unsupported argument '-m405' to option 'Wa,'

[tpimh]

I'm trying to compile ppc32 kernel using AS=clang (integrated-as in being used). The following error messages are produced when compiling scripts/mod/empty.o:

clang: error: unsupported argument '-m405' to option 'Wa,'

This arguments are added here:

linux/arch/powerpc/Makefile

Line 247 in 45824fc

cpu-as-$(CONFIG_4xx) += -Wa,-m405

The compilation continues without error if the option is removed:

--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -244,7 +244,6 @@ KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
 # often slow when they are implemented at all
 KBUILD_CFLAGS		+= $(call cc-option,-mno-string)
 
-cpu-as-$(CONFIG_4xx)		+= -Wa,-m405
 cpu-as-$(CONFIG_ALTIVEC)	+= $(call as-option,-Wa$(comma)-maltivec)
 cpu-as-$(CONFIG_E200)		+= -Wa,-me200
 cpu-as-$(CONFIG_E500)		+= -Wa,-me500

[nickdesaulniers]

Is this important to have in Clang?

[tpimh]

In my opinion just changing this line to $(call as-option,-Wa$(comma)-m405) should be okay, but it should be tested on real hardware.

[nickdesaulniers]

altivec has that option, so I guess it's ok. If anyone has the hardware to test on, or cares, and this results in a bug, we can fix this then. Otherwise please send the diff you describe.

[tpimh]

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 83522c9fc..1efa99f6c 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -244,15 +244,16 @@ KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
 # often slow when they are implemented at all
 KBUILD_CFLAGS		+= $(call cc-option,-mno-string)
 
-cpu-as-$(CONFIG_4xx)		+= -Wa,-m405
+cpu-as-$(CONFIG_4xx)		+= $(call as-option,-Wa$(comma)-m405)
 cpu-as-$(CONFIG_ALTIVEC)	+= $(call as-option,-Wa$(comma)-maltivec)
-cpu-as-$(CONFIG_E200)		+= -Wa,-me200
-cpu-as-$(CONFIG_E500)		+= -Wa,-me500
+cpu-as-$(CONFIG_E200)		+= $(call as-option,-Wa$(comma)-me200)
+cpu-as-$(CONFIG_E500)		+= $(call as-option,-Wa$(comma)-me500)
 
 # When using '-many -mpower4' gas will first try and find a matching power4
 # mnemonic and failing that it will allow any valid mnemonic that GAS knows
 # about. GCC will pass -many to GAS when assembling, clang does not.
-cpu-as-$(CONFIG_PPC_BOOK3S_64)	+= -Wa,-mpower4 -Wa,-many
+cpu-as-$(CONFIG_PPC_BOOK3S_64)	+= $(call as-option,-Wa$(comma)-mpower4) \
+					$(call as-option,-Wa$(comma)-many)
 cpu-as-$(CONFIG_PPC_E500MC)	+= $(call as-option,-Wa$(comma)-me500mc)
 
 KBUILD_AFLAGS += $(cpu-as-y)

This patch also addresses #672 as it's a very similar issue with ppc64. Not sure if my padding and newlines are okay.

[nickdesaulniers]

still reproducible with:
ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- make CC=clang LLVM_IAS=1 -j71 ppc44x_defconfig all

[nickdesaulniers]

perhaps we can wrap all -m<machine name> flags in LLVM_IAS test block?

[nickdesaulniers]

We should also test if this flag produces changes in the resulting disassembly of the object files that are created with this flag.

[nickdesaulniers]

https://sourceware.org/binutils/docs/as/PowerPC_002dOpts.html#PowerPC_002dOpts

[nickdesaulniers]

cc @aik

[nickdesaulniers]

Reminiscent of #1771 but not fixed by llvm/llvm-project@de8deb5.

cc @qiongsiwu @nemanjai

[qiongsiwu]

Sorry about the delay! I was mostly offline during the Christmas break. Thanks for relaying the issue!

I took a quick look at how clang parses the integrated assembler option (https://github.com/llvm/llvm-project/blob/fa715e2dad4adfe7f8922c299250b4463e1253c9/clang/lib/Driver/ToolChains/Clang.cpp#L2428), and experimented a bit myself. It seems that clang does not support the -Wa,-mxxx style options for the CPU model.

For example,

$ clang -Wa,-m405 test.c                                                                            
clang-16: error: unsupported argument '-m405' to option '-Wa,'
$ clang -Wa,-mppc64 test.c                                                                        
clang-16: error: unsupported argument '-mppc64' to option '-Wa,'
$ clang -Wa,-me200 test.c                                                                   
clang-16: error: unsupported argument '-me200' to option '-Wa,'

However, clang does support the -Wa,-mcpu= options. The following can compile without errors.

$ clang -Wa,-mcpu=405 test.c
$ clang -Wa,-mcpu=ppc64 test.c
$ clang -Wa,-mcpu=e200 test.c

So it seems to me that we will need to add support to the -Wa,-mxxx style option to clang to fix this issue. I am not sure if that is something we'd like to add to clang. @nemanjai what do you think?

[nickdesaulniers]

bump

[nemanjai]

The -Wa,-mxxx options are used because GCC does not produce object code and rather uses an assembler to assemble the asm output it produces. I think any attempt to wrench the handling of -Wa options to the integrated assembler that is part of Clang would be awkward at best.
Would it be feasible for builds that use Clang to specify the CPU simply with -mcpu=... on the compiler command line?

[mpe]

This should be fixed in v6.3-rc as of torvalds/linux@bfb03af.