On 8/7/23 07:28, Helge Deller wrote:
The tcg uses tgen_arithi(ARITH_AND) during fast CPU TLB lookups,
which e.g. translates to:
0x7ff5b011556a: 48 81 e6 00 f0 ff ff andq $0xfffffffffffff000, %rsi
In case the upper 48 bits are all set, the shorter sequence to operate
on the lower 16 bits of the target reg (si) can be used, which will then
be a 2 bytes shorter instruction sequence:
0x7f4488097b31: 66 81 e6 00 f0 andw $0xf000, %si
Signed-off-by: Helge Deller <deller@gmx.de>
Current Intel optimization guidelines
https://www.intel.com/content/www/us/en/content-details/671488/intel-64-and-ia-32-architectures-optimization-reference-manual.html
Section 3.4.2.3, Length Changing Prefixes, suggests that using 16-byte operands
slows decode from 1 cycle to 6 cycles.
Section 3.5.2.3, Partial Register Stalls, says that Skylake has fixed the major
issues that older microarchitectures had with such stalls, but that these
operations have two additional cycles of delay.
So on balance I don't think this is a good tradeoff.