[PowerPC] fix broken JIT-compiled code with tail call optimization

[inouehrs]

Summary

The relocation for branch instructions in the dynamic loader of ExecutionEngine assumes branch instructions with R_PPC64_REL24 relocation type are only bl. However, with the tail call optimization, b instructions can be also used to jump into another function.
This patch makes the relocation to keep bits in the branch instruction other than the jump offset to avoid relocation rewrites a b instruction into bl.

This problem makes NNVM, a deep learning optimizer which uses LLVM as backend https://github.com/dmlc/nnvm, crash on POWER.

[hfinkel]

LGTM

llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp

if (SignExtend64<32>(Result) != Result)
       llvm_unreachable("Relocation R_PPC64_ADDR32 overflow");
     writeInt32BE(LocalAddress, Result);
   } break;
   case ELF::R_PPC64_REL24: {
     uint64_t FinalAddress = Section.getLoadAddressWithOffset(Offset);
     int64_t delta = static_cast<int64_t>(Value - FinalAddress + Addend);
     if (SignExtend64<26>(delta) != delta)
       llvm_unreachable("Relocation R_PPC64_REL24 overflow");
-    // Generates a 'bl <address>' instruction
-    writeInt32BE(LocalAddress, 0x48000001 | (delta & 0x03FFFFFC));
+    // We preserve bits other than LI field, i.e. PO and AA/LK fields.
+    uint32_t Inst = readBytesUnaligned(LocalAddress, 4);
+    writeInt32BE(LocalAddress, (Inst & 0xFC000003) | (delta & 0x03FFFFFC));
   } break;
   case ELF::R_PPC64_REL32: {
     uint64_t FinalAddress = Section.getLoadAddressWithOffset(Offset);
     int64_t delta = static_cast<int64_t>(Value - FinalAddress + Addend);
     if (SignExtend64<32>(delta) != delta)
       llvm_unreachable("Relocation R_PPC64_REL32 overflow");
     writeInt32BE(LocalAddress, delta);
   } break;

llvm/trunk/test/ExecutionEngine/RuntimeDyld/PowerPC/ppc64_reloc.s

+# test for little endian
+# RUN: rm -rf %t && mkdir -p %t
+# RUN: llvm-mc -triple=powerpc64le-unknown-linux-gnu -filetype=obj -o %t/ppc64_reloc.o %s
+# RUN: llvm-mc -triple=powerpc64le-unknown-linux-gnu -filetype=obj -o %t/ppc64_elf_module_b.o %S/Inputs/ppc64_elf_module_b.s
+# RUN: llvm-rtdyld -triple=powerpc64le-unknown-linux-gnu -verify -check=%s %t/ppc64_reloc.o %t/ppc64_elf_module_b.o
+# test for big endian
+# RUN: rm -rf %t && mkdir -p %t
+# RUN: llvm-mc -triple=powerpc64-unknown-linux-gnu -filetype=obj -o %t/ppc64_reloc.o %s
+# RUN: llvm-mc -triple=powerpc64-unknown-linux-gnu -filetype=obj -o %t/ppc64_elf_module_b.o %S/Inputs/ppc64_elf_module_b.s
+# RUN: llvm-rtdyld -triple=powerpc64-unknown-linux-gnu -verify -check=%s %t/ppc64_reloc.o %t/ppc64_elf_module_b.o
+
+  .text
+  .abiversion 2
+  .file "test.c"
+  .globl  func
+  .p2align  4
+  .type func,@function
+func:                                   # @func
+.Lfunc_begin0:
+.Lfunc_gep0:
+  addis 2, 12, .TOC.-.Lfunc_gep0@ha
+  addi 2, 2, .TOC.-.Lfunc_gep0@l
+.Lfunc_lep0:
+  .localentry func, .Lfunc_lep0-.Lfunc_gep0
+  mflr 0
+  std 31, -8(1)
+  std 0, 16(1)
+  stdu 1, -112(1)
+  mr 31, 1
+# confirm that LK flag is set for bl
+# rtdyld-check: (*{4}call_bl) & 1 = 1
+call_bl:
+  bl foo
+  nop
+
+  li 3, 0
+  addi 1, 1, 112
+  ld 0, 16(1)
+  ld 31, -8(1)
+  mtlr 0
+# confirm that LK flag is not set for b
+# rtdyld-check: (*{4}call_b) & 1 = 0
+call_b:
+  b foo
+  .long 0
+  .quad 0
+.Lfunc_end0:
+  .size func, .Lfunc_end0-.Lfunc_begin0