diff --git a/lld/ELF/Arch/AVR.cpp b/lld/ELF/Arch/AVR.cpp
--- a/lld/ELF/Arch/AVR.cpp
+++ b/lld/ELF/Arch/AVR.cpp
@@ -43,6 +43,7 @@
 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,
@@ -196,3 +197,28 @@
   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 hasLinkRelaxFlag = 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))
+      hasLinkRelaxFlag = false;
+  }
+
+  if (!hasLinkRelaxFlag)
+    flags &= ~EF_AVR_LINKRELAX_PREPARED;
+
+  return flags;
+}
diff --git a/lld/test/ELF/avr-flags.s b/lld/test/ELF/avr-flags.s
new file mode 100644
--- /dev/null
+++ b/lld/test/ELF/avr-flags.s
@@ -0,0 +1,15 @@
+; RUN: llvm-mc -filetype=obj -triple=avr -mcpu=avr5 %s -o %t-v5
+; RUN: llvm-mc -filetype=obj -triple=avr -mcpu=avrxmega3 %s -o %t-xmega3
+; RUN: ld.lld %t-v5 -o %t-v5.out
+; RUN: ld.lld %t-xmega3 -o %t-xmega3.out
+; RUN: llvm-readobj -h %t-v5.out | FileCheck --check-prefix V5 %s
+; RUN: llvm-readobj -h %t-xmega3.out | FileCheck --check-prefix XMEGA3 %s
+
+;; Ensure LLD won't silently mix object files targeting different ISAs.
+; RUN: not ld.lld %t-v5 %t-xmega3 -o /dev/null 2>&1 | FileCheck --check-prefix ERR %s
+; ERR: error: {{.*}}: cannot link object files with incompatible target ISA
+
+; V5:  Flags [ (0x5)
+; V5:  EF_AVR_ARCH_AVR5 (0x5)
+; XMEGA3: Flags [ (0x67)
+; XMEGA3: EF_AVR_ARCH_XMEGA3 (0x67)