[clang][driver] Add avr-libc's default linker script to lld (#68507)

If `-fuse-ld=lld` is specified but no user linker script is offered, we
try to use avr-libc's default one for lld. (not needed for GNU ld)

Author: benshi001

clang/lib/Driver/ToolChains/AVR.cpp

@@ -554,8 +554,18 @@ void AVR::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
     CmdArgs.push_back("--end-group");
 
-    // Add user specified linker script.
-    Args.AddAllArgs(CmdArgs, options::OPT_T);
+    // Add avr-libc's linker script to lld by default, if it exists.
+    if (!Args.hasArg(options::OPT_T) &&
+        Linker.find("lld") != std::string::npos) {
+      std::string Path(*AVRLibcRoot + "/lib/ldscripts/");
+      Path += *FamilyName;
+      Path += ".x";
+      if (llvm::sys::fs::exists(Path))
+        CmdArgs.push_back(Args.MakeArgString("-T" + Path));
+    }
+    // Otherwise add user specified linker script to either avr-ld or lld.
+    else
+      Args.AddAllArgs(CmdArgs, options::OPT_T);
 
     if (Args.hasFlag(options::OPT_mrelax, options::OPT_mno_relax, true))
       CmdArgs.push_back("--relax");

clang/test/Driver/Inputs/basic_avr_tree/usr/lib/avr/lib/ldscripts/avrtiny.x

@@ -58,6 +58,28 @@
 // LINKS: {{".*ld.*"}} {{.*}} "--defsym=__DATA_REGION_ORIGIN__=0x800100" "-plugin-opt=mcpu=atmega328"
 // LINKS-NOT: "-plugin-opt=thinlto"
 
+// RUN: %clang -### --target=avr -mmcu=attiny40 -fuse-ld=lld --sysroot %S/Inputs/basic_avr_tree %s 2>&1 | FileCheck -check-prefix LINKT0 %s
+// LINKT0: {{".*lld.*"}} {{.*}} {{"-T.*avrtiny.x"}}
+// LINKT0-NOT: "-m
+
+// RUN: %clang -### --target=avr -mmcu=atxmega384c3 -fuse-ld=lld --sysroot %S/Inputs/basic_avr_tree %s 2>&1 | FileCheck -check-prefix LINKT1 %s
+// LINKT1: {{".*lld.*"}} {{.*}} {{"-T.*avrxmega6.x"}}
+// LINKT1-NOT: "-m
+
+// RUN: %clang -### --target=avr -mmcu=atmega328 -fuse-ld=lld --sysroot %S/Inputs/basic_avr_tree %s 2>&1 | FileCheck -check-prefix LINKT2 %s
+// LINKT2: {{".*lld.*"}} {{.*}} "--start-group" {{.*}} "--end-group"
+// LINKT2-NOT: "-T
+// LINKT2-NOT: "-m
+
+// RUN: %clang -### --target=avr -mmcu=attiny40 --sysroot %S/Inputs/basic_avr_tree %s 2>&1 | FileCheck -check-prefix LINKT3 %s
+// LINKT3: {{".*ld.*"}} {{.*}} "-mavrtiny"
+// LINKT3-NOT: "-T
+
+// RUN: %clang -### --target=avr -mmcu=attiny40 --sysroot %S/Inputs/basic_avr_tree -fuse-ld=lld -T %S/Inputs/basic_avr_tree/usr/lib/avr/lib/ldscripts/avrxmega6.x %s 2>&1 | FileCheck -check-prefix LINKT4 %s
+// LINKT4: {{".*lld.*"}} {{.*}} {{"-T.*avrxmega6.x"}}
+// LINKT4-NOT: {{"-T.*avrtiny.x"}}
+// LINKT4-NOT: "-m
+
 // RUN: %clang -### -r --target=avr -mmcu=atmega328 --sysroot %S/Inputs/basic_avr_tree %s 2>&1 | FileCheck --check-prefix=LINKU %s
 // LINKU: {{".*ld.*"}} {{.*}} "-r" {{.*}} "-mavr5"
 // LINKU-NOT: "--gc-sections"