[ELF] Make TargetInfo::writeIgotPlt a no-op

[MaskRay]

Summary

RELA targets don't read initial .got.plt entries.
REL targets (ARM, x86-32) write the address of the IFUNC resolver to the
entry (write32le(buf, s.getVA())).

The default writeIgotPlt() is not meaningful. Make it a no-op. AArch64
and x86-64 will have 0 as initial .got.plt entries associated with
IFUNC.

[merge_guards_bot]

Unit tests: pass. 61723 tests passed, 0 failed and 779 were skipped.

clang-tidy: fail. Please fix clang-tidy findings.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[peter.smith]

LGTM. ld.bfd, at least on AARCH64 will write the address of PLT[0] into the .got.plt, but this may just be out of convenience as it will be overwritten by the IRELATIVE relocation before it can be used.

lld/ELF/Target.h

 public:
   virtual uint32_t calcEFlags() const { return 0; }
   virtual RelExpr getRelExpr(RelType type, const Symbol &s,
                              const uint8_t *loc) const = 0;
   virtual RelType getDynRel(RelType type) const { return 0; }
   virtual void writeGotPltHeader(uint8_t *buf) const {}
   virtual void writeGotHeader(uint8_t *buf) const {}
   virtual void writeGotPlt(uint8_t *buf, const Symbol &s) const {};
-  virtual void writeIgotPlt(uint8_t *buf, const Symbol &s) const;
+  virtual void writeIgotPlt(uint8_t *buf, const Symbol &s) const {}
   virtual int64_t getImplicitAddend(const uint8_t *buf, RelType type) const;
   virtual int getTlsGdRelaxSkip(RelType type) const { return 1; }
 
   // If lazy binding is supported, the first entry of the PLT has code
   // to call the dynamic linker to resolve PLT entries the first time
   // they are called. This function writes that code.
   virtual void writePltHeader(uint8_t *buf) const {}
 

lld/ELF/Target.cpp

bool TargetInfo::adjustPrologueForCrossSplitStack(uint8_t *loc, uint8_t *end,
                                                   uint8_t stOther) const {
   llvm_unreachable("Target doesn't support split stacks.");
 }
 
 bool TargetInfo::inBranchRange(RelType type, uint64_t src, uint64_t dst) const {
   return true;
 }
 
-void TargetInfo::writeIgotPlt(uint8_t *buf, const Symbol &s) const {
-  writeGotPlt(buf, s);
-}
-
 RelExpr TargetInfo::adjustRelaxExpr(RelType type, const uint8_t *data,
                                     RelExpr expr) const {
   return expr;
 }
 
 void TargetInfo::relaxGot(uint8_t *loc, RelType type, uint64_t val) const {
   llvm_unreachable("Should not have claimed to be relaxable");
 }

lld/test/ELF/aarch64-gnu-ifunc-plt.s

 // CHECK-NEXT:     0x230460 R_AARCH64_JUMP_SLOT zed2 0x0
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
 // Check that .got.plt entries point back to PLT header
 // GOTPLT: Contents of section .got.plt:
 // GOTPLT-NEXT:  230440 00000000 00000000 00000000 00000000
 // GOTPLT-NEXT:  230450 00000000 00000000 f0022100 00000000
-// GOTPLT-NEXT:  230460 f0022100 00000000 f0022100 00000000
-// GOTPLT-NEXT:  230470 f0022100 00000000
+// GOTPLT-NEXT:  230460 f0022100 00000000 00000000 00000000
+// GOTPLT-NEXT:  230470 00000000 00000000
 
 // Check that the PLTRELSZ tag does not include the IRELATIVE relocations
 // CHECK: DynamicSection [
 // CHECK:   0x0000000000000008 RELASZ               48 (bytes)
 // CHECK:   0x0000000000000002 PLTRELSZ             48 (bytes)
 
 // Check that a PLT header is written and the ifunc entries appear last
 // DISASM: Disassembly of section .text:

lld/test/ELF/gnu-ifunc-plt.s

 // CHECK-NEXT:     0x203448 R_X86_64_JUMP_SLOT bar2 0x0
 // CHECK-NEXT:     0x203450 R_X86_64_JUMP_SLOT zed2 0x0
 // CHECK-NEXT:   }
 
 // Check that .got.plt entries point back to PLT header
 // GOTPLT: Contents of section .got.plt:
 // GOTPLT-NEXT:  203430 40232000 00000000 00000000 00000000
 // GOTPLT-NEXT:  203440 00000000 00000000 06132000 00000000
-// GOTPLT-NEXT:  203450 16132000 00000000 26132000 00000000
-// GOTPLT-NEXT:  203460 36132000 00000000
+// GOTPLT-NEXT:  203450 16132000 00000000 00000000 00000000
+// GOTPLT-NEXT:  203460 00000000 00000000
 
 // Check that the PLTRELSZ tag does not include the IRELATIVE relocations
 // CHECK: DynamicSection [
 // CHECK:   0x0000000000000008 RELASZ               48 (bytes)
 // CHECK:   0x0000000000000002 PLTRELSZ             48 (bytes)
 
 // Check that a PLT header is written and the ifunc entries appear last
 // DISASM: Disassembly of section .text: