[LLD][ELF][AVR] Propagate ELF flags to the linked image

[LemonBoy]

Summary

The e_flags for a ELF file targeting the AVR ISA contains two fields at the time of writing:

• A 7-bit integer field specifying the ISA revision being targeted
• A 1-bit flag specifying whether the object files being linked are suited for applying the relaxations at link time

The linked ELF file is blessed with the arch revision shared among all the files.
The behaviour in case of mismatch is purposefully different than the one implemented in libbfd: LLD will raise a fatal error while libbfd silently picks a default value of avr2.
The relaxation-ready flag is handled as done by libbfd, in order for it to appear in the linked object every source object must be tagged with it.

[LemonBoy]

Ping (& fix one small style issue pointed out by clang-tidy)

[MaskRay]

llvm/BinaryFormat/ELF.h is typically added along with tests to llvm-readobj/yaml2obj/obj2yaml. See D95728 for an example.

lld/ELF/Arch/AVR.cpp

213

The code style does not tend to insert so many empty lines.

[LemonBoy]

In D99754#2678502, @MaskRay wrote:

llvm/BinaryFormat/ELF.h is typically added along with tests to llvm-readobj/yaml2obj/obj2yaml. See D95728 for an example.

Apologies for the missing tests, I've just realized I forgot to git add them before sending out the patch.
This said I'm not using yaml2obj to generate the test cases but I see that's a better strategy than using llvm-mc, I'll patch the tools (and ELF.h) in a separate preparatory patch.

[MaskRay]

I think assembly tests in fine in lld/test/ELF and they typically improve readability. yaml2obj tests tend to be verbose.

If you look at my SHF_GNU_RETAIN change or earlier contributions, I usually organize features in three patches:

• BinaryFormat/ELF.h yaml2obj/obj2yaml/llvm-readobj
• llvm-mc and tests in llvm/test/MC
• lld/ELF

[LemonBoy]

Clean up. To be applied on top of D100250.

[LemonBoy]

Rebase to kick off the automatic builds.

[MaskRay]

lld/test/ELF/avr-flags.s

8 ↗

(On Diff #337748)

Newer tests use ;; for non-CHECK non-RUN comments.

10 ↗

(On Diff #337748)

See some newer tests where error: is always added.

[LemonBoy]

Minor touch-ups for the test case.

lld/ELF/Arch/AVR.cpp

 using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
 namespace {
 class AVR final : public TargetInfo {
 public:
   AVR();
+  uint32_t calcEFlags() const override;
   RelExpr getRelExpr(RelType type, const Symbol &s,
                      const uint8_t *loc) const override;
   void relocate(uint8_t *loc, const Relocation &rel,
                 uint64_t val) const override;
 };
 } // namespace
 
 AVR::AVR() { noneRel = R_AVR_NONE; }
error(getErrorLocation(loc) + "unrecognized relocation " +
           toString(rel.type));
   }
 }
 
 TargetInfo *elf::getAVRTargetInfo() {
   static AVR target;
   return ⌖
 }
+
+static uint32_t getEFlags(InputFile *file) {
+  return cast<ObjFile<ELF32LE>>(file)->getObj().getHeader().e_flags;
+}
+
+uint32_t AVR::calcEFlags() const {
+  assert(!objectFiles.empty());
+
+  uint32_t flags = getEFlags(objectFiles[0]);
+  bool hasLinkerRelaxFlag = flags & EF_AVR_LINKRELAX_PREPARED;
+
+  for (InputFile *f : makeArrayRef(objectFiles).slice(1)) {
+    uint32_t objFlags = getEFlags(f);
+
+    if ((objFlags & EF_AVR_ARCH_MASK) != (flags & EF_AVR_ARCH_MASK))
+      error(toString(f) +
+            ": cannot link object files with incompatible target ISA");
+
+    if (!(objFlags & EF_AVR_LINKRELAX_PREPARED))
+      hasLinkerRelaxFlag = false;
+  }
+
+  if (!hasLinkerRelaxFlag)
+    flags &= ~EF_AVR_LINKRELAX_PREPARED;
+
+  return flags;
+}

llvm/include/llvm/BinaryFormat/ELF.h

enum : unsigned {
   EF_AVR_ARCH_AVR6 = 6,
   EF_AVR_ARCH_AVRTINY = 100,
   EF_AVR_ARCH_XMEGA1 = 101,
   EF_AVR_ARCH_XMEGA2 = 102,
   EF_AVR_ARCH_XMEGA3 = 103,
   EF_AVR_ARCH_XMEGA4 = 104,
   EF_AVR_ARCH_XMEGA5 = 105,
   EF_AVR_ARCH_XMEGA6 = 106,
-  EF_AVR_ARCH_XMEGA7 = 107
+  EF_AVR_ARCH_XMEGA7 = 107,
+
+  EF_AVR_ARCH_MASK = 0x7f, // EF_AVR_ARCH_xxx selection mask
+
+  EF_AVR_LINKRELAX_PREPARED = 0x80, // The file is prepared for linker
+                                    // relaxation to be applied
 };
 
 // ELF Relocation types for AVR
 enum {
 #include "ELFRelocs/AVR.def"
 };
 
 // Mips Specific e_flags