Subject: Re: [v2] powerpc/32: Avoid unsupported flags with clang

From: Michael Ellerman <[email-masked]>
Date: Sat, 22 Dec 2018 20:55:04 +1100 (AEDT)

On Mon, 2018-11-12 at 05:28:06 UTC, Joel Stanley wrote:
> When building for ppc32 with clang these flags are unsupported:
> 
>   -ffixed-r2 and -mmultiple
> 
> llvm's lib/Target/PowerPC/PPCRegisterInfo.cpp marks r2 as reserved on
> when building for SVR4ABI and !ppc64:
> 
>   // The SVR4 ABI reserves r2 and r13
>   if (Subtarget.isSVR4ABI()) {
>     // We only reserve r2 if we need to use the TOC pointer. If we have no
>     // explicit uses of the TOC pointer (meaning we're a leaf function with
>     // no constant-pool loads, etc.) and we have no potential uses inside an
>     // inline asm block, then we can treat r2 has an ordinary callee-saved
>     // register.
>     const PPCFunctionInfo *FuncInfo = MF.getInfo<PPCFunctionInfo>();
>     if (!TM.isPPC64() || FuncInfo->usesTOCBasePtr() || MF.hasInlineAsm())
>       markSuperRegs(Reserved, PPC::R2);  // System-reserved register
>     markSuperRegs(Reserved, PPC::R13); // Small Data Area pointer register
>   }
> 
> This means we can safely omit -ffixed-r2 when building for 32-bit
> targets.
> 
> The -mmultiple/-mno-multiple flags are not supported by clang, so
> platforms that might support multiple miss out on using multiple word
> instructions.
> 
> We wrap these flags in cc-option so that when Clang gains support the
> kernel will be able use these flags.
> 
> Clang 8 can then build a ppc44x_defconfig which boots in Qemu:
> 
>   make CC=clang-8 ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-  ppc44x_defconfig
>   ./scripts/config -e CONFIG_DEVTMPFS -d DEVTMPFS_MOUNT
>   make CC=clang-8 ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-
> 
>   qemu-system-ppc -M bamboo \
>    -kernel arch/powerpc/boot/zImage \
>    -dtb arch/powerpc/boot/dts/bamboo.dtb \
>    -initrd ~/ppc32-440-rootfs.cpio \
>    -nographic -serial stdio -monitor pty -append "console=ttyS0"
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/261
> Link: https://bugs.llvm.org/show_bug.cgi?id=39556
> Link: https://bugs.llvm.org/show_bug.cgi?id=39555
> Signed-off-by: Joel Stanley <[email-masked]>
> Reviewed-by: Nick Desaulniers <[email-masked]>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/72e7bcc2cdf82bf03caaa5e6c9b013

cheers