[ELF] Infer EI_OSABI from object files

The first object file whose EI_OSABI is not ELFOSABI_NONE is selected.
This is useful for some OSes to identify themselves. This achieves
similar effects to BFD emulations `ld.lld -m *_fbsd` but is more
lightweight.

Pull Request: llvm#97144

Author: MaskRay

lld/ELF/Driver.cpp

@@ -2021,16 +2021,22 @@ void LinkerDriver::inferMachineType() {
   if (config->ekind != ELFNoneKind)
     return;
 
+  bool inferred = false;
   for (InputFile *f : files) {
     if (f->ekind == ELFNoneKind)
       continue;
-    config->ekind = f->ekind;
-    config->emachine = f->emachine;
+    if (!inferred) {
+      inferred = true;
+      config->ekind = f->ekind;
+      config->emachine = f->emachine;
+      config->mipsN32Abi = config->emachine == EM_MIPS && isMipsN32Abi(f);
+    }
     config->osabi = f->osabi;
-    config->mipsN32Abi = config->emachine == EM_MIPS && isMipsN32Abi(f);
-    return;
+    if (f->osabi != ELFOSABI_NONE)
+      return;
   }
-  error("target emulation unknown: -m or at least one .o file required");
+  if (!inferred)
+    error("target emulation unknown: -m or at least one .o file required");
 }
 
 // Parse -z max-page-size=<value>. The default value is defined by

lld/test/ELF/basic-freebsd.s

@@ -1,15 +1,24 @@
 # REQUIRES: x86
 # Verify that OSABI is set to the correct value.
 
-# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-freebsd %s -o %t
-# RUN: ld.lld %t -o %t2
-# RUN: llvm-readobj --file-headers %t2 | FileCheck %s
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 empty.s -o empty.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-freebsd a.s -o a.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-linux gnu.s -o gnu.o
+# RUN: ld.lld a.o -o out
+# RUN: llvm-readobj --file-headers out | FileCheck %s
+# RUN: ld.lld empty.o a.o gnu.o empty.o -o out2
+# RUN: llvm-readobj --file-headers out2 | FileCheck %s
 
+#--- empty.s
+#--- a.s
 .globl _start
 _start:
   mov $1, %rax
   mov $42, %rdi
   syscall
+#--- gnu.s
+.section retain,"aR"
 
 # CHECK: ElfHeader {
 # CHECK-NEXT:   Ident {