Hi Julien and Stefano
I found that the “cc-option/cc-option-add” is not working for “-march=xxx” option on a few very common aarch64 compilers.
Here is what I got when trying to compile XEN on r82 platform.
```
diff --git a/xen/arch/arm/arch.mk b/xen/arch/arm/arch.mk
index 11caec86ba..d2d71baef4 100644
--- a/xen/arch/arm/arch.mk
+++ b/xen/arch/arm/arch.mk
@@ -10,6 +10,10 @@ $(call cc-option-add,CFLAGS,CC,-Wnested-externs)
CFLAGS-$(CONFIG_ARM_32) += -msoft-float
CFLAGS-$(CONFIG_ARM_32) += -mcpu=cortex-a15
+#ifeq ($(CONFIG_ARM64_V8R),y)
+ CFLAGS-$(CONFIG_ARM_64) += $(call cc-option,$(CC),-march=armv8-r,-march=armv8.4-a)
+#endif
+
CFLAGS-$(CONFIG_ARM_64) += -mcpu=generic
CFLAGS-$(CONFIG_ARM_64) += -mgeneral-regs-only # No fp registers etc
$(call cc-option-add,CFLAGS-$(CONFIG_ARM_64),CC,-mno-outline-atomics)
```
My gcc compiler version is as follows:
```
Using built-in specs.
COLLECT_GCC=aarch64-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/aarch64-linux-gnu/9/lto-wrapper
Target: aarch64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.3.0-17ubuntu1~20.04'
--with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,
go,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9
--enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-libquadmath --disable-libquadmath-support --enable-plugin
--enable-default-pie --with-system-zlib --without-target-system-zlib --enable-libpth-m2 --enable-multiarch
--enable-fix-cortex-a53-843419 --disable-werror --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=aarch64-linux-gnu --program-prefix=aarch64-linux-gnu- --includedir=/usr/aarch64-linux-gnu/include
Thread model: posix
gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)
```
However. even if my gcc compiler is indeed not supporting “-march=armv8-r”, above check will not identify it
“unrecognized option” and seek to the “-march=armv8.4-a” as expected.
See the resulting errors, it still uses “-march=armv8-r”:
```
make[3]: Entering directory '/home/penny/FVP/r82/fvp_aemv8r_cortexr82_0.0_6347_gcc6.4/xen/xen'
aarch64-linux-gnu-gcc -MMD -MP -MF ./.asm-offsets.s.d -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall
-Wstrict-prototypes -Wdeclaration-after-statement -Wno-unused-but-set-variable -Wno-unused-local-typedefs
-O1 -fno-omit-frame-pointer -nostdinc -fno-builtin -fno-common -Werror -Wredundant-decls -Wno-pointer-arith -Wvla
-pipe -D__XEN__ -include /home/penny/FVP/r82/fvp_aemv8r_cortexr82_0.0_6347_gcc6.4/xen/xen/include/xen/config.h
-g -march=armv8-r -mgeneral-regs-only -mno-outline-atomics -I/home/penny/FVP/r82/fvp_aemv8r_cortexr82_0.0_6347_gcc6.4
/xen/xen/include -fno-stack-protector -fno-exceptions -fno-asynchronous-unwind-tables -fcf-protection=none
-Wnested-externs '-D__OBJECT_FILE__="asm-offsets.s"' -S -g0 -o asm-offsets.s.new -MQ asm-offsets.s arch/arm/arm64/asm-offsets.c
cc1: error: unknown value ‘armv8-r’ for ‘-march’
cc1: note: valid arguments are: armv8-a armv8.1-a armv8.2-a armv8.3-a armv8.4-a armv8.5-a; did you mean ‘armv8-a’?
make[3]: *** [Makefile:413: asm-offsets.s] Error 1
```
And the reason is that in the implementation of “cc-option”: To handle this we do a test compile, passing the option-under-test,
on a code fragment that will always produce a warning (integer assigned to pointer). We then grep for the option-under-test in
the compiler's output, the presence of which would indicate an "unrecognized command-line option" warning/error.
```
cc-option = $(shell if test -z "`echo 'void*p=1;' | \
$(1) $(2) -S -o /dev/null -x c - 2>&1 | grep -- $(2) -`"; \
then echo "$(2)"; else echo "$(3)"; fi ;)
```
Here, that is, we are trying to grep “-march=armv8-r” in output message of the test compile, and since the output
message(unknown value ‘armv8-r’ for ‘-march’) doesn’t contain the COMPLETE string, it thought the compiler supported this
option, but it actually not…
Tracing back to the commit about the implementation of “cc-option”, it discards the linux method(using exit code to tell) for dealing with
disable-warning options.
See
https://github.com/xen-project/xen/commit/28f8fb7d2b3fde2f5cbe5526ac4f1c932e3f5d26 for more details.
To fix this issue, I could loose the “grep check”, instead trying to grep the COMPLETE option string(“-march=armv8-r”), only partial(armv8-r) is
enough. But, hmmm, the way of greping output message to check is kinds of reliable, IMO, since there is no standard for that.
What do you suggest?
Cheers
--
Penny Zheng
