[mips] Deduce MIPS specific ELF header flags from emulation

In case of linking binary blobs which do not have any ELF headers, we can
deduce MIPS ABI  ELF header flags from an `emulation` option.

Patch by Kyle Evans.

llvm-svn: 372513

lld/ELF/Arch/MipsArchTree.cpp

@@ -294,12 +294,30 @@ static uint32_t getArchFlags(ArrayRef<FileFlags> files) {
   return ret;
 }
 
+// If we don't have any input files, we'll have to rely on the information we
+// can derive from emulation information, since this at least gets us ABI.
+static uint32_t getFlagsFromEmulation() {
+  uint32_t ret = 0;
+
+  if (config->emulation.empty())
+    return 0;
+
+  if (config->ekind == ELF32BEKind || config->ekind == ELF32LEKind) {
+    if (config->mipsN32Abi)
+      ret |= EF_MIPS_ABI2;
+    else
+      ret |= EF_MIPS_ABI_O32;
+  }
+
+  return ret;
+}
+
 template <class ELFT> uint32_t elf::calcMipsEFlags() {
   std::vector<FileFlags> v;
   for (InputFile *f : objectFiles)
     v.push_back({f, cast<ObjFile<ELFT>>(f)->getObj().getHeader()->e_flags});
   if (v.empty())
-    return 0;
+    return getFlagsFromEmulation();
   checkFlags(v);
   return getMiscFlags(v) | getPicFlags(v) | getArchFlags(v);
 }

lld/test/ELF/mips-elf-flags-binary.s

@@ -0,0 +1,25 @@
+# REQUIRES: mips
+# Check deducing MIPS specific ELF header flags from `emulation`.
+
+# RUN: echo -n "BLOB" > %t.binary
+# RUN: ld.lld -m elf32btsmip -r -b binary %t.binary -o %t.out
+# RUN: llvm-readobj -h %t.out | FileCheck -check-prefix=O32 %s
+
+# RUN: echo -n "BLOB" > %t.binary
+# RUN: ld.lld -m elf32btsmipn32 -r -b binary %t.binary -o %t.out
+# RUN: llvm-readobj -h %t.out | FileCheck -check-prefix=N32 %s
+
+# RUN: echo -n "BLOB" > %t.binary
+# RUN: ld.lld -m elf64btsmip -r -b binary %t.binary -o %t.out
+# RUN: llvm-readobj -h %t.out | FileCheck -check-prefix=N64 %s
+
+# O32:      Flags [
+# O32-NEXT:   EF_MIPS_ABI_O32
+# O32-NEXT: ]
+
+# N32:      Flags [
+# N32-NEXT:   EF_MIPS_ABI2
+# N32-NEXT: ]
+
+# N64:      Flags [
+# N64-NEXT: ]