Nick Desaulniers [off-list ref] writes:

On Tue, May 21, 2024 at 5:39 AM Michael Ellerman [off-list ref] wrote:

quoted

Building ppc64le_defconfig with GCC 14 fails with assembler errors:

    CC      fs/readdir.o
  /tmp/ccdQn0mD.s: Assembler messages:
  /tmp/ccdQn0mD.s:212: Error: operand out of domain (18 is not a multiple of 4)
  /tmp/ccdQn0mD.s:226: Error: operand out of domain (18 is not a multiple of 4)
  ... [6 lines]
  /tmp/ccdQn0mD.s:1699: Error: operand out of domain (18 is not a multiple of 4)

A snippet of the asm shows:

  # ../fs/readdir.c:210:         unsafe_copy_dirent_name(dirent->d_name, name, namlen, efault_end);
         ld 9,0(29)       # MEM[(u64 *)name_38(D) + _88 * 1], MEM[(u64 *)name_38(D) + _88 * 1]
  # 210 "../fs/readdir.c" 1
         1:      std 9,18(8)     # put_user       # *__pus_addr_52, MEM[(u64 *)name_38(D) + _88 * 1]

The 'std' instruction requires a 4-byte aligned displacement because
it is a DS-form instruction, and as the assembler says, 18 is not a
multiple of 4.

The fix is to change the constraint on the memory operand to put_user(),
from "m" which is a general memory reference to "YZ".

The "Z" constraint is documented in the GCC manual PowerPC machine
constraints, and specifies a "memory operand accessed with indexed or
indirect addressing". "Y" is not documented in the manual but specifies
a "memory operand for a DS-form instruction". Using both allows the
compiler to generate a DS-form "std" or X-form "stdx" as appropriate.

The change has to be conditional on CONFIG_PPC_KERNEL_PREFIXED because
the "Y" constraint does not guarantee 4-byte alignment when prefixed
instructions are enabled.

Unfortunately clang doesn't support the "Y" constraint so that has to be
behind an ifdef.

Filed: https://github.com/llvm/llvm-project/issues/92939

Thanks. I will file one to have the GCC constraint documented.

cheers