While that pattern is indirectly handled via
reassociateShiftAmtsOfTwoSameDirectionShifts(),
that incursme one-use restriction on truncation,
which is pointless since we know that we'll produce a single instruction.
Additionally, *if* we are only looking for sign bit,
we don't need shifts to be identical,
which isn't the case in general,
and is the blocker for me in bug in question:
https://bugs.llvm.org/show_bug.cgi?id=43595
llvm-svn: 374726
Initially (D65380) i believed that if we have rightshift-trunc-rightshift,
we can't do any folding. But as it usually happens, i was wrong.
https://rise4fun.com/Alive/GEwhttps://rise4fun.com/Alive/gN2O
In https://bugs.llvm.org/show_bug.cgi?id=43564 we happen to have
this very sequence, of two right shifts separated by trunc.
And "just" so that happens, we apparently can fold the pattern
if the total shift amount is either 0, or it's equal to the bitwidth
of the innermost widest shift - i.e. if we are left with only the
original sign bit. Which is exactly what is wanted there.
llvm-svn: 373801
Roman Lebedev