Commit bb87364f authored by Fangrui Song's avatar Fangrui Song
Browse files

[ELF][PPC64] Improve "call lacks nop" diagnostic and make it compatible with GCC<5.5 and GCC<6.4 

GCC before r245813 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79439)
did not emit nop after b/bl. This can happen with recursive calls.
r245813 was back ported to GCC 5.5 and GCC 6.4.

This is common, for example, libstdc++.a(locale.o) shipped with GCC 4.9
and many objects in netlib lapack can cause lld to error.  gold allows
such calls to the same section. Our __plt_foo symbol's `section` field
is used for ThunkSection, so we can't implement a similar loosen rule
easily. But we can make use of its `file` field which is currently NULL.

Differential Revision: https://reviews.llvm.org/D71639
parent fb2944bd

lld/ELF/InputSection.cpp

+10 −2
Original line number Diff line number Diff line
@@ -971,8 +971,16 @@ void InputSectionBase::relocateAlloc(uint8_t *buf, uint8_t *bufEnd) { 


      // Patch a nop (0x60000000) to a ld.

      if (rel.sym->needsTocRestore) {

        if (bufLoc + 8 > bufEnd || read32(bufLoc + 4) != 0x60000000) {

          error(getErrorLocation(bufLoc) + "call lacks nop, can't restore toc");

        // gcc/gfortran 5.4, 6.3 and earlier versions do not add nop for

        // recursive calls even if the function is preemptible. This is not

        // wrong in the common case where the function is not preempted at

        // runtime. Just ignore.

        if ((bufLoc + 8 > bufEnd || read32(bufLoc + 4) != 0x60000000) &&

            rel.sym->file != file) {

          // Use substr(6) to remove the "__plt_" prefix.

          errorOrWarn(getErrorLocation(bufLoc) + "call to " +

                      lld::toString(*rel.sym).substr(6) +

                      " lacks nop, can't restore toc");

          break;

        }

        write32(bufLoc + 4, 0xe8410018); // ld %r2, 24(%r1)

lld/ELF/Thunks.cpp

+1 −0
Original line number Diff line number Diff line
@@ -786,6 +786,7 @@ void PPC64PltCallStub::addSymbols(ThunkSection &isec) { 
  Defined *s = addSymbol(saver.save("__plt_" + destination.getName()), STT_FUNC,

                         0, isec);

  s->needsTocRestore = true;

  s->file = destination.file;

}



void PPC64LongBranchThunk::writeTo(uint8_t *buf) {

lld/test/ELF/ppc64-bsymbolic-toc-restore.s

+1 −1
Original line number Diff line number Diff line
@@ -12,7 +12,7 @@ 
# RUN: llvm-objdump -d -r %t | FileCheck %s

# RUN: not ld.lld -shared %t1.o %t2.o -o %t 2>&1 | FileCheck --check-prefix=FAIL %s



# FAIL: call lacks nop, can't restore toc

# FAIL: call to def lacks nop, can't restore toc



# Test to document the toc-restore behavior with -Bsymbolic option. Since

# -Bsymbolic causes the call to bind to the internal defintion we know the

lld/test/ELF/ppc64-error-toc-restore.s

+1 −1
Original line number Diff line number Diff line
@@ -11,7 +11,7 @@ 
// RUN: not ld.lld %t.o %t2.so -o %t 2>&1 | FileCheck %s



# Calling external function bar needs a nop

// CHECK: call lacks nop, can't restore toc

// CHECK: call to foo lacks nop, can't restore toc

    .text

    .abiversion 2

lld/test/ELF/ppc64-error-toc-tail-call.s

+7 −1
Original line number Diff line number Diff line
@@ -11,10 +11,16 @@ 
// RUN: not ld.lld %t.o %t2.so -o %t 2>&1 | FileCheck %s



# A tail call to an external function without a nop should issue an error.

// CHECK: call lacks nop, can't restore toc

// CHECK: call to foo lacks nop, can't restore toc

// CHECK-NOT: lacks nop

    .text

    .abiversion 2



.global _start

_start:

  b foo



  // gcc/gfortran 5.4, 6.3 and earlier versions do not add nop for recursive

  // calls.

  b _start

  b _start