[ELF] Disable ICF for synthetic sections

[andrewng]

Summary

Do not include synthetic sections in ICF.

Fixes bug PR36910.

Alternative to D44898.

[ruiu]

ELF/ICF.cpp

168

Please explain why. We don't merge synthetic sections because their Data member doesn't have a meaningful value.

170

Maybe it's better to write this as two if's because the boolean expression is a bit too complicated.

[andrewng]

Updated to address review comments.

[ruiu]

LGTM

Thanks!

ELF/ICF.cpp

173

You can use isa instead of dyn_cast.

ELF/ICF.cpp

 // http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36912.pdf
 //
 //===----------------------------------------------------------------------===//
 
 #include "ICF.h"
 #include "Config.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
+#include "SyntheticSections.h"
 #include "lld/Common/Threads.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Object/ELF.h"
 #include <algorithm>
 #include <atomic>
 
 using namespace lld;
template <class ELFT> static uint32_t getHash(InputSection *S) {
   return hash_combine(S->Flags, S->getSize(), S->NumRelocations, S->Data);
 }
 
 // Returns true if section S is subject of ICF.
 static bool isEligible(InputSection *S) {
   // Don't merge read only data sections unless
   // --ignore-data-address-equality was passed.
   if (!(S->Flags & SHF_EXECINSTR) && !Config->IgnoreDataAddressEquality)
     return false;
 
+  // Don't merge synthetic sections as their Data member is not valid and empty.
+  // The Data member needs to be valid for ICF as it is used by ICF to determine
+  // the equality of section contents.
+  if (dyn_cast<SyntheticSection>(S))
+    return false;
+
   // .init and .fini contains instructions that must be executed to
   // initialize and finalize the process. They cannot and should not
   // be merged.
   return S->Live && (S->Flags & SHF_ALLOC) && !(S->Flags & SHF_WRITE) &&
          S->Name != ".init" && S->Name != ".fini";
 }
 
 // Split an equivalence class into smaller classes.

test/ELF/icf-merged-sections.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
-# RUN: ld.lld %t.o -o %t --icf=all --ignore-data-address-equality --print-icf-sections | FileCheck %s --check-prefix ICF
-# RUN: llvm-objdump -s -d -print-imm-hex %t | FileCheck %s
+# RUN: ld.lld %t.o -o %t --icf=all --ignore-data-address-equality --print-icf-sections | FileCheck -allow-empty --check-prefix=NOICF %s
+# RUN: llvm-readobj -s -section-data %t | FileCheck %s
 
-# ICF: selected section <internal>:(.rodata)
-# ICF-NEXT: removing identical section <internal>:(.rodata)
+# Check that merge synthetic sections are not merged by ICF.
 
-# CHECK: {{^}}.text:
-# CHECK-NEXT: movq 0x[[ADDR:[0-9a-f]+]], %rax
-# CHECK-NEXT: movq 0x[[ADDR]], %rax
-# CHECK: Contents of section .rodata:
-# CHECK-NEXT: 2a000000 00000000 67452301 10325476
+# NOICF-NOT: selected section <internal>:(.rodata)
 
-.section .rodata, "a"
-  .quad 42
+# CHECK:      Name: .rodata
+# CHECK-NEXT: Type: SHT_PROGBITS
+# CHECK-NEXT: Flags [
+# CHECK-NEXT:   SHF_ALLOC
+# CHECK-NEXT:   SHF_MERGE
+# CHECK-NEXT: ]
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Offset:
+# CHECK-NEXT: Size: 16
+# CHECK-NEXT: Link:
+# CHECK-NEXT: Info:
+# CHECK-NEXT: AddressAlignment: 8
+# CHECK-NEXT: EntrySize: 0
+# CHECK-NEXT: SectionData (
+# CHECK-NEXT:   0000: 67452301 10325476 67452301 10325476
 
 .section .rodata.cst4,"aM",@progbits,4
 rodata4:
   .long 0x01234567
   .long 0x76543210
   .long 0x01234567
   .long 0x76543210
 
 .section .rodata.cst8,"aM",@progbits,8
 rodata8:
   .long 0x01234567
   .long 0x76543210
-
-.section .text,"ax"
-  movq rodata4, %rax
-  movq rodata8, %rax