ELF ICF: Merge only functions.

[ruiu]

Summary

Previously, LLD merged all read-only sections. So the following
program prints out "true" if -icf=all is specified.

static const int foo = 1;
static const int bar = 1;
int main() { printf("%s\n", &foo == &bar ? "true" : "false"); }

This is somewhat counter-intuitive, and it actually caused nasty issues.
One example is https://bugs.chromium.org/p/chromium/issues/detail?id=682773#c24.

This patch changes the way how it works. Now ICF merges only functions
(i.e. executable sections).

[ruiu]

• Updated the test to be more realistic -- now it is a data section rather than a text section without x bit.

[rnk]

lgtm

Thanks! I think experience has shown that this isn't worth it.

[krasin]

Thank you, Rui!

[krasin]

Do we wait for anything or anybody? If not, let's submit the CL. That should fix ThinLTO bot in Chrome.

lld/trunk/ELF/ICF.cpp

template <class ELFT> static uint32_t getHash(InputSection *S) {
   return hash_combine(S->Flags, S->template getSize<ELFT>(), S->NumRelocations);
 }
 
 // Returns true if section S is subject of ICF.
 static bool isEligible(InputSection *S) {
   // .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";
+  return S->Live && (S->Flags & SHF_ALLOC) && (S->Flags & SHF_EXECINSTR) &&
+         !(S->Flags & SHF_WRITE) && S->Name != ".init" && S->Name != ".fini";
 }
 
 // Split an equivalence class into smaller classes.
 template <class ELFT>
 void ICF<ELFT>::segregate(size_t Begin, size_t End, bool Constant) {
   // This loop rearranges sections in [Begin, End) so that all sections
   // that are equal in terms of equals{Constant,Variable} are contiguous
   // in [Begin, End).

lld/trunk/test/ELF/icf9.s

+# REQUIRES: x86
+
+### Make sure that we do not merge data.
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t2 --icf=all --verbose | FileCheck %s
+
+# CHECK-NOT: selected .data.d1
+# CHECK-NOT: selected .data.d2
+
+.globl _start, d1, d2
+_start:
+  ret
+
+.section .data.f1, "a"
+d1:
+  .byte 1
+
+.section .data.f2, "a"
+d2:
+  .byte 1