Messages in this thread | | | From | Brian Gerst <> | Date | Mon, 11 May 2020 14:52:16 -0400 | Subject | Re: [PATCH v5] x86: bitops: fix build regression |
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On Fri, May 8, 2020 at 2:32 PM Nick Desaulniers <ndesaulniers@google.com> wrote: > > This is easily reproducible via CC=clang+CONFIG_STAGING=y+CONFIG_VT6656=m. > > It turns out that if your config tickles __builtin_constant_p via > differences in choices to inline or not, these statements produce > invalid assembly: > > $ cat foo.c > long a(long b, long c) { > asm("orb\t%1, %0" : "+q"(c): "r"(b)); > return c; > } > $ gcc foo.c > foo.c: Assembler messages: > foo.c:2: Error: `%rax' not allowed with `orb' > > Use the `%b` "x86 Operand Modifier" to instead force register allocation > to select a lower-8-bit GPR operand. > > The "q" constraint only has meaning on -m32 otherwise is treated as > "r". Not all GPRs have low-8-bit aliases for -m32. > > Cc: Jesse Brandeburg <jesse.brandeburg@intel.com> > Link: https://github.com/ClangBuiltLinux/linux/issues/961 > Link: https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/ > Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers > Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast") > Reported-by: Sedat Dilek <sedat.dilek@gmail.com> > Reported-by: kernelci.org bot <bot@kernelci.org> > Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com> > Suggested-by: Brian Gerst <brgerst@gmail.com> > Suggested-by: H. Peter Anvin <hpa@zytor.com> > Suggested-by: Ilie Halip <ilie.halip@gmail.com> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-By: Brian Gerst <brgerst@gmail.com>
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