[LLD][ELF] Mark ARM Exceptions that refer to folded code as not live

[peter.smith]

Summary

ARM Exception Index Table sections .ARM.exidx have an implicit dependency on code sections via SHF_LINK_ORDER. When code sections are folded by ICF we must mark the unique .ARM.exidx table that describes it as not live to prevent an illegal entry in the exception table.

Note that we do not try and follow the relocations from the .ARM.exidx section to the .ARM.extab sections to mark these as not live. Leaving these sections is not a correctness problem. In theory these could be removed via an application of garbage collection.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32614

[ruiu]

LGTM. I thought for a while to try to find a better way, but looks like this is the simplest way of doing it.

ELF/ICF.cpp

382–391 ↗

(On Diff #94806)

Looks like this can be

if (Config->EMachine == EM_ARM)
  for (InputSectionBase *Sec : InputSections)
    if (auto *S = dyn_cast<InputSection>(Sec))
      if (S->Flags & SHF_LINK_ORDER)
        S->getLinkOrderDep()->Live = S->Live;

lld/trunk/ELF/ICF.cpp

if (End - Begin == 1)
       return;
 
     log("selected " + Sections[Begin]->Name);
     for (size_t I = Begin + 1; I < End; ++I) {
       log("  removed " + Sections[I]->Name);
       Sections[Begin]->replace(Sections[I]);
     }
   });
+
+  // Mark ARM Exception Index table sections that refer to folded code
+  // sections as not live. These sections have an implict dependency
+  // via the link order dependency.
+  if (Config->EMachine == EM_ARM)
+    for (InputSectionBase *Sec : InputSections)
+      if (auto *S = dyn_cast<InputSection>(Sec))
+        if (S->Flags & SHF_LINK_ORDER)
+          S->Live = S->getLinkOrderDep()->Live;
 }
 
 // ICF entry point function.
 template <class ELFT> void elf::doIcf() { ICF<ELFT>().run(); }
 
 template void elf::doIcf<ELF32LE>();
 template void elf::doIcf<ELF32BE>();
 template void elf::doIcf<ELF64LE>();
 template void elf::doIcf<ELF64BE>();

lld/trunk/test/ELF/arm-icf-exidx.s

+// REQUIRES: arm
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: ld.lld %t -o %t2 --icf=all
+// RUN: llvm-objdump -s -d -triple=armv7a-none-linux-gnueabi %t2 | FileCheck %s
+
+ .syntax unified
+ .section        .text.f,"axG",%progbits,f,comdat
+f:
+ .fnstart
+ bx      lr
+ .fnend
+
+ .section        .text.g,"axG",%progbits,g,comdat
+g:
+ .fnstart
+ bx      lr
+ .fnend
+
+ .section .text.h
+ .global __aeabi_unwind_cpp_pr0
+__aeabi_unwind_cpp_pr0:
+ bx lr
+
+// CHECK: Disassembly of section .text:
+// CHECK-NEXT: f:
+// CHECK-NEXT:    11000:        1e ff 2f e1     bx      lr
+// CHECK: __aeabi_unwind_cpp_pr0:
+// CHECK-NEXT:    11004:        1e ff 2f e1     bx      lr
+
+// CHECK: Contents of section .ARM.exidx:
+// CHECK-NEXT:  100d4 2c0f0000 b0b0b080 280f0000 01000000