ELF: Do not ICF sections named with a C identifier.

[pcc]

Summary

A user program may enumerate sections named with a C identifier using
start_* and stop_* symbols. We cannot ICF any such sections because
that could change program semantics.

[ruiu]

LGTM

ELF/ICF.cpp

static bool isEligible(InputSection *S) {
   if (isa<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.
   if (S->Name == ".init" || S->Name == ".fini")
     return false;
 
+  // A user program may enumerate sections named with a C identifier using
+  // __start_* and __stop_* symbols. We cannot ICF any such sections because
+  // that could change program semantics.
+  if (isValidCIdentifier(S->Name))
+    return false;
+
   return true;
 }
 
 // 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

test/ELF/icf-c-identifier.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t2 --icf=all --print-icf-sections | count 0
+
+.section foo,"ax",@progbits,unique,0
+.byte 42
+
+.section foo,"ax",@progbits,unique,1
+.byte 42

test/ELF/icf-different-output-sections.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld %t -o %t2 --icf=all --print-icf-sections | count 0
 
-.section foo,"ax"
+.section .foo,"ax"
 .byte 42
 
-.section bar,"ax"
+.section .bar,"ax"
 .byte 42