we activate code (CODE_ON) only when the target labels are used,
which doesn't happen during nocode_wanted regions. So, we cannot
just switch off code either during nocode_wanted regions, nothing
would switch it on again (except in the happy coincidences when we
outright save/restore nocode_wanted). See the testcase for one
example, reduced from code generated by yarpgen: in
ext = (xxx || 1) // #1
|| ((xxx && 1) || 1) // #2
code is first suppressed in #1 normally, then (before this commit)
was suppressed via CODE_OFF during #2 (via indirect gjmp),
then the suppression from #1 was undone, but nothing undoes the
suppression from #2 anymore as everything therein was generated
while nocode_wanted was active.
So, we would either need to save/restore nocode_wanted around
some more expressions, activate CODE_ON also with unused labels (breaks
some optimizations we want), or deactivate CODE_OFF only when not
already in nocode_wanted state. This does the latter.
A more automatic approach to code suppression (aka. nocode_wanted)
The simple rules are:
- Clear 'nocode_wanted' at (im/explicit) label IF it was used
- Set 'nocode_wanted' after unconditional jumps
Also in order to test this then I did add the "function might
return no value" warning, and then to make that work again I
did add the __attribute__((noreturn)).
Also moved the look ahead label check into the type parser
to gain a little speed.
Some more subtle issues with code suppression:
- outputting asms but not their operand setup is broken
- but global asms must always be output
- statement expressions are transparent to code suppression
- vtop can't be transformed from VT_CMP/VT_JMP when nocode_wanted
Also remove .exe files from tests2 if they don't fail.
