https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64537
--- Comment #4 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
b is used twice, once shifted left by 3 and once directly.
We could write this as
subs x3, x0, x1, sxth 3
beq .L5
add w0, w2, w1, sxth <= Now extended
add w0, w0, w3
ret
.p2align 2
.L5:
add w0, w2, w0
ret
which in this specific case would perhaps be more efficient, but in practice
it's quite hard to get this sort of multiple-use right.
I think this is a special case, however, of the more common 'un-cse' type of
problem, where multiple uses of an extended (or shifted) value are always
commoned up.
Note that modern CPUs may take an extra cycle to perform an ALU-with-shift type
operation, eliminating the benefit of sinking multiple uses down into the ALU
operations themselves.
