ELF: Allow ICF on .data.rel.ro sections.

[ruiu]

lld/ELF/ICF.cpp

165–167 ↗

(On Diff #148120)

This expressionis perhaps a bit too long. I'd split it by defining bool IsWritable or something, with a comment explaning that .data.rel.ro is essentially read-only.

[pcc]

• Add comment

[ruiu]

LGTM

lld/trunk/ELF/ICF.cpp

 // Returns a hash value for S. Note that the information about
 // relocation targets is not included in the hash value.
 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) {
-  if (!S->Live || S->KeepUnique || !(S->Flags & SHF_ALLOC) ||
-      (S->Flags & SHF_WRITE))
+  if (!S->Live || S->KeepUnique || !(S->Flags & SHF_ALLOC))
+    return false;
+
+  // Don't merge writable sections. .data.rel.ro sections are marked as writable
+  // but are semantically read-only.
+  if ((S->Flags & SHF_WRITE) && S->Name != ".data.rel.ro" &&
+      !S->Name.startswith(".data.rel.ro."))
     return false;
 
   // 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.

lld/trunk/test/ELF/icf-relro.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t2 --icf=all --ignore-data-address-equality --print-icf-sections | FileCheck %s
+
+# CHECK: selected section {{.*}}:(.data.rel.ro)
+# CHECK:   removing identical section {{.*}}:(.data.rel.ro.foo)
+
+.section .data.rel.ro,"aw"
+.quad foo
+
+.section .data.rel.ro.foo,"aw"
+foo:
+.quad foo