME-EL-NEXT:      20018:       7c fc 18 80     lw      $3, -32744($gp)
 # FIXME-EL-NEXT:      2001c:       63 fc 18 80     lw      $3, -32744($3)
 # FIXME-EL-NEXT:      20020:       70 8f           beqz16  $6, -32
 # FIXME-EL-NEXT:      20022:       7e 00 00 00     sll     $3, $fp, 0
 # FIXME-EL-NEXT:      20026:       ec cf           b16     -40
 # FIXME-EL-NEXT:      20028:       00 00 00 00     nop
 # FIXME-EL-NEXT:      2002c:       00 94 e8 ff     b       -44
 
-# SYM: 00037ff0         *ABS*           00000000 .hidden _gp
+# SYM: 00037ff0         .got            00000000 .hidden _gp
 # SYM: 00020000 g F     .text           00000000 foo
 # SYM: 00020010         .text           00000000 __start
 
   .text
   .set micromips
   .global __start
 __start:
   lui     $3, %hi(_gp_disp)       # R_MICROMIPS_HI16
   addiu   $3, $3, %lo(_gp_disp)   # R_MICROMIPS_LO16
 
   lw      $3, %call16(foo)($gp)   # R_MICROMIPS_CALL16
   lw      $3, %got(foo)($3)       # R_MICROMIPS_GOT16
 
   beqz16  $6, foo                 # R_MICROMIPS_PC7_S1
   b16     foo                     # R_MICROMIPS_PC10_S1
   b       foo                     # R_MICROMIPS_PC16_S1

test/ELF/mips-n32-rels.s

 #                                                       ^-- 0 - 0xfffe8010
 #                                                       ^-- %lo(0x17ff0)
 
 # CHECK:      Contents of section .rodata:
 # CHECK-NEXT:  100d4 00020004
 #                    ^-- loc
 
 # CHECK: 00020004      .text   00000000 loc
-# CHECK: 00037ff0      *ABS*   00000000 .hidden _gp
+# CHECK: 00037ff0      .got    00000000 .hidden _gp
 # CHECK: 00020000 g  F .text   00000000 __start
 
 # ELF:      Format: ELF32-mips
 # ELF-NEXT: Arch: mips
 # ELF-NEXT: AddressSize: 32bit
 # ELF-NEXT: LoadName:
 # ELF-NEXT: ElfHeader {
 # ELF-NEXT:   Ident {