[lld][ELF] Avoid folding sections with unmergeable symbols

Author: pranavk

lld/ELF/ICF.cpp

@@ -287,7 +287,11 @@ bool ICF<ELFT>::constantEq(const InputSection *secA, Relocs<RelTy> ra,
     // Relocations referring to InputSections are constant-equal if their
     // section offsets are equal.
     if (isa<InputSection>(da->section)) {
-      if (da->value + addA == db->value + addB)
+      // Our symbol folding logic later merges symbols in two folded sections
+      // We should not merge sections in the first place if their symbols 
+      // cannot be merged together.
+      bool canMergeSymbols = addA == addB;
+      if (da->value + addA == db->value + addB && canMergeSymbols)
         continue;
       return false;
     }

lld/test/ELF/icf-addend.s

@@ -0,0 +1,33 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -o /dev/null --icf=all --print-icf-sections | FileCheck %s
+
+# Check that ICF doesn't fold different symbols when functions referencing them
+# can be folded because they are pointing to the same address.
+
+# CHECK: selected section {{.*}}:(.text.f1)
+# CHECK:   removing identical section {{.*}}:(.text.f2)
+# CHECK-NOT: redirecting 'y' in symtab to 'x'
+# CHECK-NOT: redirecting 'y' to 'x'
+
+.globl x, y
+
+.section .rodata,"a",@progbits
+x:
+.long 11
+y:
+.long 12
+
+.section .text.f1,"ax",@progbits
+f1:
+movq x+4(%rip), %rax 
+
+.section .text.f2,"ax",@progbits
+f2:
+movq y(%rip), %rax
+
+.section .text._start,"ax",@progbits
+.globl _start
+_start:
+call f1
+call f2