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

[Peter Wang]

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.
> 
> Julien.

> @@ -442,12 +443,33 @@ gather_c_compiler_flags(Globals, PIC, AllCFlags) :-
>           AppleGCCRegWorkaroundOpt = ""
>       ),
> 
> -    % Workaround performance problem(s) with gcc that causes the C files
> -    % generated in debugging grades to compile very slowly at -O1 and above.
> -    % (Changes here need to be reflected in scripts/mgnuc.in.)
> +    % Last resort workarounds for C compiler bugs.
> +    % Changes here need to be reflected in scripts/mgnuc.in.
> +    %
> +    globals.lookup_bool_option(Globals, exec_trace, ExecTrace),
>       ( if
> -        globals.lookup_bool_option(Globals, exec_trace, yes),
> -        arch_is_apple_darwin(TargetArch)
> +        % We need to disable C compiler optimizations in debugging grades
> +        % in either of the two situations described below.
> +        ExecTrace = yes,
> +        (
> +            % 1. On Apple Darwin systems there are performance problems with
> +            % GCC that cause it to compile the C files generated in debugging
> +            % grades very slowly at -O1 or greater.
> +            %
> +            % XXX we are also enabling this for clang; does it have the
> +            % same performance problems?
> +            %
> +            arch_is_apple_darwin(TargetArch)
> +        ;
> +            % 2. There is a bug in GCC 9.[12] that results in an internal error
> +            % in the LRA pass when compiling generated C files in debugging
> +            % grades that also use global registers on x86_64 machines.
> +            % See: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91430>
> +            %
> +            GlobalRegisters = yes,
> +            C_CompilerType = cc_gcc(yes(9), _, _),
> +            string.prefix(TargetArch, "x86_64")
> +        )

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.
I'll try verify a gcc snapshot tomorrow.

Looks fine otherwise.

Peter