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[ELF][MIPS] Make R_MIPS_LO16 a relative relocation if it references _gp_disp symbol

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The _gp_disp symbol designates offset between start of function and 'gp'
pointer into GOT. The following code is a typical MIPS function preamble
used to setup $gp register:

lui    $gp, %hi(_gp_disp)
addi   $gp, $gp, %lo(_gp_disp)

To calculate R_MIPS_HI16 / R_MIPS_LO16 relocations results we use
the following formulas:

%hi(_gp - P + A)
%lo(_gp - P + A + 4),
where _gp is a value of _gp symbol, A is addend, and P current address.

The R_MIPS_LO16 relocation references _gp_disp symbol is always the second
instruction. That is why we need four byte adjustments. The patch assigns
R_PC type for R_MIPS_LO16 relocation and adjusts its addend by 4. That fix
R_MIPS_LO16 calculation.

For details see p. 4-19 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf

Differential Revision: http://reviews.llvm.org/D19115

llvm-svn: 266368
This commit is contained in:
Simon Atanasyan
2016-04-14 21:10:05 +00:00
parent 9a15293ec1
commit 1ca263c890
3 changed files with 26 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

1
lld/ELF/Target.cpp
View File

@@ -1579,6 +1579,7 @@ RelExpr MipsTargetInfo<ELFT>::getRelExpr(uint32_t Type,
default:
return R_ABS;
case R_MIPS_HI16:
case R_MIPS_LO16:
// MIPS _gp_disp designates offset between start of function and 'gp'
// pointer into GOT. __gnu_local_gp is equal to the current value of
// the 'gp'. Therefore any relocations against them do not require

12
lld/ELF/Writer.cpp
View File

@@ -462,14 +462,22 @@ void Writer<ELFT>::scanRelocs(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels) {
if (auto *S = dyn_cast<SharedSymbol<ELFT>>(&Body))
S->File->IsUsed = true;
RelExpr Expr = Target->getRelExpr(Type, Body);
uintX_t Addend = getAddend<ELFT>(RI);
const uint8_t *BufLoc = Buf + RI.r_offset;
if (!RelTy::IsRela)
Addend += Target->getImplicitAddend(BufLoc, Type);
if (Config->EMachine == EM_MIPS)
if (Config->EMachine == EM_MIPS) {
Addend += findMipsPairedAddend<ELFT>(Buf, BufLoc, Body, &RI, E);
if (Type == R_MIPS_LO16 && Expr == R_PC)
// R_MIPS_LO16 expression has R_PC type iif the target is _gp_disp
// symbol. In that case we should use the following formula for
// calculation "AHL + GP – P + 4". Let's add 4 right here.
// For details see p. 4-19 at
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
Addend += 4;
}
RelExpr Expr = Target->getRelExpr(Type, Body);
if (unsigned Processed =
handleTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr)) {
I += (Processed - 1);

17
lld/test/ELF/mips-hilo-gp-disp.s
View File

@@ -16,6 +16,9 @@ __start:
lui $t0,%hi(_gp_disp)
addi $t0,$t0,%lo(_gp_disp)
lw $v0,%call16(_foo)($gp)
bar:
lui $t0,%hi(_gp_disp)
addi $t0,$t0,%lo(_gp_disp)
# EXE: Disassembly of section .text:
# EXE-NEXT: __start:
@@ -23,11 +26,16 @@ __start:
# ^-- %hi(0x37ff0-0x20000)
# EXE-NEXT: 20004: 21 08 7f f0 addi $8, $8, 32752
# ^-- %lo(0x37ff0-0x20004+4)
# EXE: bar:
# EXE-NEXT: 2000c: 3c 08 00 01 lui $8, 1
# ^-- %hi(0x37ff0-0x2000c)
# EXE-NEXT: 20010: 21 08 7f e4 addi $8, $8, 32740
# ^-- %lo(0x37ff0-0x20010+4)
# EXE: SYMBOL TABLE:
# EXE: 0002000c .text 00000000 bar
# EXE: 00037ff0 .got 00000000 .hidden _gp
# EXE: 00020000 .text 00000000 __start
# EXE: 00020010 .text 00000000 _foo
# SO: Disassembly of section .text:
# SO-NEXT: __start:
@@ -35,8 +43,13 @@ __start:
# ^-- %hi(0x27ff0-0x10000)
# SO-NEXT: 10004: 21 08 7f f0 addi $8, $8, 32752
# ^-- %lo(0x27ff0-0x10004+4)
# SO: bar:
# SO-NEXT: 1000c: 3c 08 00 01 lui $8, 1
# ^-- %hi(0x27ff0-0x1000c)
# SO-NEXT: 10010: 21 08 7f e4 addi $8, $8, 32740
# ^-- %lo(0x27ff0-0x10010+4)
# SO: SYMBOL TABLE:
# SO: 0001000c .text 00000000 bar
# SO: 00027ff0 .got 00000000 .hidden _gp
# SO: 00010000 .text 00000000 __start
# SO: 00010010 .text 00000000 _foo