[ELF] GotSection increment NumEntries when Target saves GlobalOffsetTable in the .got

[syzaara]

Summary

When the target saves ElfSym::GlobalOffsetTable in the .got rather than .got.plt, Target->GotHeaderEntriesNum states the number of extra entries required in the .got. Rather than having to add Target->GotHeaderEntriesNum to NumEntries in every function which refers to NumEntries, this patch changes the initial value of NumEntries in the constructor. We now add Target->GotHeaderEntriesNum to NumEntries in the constructor if the target saves the GlobalOffsetTable in the .got

[ruiu]

ELF/SyntheticSections.cpp

591–592 ↗

(On Diff #139324)

This comment just explains what the following code does, so it's not that useful. Please explain why we are doing this instead of what.

Also, please avoid explaining things too abstractly. The target that saves the global offset table to the first entry in the .got is PPC64 only, so please directly say so.

593 ↗

(On Diff #139324)

Is this possible that GlobalOffsetTable is not defined?

[ruiu]

LGTM

lld/ELF/SyntheticSections.cpp

void EhFrameSection::writeTo(uint8_t *Buf) {
   // in the output buffer, but relocateAlloc() still works because
   // getOffset() takes care of discontiguous section pieces.
   for (EhInputSection *S : Sections)
     S->relocateAlloc(Buf, nullptr);
 }
 
 GotSection::GotSection()
     : SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS,
-                       Target->GotEntrySize, ".got") {}
+                       Target->GotEntrySize, ".got") {
+  // PPC64 saves the ElfSym::GlobalOffsetTable .TOC. as the first entry in the
+  // .got. If there are no references to .TOC. in the symbol table,
+  // ElfSym::GlobalOffsetTable will not be defined and we won't need to save
+  // .TOC. in the .got. When it is defined, we increase NumEntries by the number
+  // of entries used to emit ElfSym::GlobalOffsetTable.
+  if (ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt)
+    NumEntries += Target->GotHeaderEntriesNum;
+}
 
 void GotSection::addEntry(Symbol &Sym) {
-  Sym.GotIndex = Target->GotHeaderEntriesNum + NumEntries;
+  Sym.GotIndex = NumEntries;
   ++NumEntries;
 }
 
 bool GotSection::addDynTlsEntry(Symbol &Sym) {
   if (Sym.GlobalDynIndex != -1U)
     return false;
   Sym.GlobalDynIndex = NumEntries;
   // Global Dynamic TLS entries take two GOT slots.
uint64_t GotSection::getGlobalDynAddr(const Symbol &B) const {
   return this->getVA() + B.GlobalDynIndex * Config->Wordsize;
 }
 
 uint64_t GotSection::getGlobalDynOffset(const Symbol &B) const {
   return B.GlobalDynIndex * Config->Wordsize;
 }
 
 void GotSection::finalizeContents() {
-  Size = (NumEntries + Target->GotHeaderEntriesNum) * Config->Wordsize;
+  Size = NumEntries * Config->Wordsize;
 }
 
 bool GotSection::empty() const {
   // We need to emit a GOT even if it's empty if there's a relocation that is
   // relative to GOT(such as GOTOFFREL) or there's a symbol that points to a GOT
   // (i.e. _GLOBAL_OFFSET_TABLE_) that the target defines relative to the .got.
   return NumEntries == 0 && !HasGotOffRel &&
          !(ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt);

lld/test/ELF/ppc64_entry_point.s

 # REQUIRES: ppc
 # RUN: llvm-mc -filetype=obj -triple=powerpc64le-unknown-linux %s -o %t
 # RUN: ld.lld %t -o %t2
-# RUN: llvm-objdump -d %t2 | FileCheck %s
+# RUN: llvm-objdump -D %t2 | FileCheck %s
 
 .text
 .abiversion 2
 .globl  _start
 .p2align        4
 .type   _start,@function
 
 _start:
.Lfunc_gep0:
   sc
 .Lfunc_end0:
     .size   _start, .Lfunc_end0-.Lfunc_begin0
 
 // CHECK: 10010000:       01 10 80 3c     lis 4, 4097
 // CHECK-NEXT: 10010004:       00 00 84 38     addi 4, 4, 0
 // CHECK-NEXT: 10010008:       02 00 a0 3c     lis 5, 2
 // CHECK-NEXT: 1001000c:       00 80 a5 38     addi 5, 5, -32768
+// CHECK: Disassembly of section .got:
+// CHECK-NEXT: .got:
+// CHECK-NEXT: 10020000:       00 80 02 10