[PATCH v5 5/6] Makefile: Support building with Clang and LLVM binutils

[Jessica Clarke]

This is intended to mirror the Linux kernel. Building with CC=clang will
use Clang as the compiler but default to using the existing binutils.
Building with LLVM=1 will default to using Clang and LLVM binutils.

Whilst GCC will accept the -N linker option and forward it on to the
linker, Clang will not, and so in order to support both compilers we
must use -Wl, to forward it to the linker as is required for most other
linker options.

Note that there is currently a bug when using Clang as the compiler and
riscv64-linux-gnu-ld as the linker for FW_PIC=y. At first glance this
appears to be a bug in GNU binutils, but this could also be Clang or
OpenSBI at fault in some subtle way. Thus, for now, advise that this
combination be avoided.

Signed-off-by: Jessica Clarke <jrtc27 at jrtc27.com>
---
 Makefile  | 66 +++++++++++++++++++++++++++++++++++++++++++++++++------
 README.md | 48 ++++++++++++++++++++++++++++++++++++++--
 2 files changed, 105 insertions(+), 9 deletions(-)

diff --git a/Makefile b/Makefile
index ba06313..5c188d5 100644
--- a/Makefile
+++ b/Makefile
@@ -76,26 +76,54 @@ OPENSBI_VERSION_MINOR=`grep "define OPENSBI_VERSION_MINOR" $(include_dir)/sbi/sb
 OPENSBI_VERSION_GIT=$(shell if [ -d $(src_dir)/.git ]; then git describe 2> /dev/null; fi)
 
 # Setup compilation commands
+ifneq ($(LLVM),)
+CC		=	clang
+AR		=	llvm-ar
+LD		=	ld.lld
+OBJCOPY		=	llvm-objcopy
+else
 ifdef CROSS_COMPILE
 CC		=	$(CROSS_COMPILE)gcc
-CPP		=	$(CROSS_COMPILE)cpp
 AR		=	$(CROSS_COMPILE)ar
 LD		=	$(CROSS_COMPILE)ld
 OBJCOPY		=	$(CROSS_COMPILE)objcopy
 else
 CC		?=	gcc
-CPP		?=	cpp
 AR		?=	ar
 LD		?=	ld
 OBJCOPY		?=	objcopy
 endif
+endif
+CPP		=	$(CC) -E
 AS		=	$(CC)
 DTC		=	dtc
 
-# Guess the compillers xlen
-OPENSBI_CC_XLEN := $(shell TMP=`$(CC) -dumpmachine | sed 's/riscv\([0-9][0-9]\).*/\1/'`; echo $${TMP})
+ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
+CC_IS_CLANG	=	y
+else
+CC_IS_CLANG	=	n
+endif
+
+ifneq ($(shell $(LD) --version 2>&1 | head -n 1 | grep LLD),)
+LD_IS_LLD	=	y
+else
+LD_IS_LLD	=	n
+endif
+
+ifeq ($(CC_IS_CLANG),y)
+ifneq ($(CROSS_COMPILE),)
+CLANG_TARGET	=	--target=$(notdir $(CROSS_COMPILE:%-=%))
+endif
+endif
+
+# Guess the compiler's XLEN
+OPENSBI_CC_XLEN := $(shell TMP=`$(CC) $(CLANG_TARGET) -dumpmachine | sed 's/riscv\([0-9][0-9]\).*/\1/'`; echo $${TMP})
+
+# Guess the compiler's ABI and ISA
+ifneq ($(CC_IS_CLANG),y)
 OPENSBI_CC_ABI := $(shell TMP=`$(CC) -v 2>&1 | sed -n 's/.*\(with\-abi=\([a-zA-Z0-9]*\)\).*/\2/p'`; echo $${TMP})
 OPENSBI_CC_ISA := $(shell TMP=`$(CC) -v 2>&1 | sed -n 's/.*\(with\-arch=\([a-zA-Z0-9]*\)\).*/\2/p'`; echo $${TMP})
+endif
 
 # Setup platform XLEN
 ifndef PLATFORM_RISCV_XLEN
@@ -106,8 +134,21 @@ ifndef PLATFORM_RISCV_XLEN
   endif
 endif
 
+ifeq ($(CC_IS_CLANG),y)
+ifeq ($(CROSS_COMPILE),)
+CLANG_TARGET	=	--target=riscv$(PLATFORM_RISCV_XLEN)-unknown-elf
+endif
+endif
+
+ifeq ($(LD_IS_LLD),y)
+RELAX_FLAG	=	-mno-relax
+USE_LD_FLAG	=	-fuse-ld=lld
+else
+USE_LD_FLAG	=	-fuse-ld=bfd
+endif
+
 # Check whether the linker supports creating PIEs
-OPENSBI_LD_PIE := $(shell $(CC) -fPIE -nostdlib -Wl,-pie -x c /dev/null -o /dev/null >/dev/null 2>&1 && echo y || echo n)
+OPENSBI_LD_PIE := $(shell $(CC) $(CLANG_TARGET) $(RELAX_FLAG) $(USE_LD_FLAG) -fPIE -nostdlib -Wl,-pie -x c /dev/null -o /dev/null >/dev/null 2>&1 && echo y || echo n)
 
