[LLD] Combine read-only and writable input sections into single output section

[kettenis]

Summary

On OpenBSD crtbegin.o has a read-only .ctors section whereas crtend.o has a writable .ctors.
This results in two separate .ctors sections in the output, making the resulting executable crash
because the sentinel from crtend.o is stored in the second .ctors section which doesn't immediately
follow the first .ctors section. This can be easily fixed by masking off the SHF_WRITE flag when
creating the key that is used to look up the output section. This will combine the input sections
into a single output sections that is marked as writable. This matches the behaviour of the GNU
toolchain.

[kettenis]

Didn't write a test for this yet as I couldn't figure out how to create a test with multiple input object files
Is there an example of such a test in the lld test suite?

[emaste]

This needs a test case.

See also http://llvm.org/pr30415.

[emaste]

Our messages crossed paths. There are tests that use separate input files which are in the Inputs/ directory.

Others might comment on the best / canonical example to start from, but one example is relocatable.s.

[kettenis]

Added a test case.

[kettenis]

Note that the cited PR is about merging segments instead of sections; a not entirely unrelated, but different issue.

[kettenis]

Rafael pointed out that the current behaviour matches the ELF specification.

I verified that the Solaris ld also doesn't concatenate the sections in this case.

Consider this diff withdrawn.

[kettenis]

Rafael pointed out that the current behaviour matches the ELF specification.

I verified that the Solaris ld also doesn't concatenate the sections in this case.

ELF/OutputSections.cpp

 static typename ELFT::uint getOutFlags(InputSectionBase<ELFT> *S) {
   return S->Flags & ~SHF_GROUP & ~SHF_COMPRESSED;
 }
 
 template <class ELFT>
 static SectionKey<ELFT::Is64Bits> createKey(InputSectionBase<ELFT> *C,
                                             StringRef OutsecName) {
   typedef typename ELFT::uint uintX_t;
-  uintX_t Flags = getOutFlags(C);
+  uintX_t Flags = getOutFlags(C) & ~SHF_WRITE;
 
   // For SHF_MERGE we create different output sections for each alignment.
   // This makes each output section simple and keeps a single level mapping from
   // input to output.
   // In case of relocatable object generation we do not try to perform merging
   // and treat SHF_MERGE sections as regular ones, but also create different
   // output sections for them to allow merging at final linking stage.
   uintX_t Alignment = 0;

test/ELF/Inputs/mixed1.s

+.section .ctors, "a", @progbits
+  .long -1

test/ELF/Inputs/mixed2.s

+.section .ctors, "aw", @progbits
+  .long 0

test/ELF/mixed.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/mixed1.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/mixed2.s -o %t2.o
+# RUN: ld.lld -r %t1.o %t2.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck %s
+
+# CHECK:      Contents of section .ctors:
+# CHECK-NEXT: ffffffff 00000000