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target_shift_truncation_mask for all shifts?!

Hi Richard,

I've recently experimented with TARGET_SHIFT_TRUNCATION_MASK
macro and have posted a patch defining it for S/390. 
On S/390 only the least significant six bits of a shift count 
operand are used and I therefore expected the modulo operation 
in the following example to be optimized away:

f (long t, unsigned int shift_count)
  return t << (shift_count % 64);

But I understand that this optimization can't simply be added 
to simplify_rtx.c.
The macro only applies to rtxes coming from named patterns
and not to shifts emitted anywhere else in the optimization process because
combine and co are currently not aware that macro exists. Because 
nobody is able to determine the origin of a shift rtx it would
lead to wrong code performing that kind of optimization in simplify-rtx
for all incoming shifts.


you proposed to take care of this in the 4.1 (formerly 3.6) timeframe fixing
all places where shift rtxes are generated besides optabs.
Is this still on your todo list?



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