[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

2 files changed, 11 insertions, 2 deletions

diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index 959dcb8533d..7dceddb5659 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -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)
diff --git a/lld/ELF/Thunks.cpp b/lld/ELF/Thunks.cpp
index 446c88c92d9..7b927a434e3 100644
--- a/lld/ELF/Thunks.cpp
+++ b/lld/ELF/Thunks.cpp
@@ -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) {