Add a prepare_pch_save target hook

[Richard Sandiford]

Mike Stump <mikestump@comcast.net> writes:
> On Dec 4, 2011, at 4:02 AM, Richard Sandiford <rdsandiford@googlemail.com> wrote:
>> A while back I added the target_globals structure, to allow a backend
>> to switch between two very different ISA modes without paying the full
>> target_reinit penalty each time.  This made a huge difference to compile
>> time, but had a drawback: the target_globals structure contained both
>> GGC and non-GGC data.  This meant that secondary target_globals structures
>> like mips16_globals couldn't be saved correctly in PCH files.
>
>> +  /* We are called in a context where the current MIPS16 vs. non-MIPS16
>> +     setting should be irrelevant.  The question then is: which setting
>> +     makes most sense at load time?
>> +
>> +     The PCH is loaded before the first token is read.  We should never
>> +     have switched into MIPS16 mode by that point,
>
> If there is any way to say:
>
>   #pragma mips16
>
> to globally switch state into mips16 mode, then I believe the patch is
> wrong.

The global switching of state happens when we start to compile a function,
which means we must have seen a C token.  The target_globals state is
non-MIPS16 up until then, even if -mips16 is passed on the command line,
or if the command line is changed by pragmas.

Besides, I think:

         #pragma GCC optimize ...X...
         #include "foo.h"

breaks the PCH contract if X is incompatible with the flags used at
the beginning of the PCH compilation.

Richard