bfd: optimize bfd_elf_hash

[Nathan Sidwell]

I happened to be poking at llvm's implementation of the sysv hash and notice 
binutils' version had similar optimization issues.

neither gcc nor llvm can spot these xforms, and the if (...) at least obscures 
the data flow IMHO anyway.

The bfd_elf_hash loop is taken straight from the sysV document, but it
is poorly optimized. This refactoring removes about 5 x86 insns from
the 15 insn loop.

1) The if (..) is meaningless -- we're xoring with that value, and of
course xor 0 is a nop. On x86 (at least) we actually compute the xor'd
value and then cmov.  Removing the if test removes the cmov.

2) The 'h ^ g' to clear the top 4 bits is not needed, as those 4 bits
will be shifted out in the next iteration.  All we need to do is sink
a mask of those 4 bits out of the loop.

3) anding with 0xf0 after shifting by 24 bits can allow betterin
encoding on RISC ISAs than masking with '0xf0 << 24' before shifting.
RISC ISAs often require materializing larger constants.

nathan

-- 
Nathan Sidwell
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