[PPC64] Preserve LocalEntry when linking

[luporl]

Summary

On PowerPC64, it is necessary to keep the LocalEntry bits in st_other,
especially when -r is used. Otherwise, when the resulting object is used
in a posterior linking, LocalEntry info will be unavailable and functions may
be called through the wrong entrypoint.

[ruiu]

Sorry for the belated response. I missed this one for some reason.

Can you add a test?

ELF/SyntheticSections.cpp

2003

I think you need to define a new function that is so trivial. I'd hardcode & 0xe0 with an explanation what that is.

[MaskRay]

ELF/Target.h

182

Perhaps add a comment referencing Arch/PPC64.cpp getPPC64GlobalEntryToLocalEntryOffset

The 3 most significant bits of st_other are used by OpenPOWER ABI.

[MaskRay]

ELF/Target.h

182

Sorry, the comment should be near line 2003 :)

[luporl]

• Addressed review's comments
• Added a test for the fix

[luporl]

In D56782#1394013, @ruiu wrote:

Sorry for the belated response. I missed this one for some reason.

Can you add a test?

Yes, I've added the ppc64-local-entry.s test.

[ruiu]

LGTM

[luporl]

Thank you @ruiu. Can you commit this change for me?

ELF/SyntheticSections.cpp

for (SymbolTableEntry &Ent : Symbols) {
     ESym->st_other = 0;
     if (Sym->isLocal()) {
       ESym->setBindingAndType(STB_LOCAL, Sym->Type);
     } else {
       ESym->setBindingAndType(Sym->computeBinding(), Sym->Type);
       ESym->setVisibility(Sym->Visibility);
     }
 
+    if (Config->EMachine == EM_PPC64)
+      ESym->st_other |= getPPC64GlobalEntryToLocalEntryBits(Sym->StOther);
+
     ESym->st_name = Ent.StrTabOffset;
     ESym->st_shndx = getSymSectionIndex(Ent.Sym);
 
     // Copy symbol size if it is a defined symbol. st_size is not significant
     // for undefined symbols, so whether copying it or not is up to us if that's
     // the case. We'll leave it as zero because by not setting a value, we can
     // get the exact same outputs for two sets of input files that differ only
     // in undefined symbol size in DSOs.

ELF/Target.h

 // a global entry point (GEP) which typically is used to intiailzie the TOC
 // pointer in general purpose register 2.  The second is a local entry
 // point (LEP) which bypasses the TOC pointer initialization code. The
 // offset between GEP and LEP is encoded in a function's st_other flags.
 // This function will return the offset (in bytes) from the global entry-point
 // to the local entry-point.
 unsigned getPPC64GlobalEntryToLocalEntryOffset(uint8_t StOther);
 
+// Return only the StOther bits used to encode PPC64
+// GlobalEntryToLocalEntry offset.
+static inline uint8_t getPPC64GlobalEntryToLocalEntryBits(uint8_t StOther) {
+  return StOther & 0xE0;
+}
+
 uint64_t getPPC64TocBase();
 uint64_t getAArch64Page(uint64_t Expr);
 
 extern TargetInfo *Target;
 TargetInfo *getTarget();
 
 template <class ELFT> bool isMipsPIC(const Defined *Sym);