[ELF][MIPS] Make R_MIPS_LO16 a relative relocation if it references _gp_disp symbol
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
diff --git a/lld/ELF/Target.cpp b/lld/ELF/Target.cpp
index 8b12d75..240eb98 100644
--- a/lld/ELF/Target.cpp
+++ b/lld/ELF/Target.cpp
@@ -1579,6 +1579,7 @@
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
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 31f2d3e..7a75d23 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -462,14 +462,22 @@
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);