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Makefile: Support building with Clang and LLVM binutils

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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@jrtc27.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Anup Patel <anup.patel@wdc.com>
This commit is contained in:
Jessica Clarke
2021-07-11 03:28:23 +01:00
committed by Anup Patel
parent 09ad811ec4
commit 2942777425
2 changed files with 105 additions and 9 deletions
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48
README.md
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@@ -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
-----------------------