On Wed, Apr 11, 2018 at 12:48 AM, James Hogan <jhogan at kernel.org> wrote: > Hi Arnd, > > On Tue, Dec 19, 2017 at 12:39:33PM +0100, Arnd Bergmann wrote: >> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h >> index 5d595cfdb2c4..66cfdad68f7e 100644 >> --- a/include/linux/compiler-gcc.h >> +++ b/include/linux/compiler-gcc.h >> @@ -205,6 +205,15 @@ >> #endif >> >> /* >> + * calling noreturn functions, __builtin_unreachable() and __builtin_trap() >> + * confuse the stack allocation in gcc, leading to overly large stack >> + * frames, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365 >> + * >> + * Adding an empty inline assembly before it works around the problem >> + */ >> +#define barrier_before_unreachable() asm volatile("") >> + >> +/* >> * Mark a position in code as unreachable. This can be used to >> * suppress control flow warnings after asm blocks that transfer >> * control elsewhere. >> @@ -214,7 +223,11 @@ >> * unreleased. Really, we need to have autoconf for the kernel. >> */ >> #define unreachable() \ >> - do { annotate_unreachable(); __builtin_unreachable(); } while (0) >> + do { \ >> + annotate_unreachable(); \ >> + barrier_before_unreachable(); \ >> + __builtin_unreachable(); \ >> + } while (0) > > Unfortunately this breaks microMIPS builds (e.g. MIPS > micro32r2_defconfig and micro32r2el_defconfig) on gcc 7.2, due to the > lack of .insn in the asm volatile. Because of the > __builtin_unreachable() there is no code following it. Without the empty > asm the compiler will apparently put the .insn there automatically, but > with the empty asm it doesn't. Therefore the assembler won't treat an > immediately preceeding label as pointing at 16-bit microMIPS > instructions which need the ISA bit set, i.e. bit 0 of the address. > This causes assembler errors since the branch target is treated as a > different ISA mode: > > arch/mips/mm/dma-default.s:3265: Error: branch to a symbol in another ISA mode > arch/mips/mm/dma-default.s:5027: Error: branch to a symbol in another ISA mode Ok, I see. > Due to a compiler bug on gcc 4.9.2 -> somewhere before 7.2, Paul > submitted these patches a while back: > https://patchwork.linux-mips.org/patch/13360/ > https://patchwork.linux-mips.org/patch/13361/ > > Your patch (suitably fixed for microMIPS) would I imagine fix that issue > too (it certainly fixes the resulting link error on microMIPS builds > with an old toolchain). > > Before I forward port those patches to add .insn for MIPS, is that sort > of approach (an arch specific asm/compiler-gcc.h to allow MIPS to > override barrier_before_unreachable()) an acceptable fix? That sounds fine to me. However, I would suggest making that asm/compiler.h instead of asm/compiler-gcc.h, so we can also use the same file to include workarounds for clang if needed. Arnd
