[PATCH] D30365: ELF ICF: Merge only functions.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu created this revision.

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).


https://reviews.llvm.org/D30365

Files:
  lld/ELF/ICF.cpp
  lld/test/ELF/icf8.s


Index: lld/test/ELF/icf8.s
===================================================================
--- /dev/null
+++ lld/test/ELF/icf8.s
@@ -0,0 +1,23 @@
+# REQUIRES: x86
+
+# 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 .text.f1
+# CHECK-NOT: selected .text.f2
+
+.globl _start, f1, f2
+_start:
+  ret
+
+.section .text.f1, "a"
+f1:
+  mov $60, %rax
+  mov $42, %rdi
+  syscall
+
+.section .text.f2, "a"
+f2:
+  mov $60, %rax
+  mov $42, %rdi
+  syscall
Index: lld/ELF/ICF.cpp
===================================================================
--- lld/ELF/ICF.cpp
+++ lld/ELF/ICF.cpp
@@ -163,8 +163,8 @@
   // .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.


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