Set lld as the default linker for Clang builds

The LLVM linker replaces the GNU linker as default for the link on Clang
builds. It is possible to override the default linker by setting the LD build
flag.

The patch also updates the TF-A doc.

Change-Id: Ic08552b9994d4fa8f0d4863e67a2726c1dce2e35
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>

Makefile

 #
-# Copyright (c) 2013-2019, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2013-2020, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -209,7 +209,10 @@ PP			=	$(CC) -E $(TF_CFLAGS_$(ARCH))
 else ifneq ($(findstring clang,$(notdir $(CC))),)
 TF_CFLAGS_aarch32	=	$(target32-directive) $(march32-directive)
 TF_CFLAGS_aarch64	=	-target aarch64-elf $(march64-directive)
-LD			=	$(LINKER)
+LD			=	ld.lld
+ifeq (, $(shell which $(LD)))
+$(error "No $(LD) in PATH, make sure it is installed or set LD to a different linker")
+endif
 AS			=	$(CC) -c -x assembler-with-cpp $(TF_CFLAGS_$(ARCH))
 CPP			=	$(CC) -E
 PP			=	$(CC) -E
@@ -317,10 +320,13 @@ endif
 
 GCC_V_OUTPUT		:=	$(shell $(CC) -v 2>&1)
 
+# LD = armlink
 ifneq ($(findstring armlink,$(notdir $(LD))),)
 TF_LDFLAGS		+=	--diag_error=warning --lto_level=O1
 TF_LDFLAGS		+=	--remove --info=unused,unusedsymbols
 TF_LDFLAGS		+=	$(TF_LDFLAGS_$(ARCH))
+
+# LD = gcc (used when GCC LTO is enabled)
 else ifneq ($(findstring gcc,$(notdir $(LD))),)
 # Pass ld options with Wl or Xlinker switches
 TF_LDFLAGS		+=	-Wl,--fatal-warnings -O1
@@ -337,11 +343,17 @@ ifneq (${ERRATA_A53_843419},1)
 endif
 TF_LDFLAGS		+= 	-nostdlib
 TF_LDFLAGS		+=	$(subst --,-Xlinker --,$(TF_LDFLAGS_$(ARCH)))
+
+# LD = gcc-ld (ld) or llvm-ld (ld.lld) or other
 else
 TF_LDFLAGS		+=	--fatal-warnings -O1
 TF_LDFLAGS		+=	--gc-sections
+# ld.lld doesn't recognize the errata flags,
+# therefore don't add those in that case
+ifeq ($(findstring ld.lld,$(notdir $(LD))),)
 TF_LDFLAGS		+=	$(TF_LDFLAGS_$(ARCH))
 endif
+endif
 
 DTC_FLAGS		+=	-I dts -O dtb
 DTC_CPPFLAGS		+=	-nostdinc -Iinclude -undef -x assembler-with-cpp

docs/getting_started/initial-build.rst

@@ -18,9 +18,10 @@ Performing an Initial Build
 
    It is possible to build TF-A using Clang or Arm Compiler 6. To do so
    ``CC`` needs to point to the clang or armclang binary, which will
-   also select the clang or armclang assembler. Be aware that the
-   GNU linker is used by default.  In case of being needed the linker
-   can be overridden using the ``LD`` variable. Clang linker version 6 is
+   also select the clang or armclang assembler. Be aware that for Arm Compiler,
+   the GNU linker is used by default. However for Clang LLVM linker (LLD)
+   is used by default. In case of being needed the linker can be overridden
+   using the ``LD`` variable. LLVM linker (LLD) version 9 is
    known to work with TF-A.
 
    In both cases ``CROSS_COMPILE`` should be set as described above.
@@ -114,4 +115,4 @@ Performing an Initial Build
 
 --------------
 
-*Copyright (c) 2019, Arm Limited. All rights reserved.*
+*Copyright (c) 2020, Arm Limited. All rights reserved.*