[m-rev.] for review: add a workaround for Mantis bug #492

[Julien Fischer]

On Thu, 16 Jan 2020, Peter Wang wrote:

> On Thu, 16 Jan 2020 18:58:55 +1100 (AEDT), Julien Fischer <jfischer at opturion.com> wrote:
>>
>> For review by anyone.
>>
>> Branches: master, 20.01
>>
>> --------------------------
>>
>> Add a workaround for Mantis bug #492.
>>
>> In debug grades that use global register variables, the generated C code is
>> triggering an internal error in GCC 9 on x86_64 machines. Force GCC to compile
>> at -O0 in this case as that seems to be the only workaround for the issue.
>>
>> scripts/mgnuc.in:
>>      Force GCC to use -O0 in the above case.
>>
>> compiler/compile_target_code.m:
>>      Do the same when GCC is invoked directly by the Mercury compiler.
>>
>>      Re-arrange some of the code that applies C compiler bug workarounds
>>      to make this possible.
>>
>>      Add an XXX about an overly broad bug workaround on darwin; I'll look
>>      into that separately.

...

> The bug is supposed to be fixed but the fix didn't make it into
> gcc 9.2, so maybe limit the workaround to gcc 9.1 and gcc 9.2.

Done.

> I'll try verify a gcc snapshot tomorrow.

Thanks.

Julien.