ELF: Fix ICF crash on absolute symbol relocations.

[pcc]

Summary

If two sections contained relocations to absolute symbols with the same
value we would crash when trying to access their sections. Add a check that
both symbols point to sections before accessing their sections, and treat
absolute symbols as equal if their values are equal.

[ruiu]

LGTM

lld/ELF/ICF.cpp

bool ICF<ELFT>::variableEq(const InputSection<ELFT> *A, ArrayRef<RelTy> RelsA,
                            const InputSection<ELFT> *B, ArrayRef<RelTy> RelsB) {
   auto Eq = [&](const RelTy &RA, const RelTy &RB) {
     // The two sections must be identical.
     SymbolBody &SA = A->getFile()->getRelocTargetSym(RA);
     SymbolBody &SB = B->getFile()->getRelocTargetSym(RB);
     if (&SA == &SB)
       return true;
 
-    // Or, the two sections must be in the same equivalence class.
     auto *DA = dyn_cast<DefinedRegular<ELFT>>(&SA);
     auto *DB = dyn_cast<DefinedRegular<ELFT>>(&SB);
     if (!DA || !DB)
       return false;
     if (DA->Value != DB->Value)
       return false;
 
+    // Either both symbols must be absolute...
+    if (!DA->Section || !DB->Section)
+      return !DA->Section && !DB->Section;
+
+    // Or the two sections must be in the same equivalence class.
     auto *X = dyn_cast<InputSection<ELFT>>(DA->Section);
     auto *Y = dyn_cast<InputSection<ELFT>>(DB->Section);
     if (!X || !Y)
       return false;
 
     // Ineligible sections are in the special equivalence class 0.
     // They can never be the same in terms of the equivalence class.
     if (X->Class[Current] == 0)

lld/test/ELF/Inputs/icf-absolute.s

+.globl a1, a2
+a1 = 1
+a2 = 1

lld/test/ELF/icf-absolute.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %S/Inputs/icf-absolute.s -o %t2
+# RUN: ld.lld %t %t2 -o %t3 --icf=all --verbose | FileCheck %s
+
+# CHECK: selected .text.f1
+# CHECK:   removed .text.f2
+
+.globl _start, f1, f2
+_start:
+  ret
+
+.section .text.f1, "ax"
+f1:
+  .byte a1
+
+.section .text.f2, "ax"
+f2:
+  .byte a2