[PATCH] D20664: [ELF][MIPS] Always resolve MIPS GP-relative relocations to 'local' definitions

[Simon Atanasyan via llvm-commits]

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In case of MIPS, GP-relative relocations always resolve to a definition in a regular input file, ignoring the one-definition rule. Such relocations are used to setup GP relative offsets in a function's prologue. So we, for example, should not attempt to create a dynamic relocation even if the target symbol is preemptible.

Fixes bug 27880.

Repository:
  rL LLVM

http://reviews.llvm.org/D20664

Files:
  ELF/Relocations.cpp
  test/ELF/mips-gprel32-relocs-so.s

Index: test/ELF/mips-gprel32-relocs-so.s
===================================================================
--- /dev/null
+++ test/ELF/mips-gprel32-relocs-so.s
@@ -0,0 +1,24 @@
+# Check R_MIPS_GPREL32 relocation calculation in case of DSO output.
+# GP-relative relocation always resolve to a definition in a regular input file.
+
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
+# RUN: ld.lld -shared -o %t.so %t.o
+# RUN: llvm-objdump -s -t %t.so | FileCheck %s
+
+# REQUIRES: mips
+
+  .text
+  .global foo
+foo:
+  nop
+
+  .section .rodata, "a"
+  .gpword foo
+
+# CHECK: Contents of section .rodata:
+# CHECK:  0114 fffe8010
+#              ^ 0x10000 - 0x27ff0
+
+# CHECK: SYMBOL TABLE:
+# CHECK: 00027ff0   .got    00000000 .hidden _gp
+# CHECK: 00010000   .text   00000000 foo
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -231,6 +231,14 @@
          Expr == R_RELAX_GOT_PC;
 }
 
+static bool isForcedLinkTimeConstant(RelExpr E) {
+  // In case of MIPS GP-relative relocations always resolve to a definition
+  // in a regular input file, ignoring the one-definition rule. So we,
+  // for example, should not attempt to create a dynamic relocation even
+  // if the target symbol is preemptible.
+  return Config->EMachine == EM_MIPS && E == R_GOTREL;
+}
+
 template <class ELFT>
 static bool isStaticLinkTimeConstant(RelExpr E, uint32_t Type,
                                      const SymbolBody &Body) {
@@ -241,6 +249,9 @@
       E == R_TLSGD || E == R_PPC_PLT_OPD)
     return true;
 
+  if (isForcedLinkTimeConstant(E))
+    return true;
+
   // These never do, except if the entire file is position dependent or if
   // only the low bits are used.
   if (E == R_GOT || E == R_PLT)
@@ -504,7 +515,7 @@
     }
 
     if (needsPlt(Expr) || Expr == R_THUNK || refersToGotEntry(Expr) ||
-        !Body.isPreemptible()) {
+        !Body.isPreemptible() || isForcedLinkTimeConstant(Expr)) {
       // If the relocation points to something in the file, we can process it.
       bool Constant = isStaticLinkTimeConstant<ELFT>(Expr, Type, Body);
 


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