 # Setup list of objects.mk files
 ifdef PLATFORM
@@ -197,7 +238,11 @@ else
 endif
 
 # Setup compilation commands flags
-GENFLAGS	=	-I$(platform_src_dir)/include
+ifeq ($(CC_IS_CLANG),y)
+GENFLAGS	+=	$(CLANG_TARGET)
+GENFLAGS	+=	-Wno-unused-command-line-argument
+endif
+GENFLAGS	+=	-I$(platform_src_dir)/include
 GENFLAGS	+=	-I$(include_dir)
 ifneq ($(OPENSBI_VERSION_GIT),)
 GENFLAGS	+=	-DOPENSBI_VERSION_GIT="\"$(OPENSBI_VERSION_GIT)\""
@@ -211,6 +256,7 @@ CFLAGS		+=	-fno-omit-frame-pointer -fno-optimize-sibling-calls
 CFLAGS		+=	-mno-save-restore -mstrict-align
 CFLAGS		+=	-mabi=$(PLATFORM_RISCV_ABI) -march=$(PLATFORM_RISCV_ISA)
 CFLAGS		+=	-mcmodel=$(PLATFORM_RISCV_CODE_MODEL)
+CFLAGS		+=	$(RELAX_FLAG)
 CFLAGS		+=	$(GENFLAGS)
 CFLAGS		+=	$(platform-cflags-y)
 CFLAGS		+=	-fno-pie -no-pie
@@ -225,18 +271,24 @@ ASFLAGS		+=	-fno-omit-frame-pointer -fno-optimize-sibling-calls
 ASFLAGS		+=	-mno-save-restore -mstrict-align
 ASFLAGS		+=	-mabi=$(PLATFORM_RISCV_ABI) -march=$(PLATFORM_RISCV_ISA)
 ASFLAGS		+=	-mcmodel=$(PLATFORM_RISCV_CODE_MODEL)
+ASFLAGS		+=	$(RELAX_FLAG)
 ASFLAGS		+=	$(GENFLAGS)
 ASFLAGS		+=	$(platform-asflags-y)
 ASFLAGS		+=	$(firmware-asflags-y)
 
 ARFLAGS		=	rcs
 
-ELFFLAGS	+=	-Wl,--build-id=none -N -static-libgcc -lgcc
+ELFFLAGS	+=	$(USE_LD_FLAG)
+ELFFLAGS	+=	-Wl,--build-id=none -Wl,-N -static-libgcc -lgcc
 ELFFLAGS	+=	$(platform-ldflags-y)
 ELFFLAGS	+=	$(firmware-ldflags-y)
 
 MERGEFLAGS	+=	-r
+ifeq ($(LD_IS_LLD),y)
+MERGEFLAGS	+=	-b elf
+else
 MERGEFLAGS	+=	-b elf$(PLATFORM_RISCV_XLEN)-littleriscv
+endif
 MERGEFLAGS	+=	-m elf$(PLATFORM_RISCV_XLEN)lriscv
 
 DTSCPPFLAGS	=	$(CPPFLAGS) -nostdinc -nostdlib -fno-builtin -D__DTS__ -x assembler-with-cpp
diff --git a/README.md b/README.md
index 03c02fb..b296f00 100644
--- a/README.md
+++ b/README.md
@@ -96,8 +96,13 @@ Required Toolchain
 ------------------
 
 OpenSBI can be compiled natively or cross-compiled on a x86 host. For
-cross-compilation, you can build your own toolchain or just download
-a prebuilt one from the [Bootlin toolchain repository].
+cross-compilation, you can build your own toolchain, download a prebuilt one
+from the [Bootlin toolchain repository] or install a distribution-provided
+toolchain; if you opt to use LLVM/Clang, most distribution toolchains will
+support cross-compiling for RISC-V using the same toolchain as your native
+LLVM/Clang toolchain due to LLVM's ability to support multiple backends in the
+same binary, so is often an easy way to obtain a working cross-compilation
+toolchain.
 
 Please note that only a 64-bit version of the toolchain is available in
 the Bootlin toolchain repository for now.
@@ -202,6 +207,45 @@ export PLATFORM_RISCV_XLEN=32
 
 will generate 32-bit OpenSBI images. And vice vesa.
 
+Building with Clang/LLVM
+------------------------
+
+OpenSBI can also be built with Clang/LLVM. To build with just Clang but keep
+the default binutils (which will still use the *CROSS_COMPILE* prefix if
+defined), override the *CC* make variable with:
+```
+make CC=clang
+```
+
+To build with a full LLVM-based toolchain, not just Clang, enable the *LLVM*
+option with:
+```
+make LLVM=1
+```
+
+When using Clang, *CROSS_COMPILE* often does not need to be defined unless
+using GNU binutils with prefixed binary names. *PLATFORM_RISCV_XLEN* will be
+used to infer a default triple to pass to Clang, so if *PLATFORM_RISCV_XLEN*
+itself defaults to an undesired value then prefer setting that rather than the
+full triple via *CROSS_COMPILE*. If *CROSS_COMPILE* is nonetheless defined,
+rather than being used as a prefix for the executable name, it will instead be
+passed via the `--target` option with the trailing `-` removed, so must be a
+valid triple.
+
+These can also be mixed; for example using a GCC cross-compiler but LLVM
+binutils would be:
+```
+make CC=riscv64-unknown-elf-gcc LLVM=1
+```
+
+These variables must be passed for all the make invocations described in this
+document.
+
+NOTE: Using Clang with a `riscv*-linux-gnu` GNU binutils linker has been seen
+to produce broken binaries with missing relocations; it is therefore currently
+recommended that this combination be avoided or *FW_PIC=n* be used to disable
+building OpenSBI as a position-independent binary.
+
 Contributing to OpenSBI
 -----------------------
 
-- 
2.31.0