[cfe-dev] [LLVMdev] fatal error: error in backend: 32-bit absolute addressing is not supported in 64-bit mode

[Tim Northover]

Hi Jack,

On 11 June 2015 at 08:21, Jack Howarth <howarth.mailing.lists at gmail.com> wrote:
>    While building FSF gcc 5.1 against Apple's public Xcode 7 beta's
> new clang-based assembler, I ran into rejected assembly instructions
> during the -m32 multilb build of libjava.

The diagnostics here don't seem to have changed in Clang, going back
to even early Xcode 6.

> natArray.s:1110:2: error: ambiguous instructions require an explicit
> suffix (could be 'filds', or 'fildl')
>         fild    8(%esi,%edi,2)

This seems reasonable to me. It seems GCC arbitrarily chooses "filds",
but there's no grand logic behind that decision. I prefer requiring an
explicit disambiguator.

> clang -cc1as: fatal error: error in backend: 32-bit absolute
> addressing is not supported in 64-bit mode

Technically, I believe MachO could support such a relocation. But it's
never one you'd actually want to use because PAGEZERO occupies the low
32-bits of the address space as a guard against null pointer accesses.
So I think this is probably better than the alternative too.

I'd probably suggest making the test use "filds (%ebp)", which should
assemble in both 32-bit and 64-bit modes and sidesteps the whole
relocation issue. "filds mem(%rip)" would be a sound 64-bit
instruction closer to the original, but wouldn't assemble on 32-bit.

Cheers.

Tim.