This new variable length pair of integer implementation is taking direct
advantage of the most common aspects of FEX's JIT in that most x86
instructions are <= 8-bytes in length, and the ARM implementations of
those are /usually/ 16 instructions in length or less. Also only
unsigned offsets in this implementation since the common case is forward
incrementing.
This converts a majority of 16-bit vl encodings in to an 8-bit encoding
instead, shaving space off the RIP reconstruction data.
An additional optimization is for the 16-bit pair of integers, we
continue this optimization through but with more bits and changing over
to signed. This captures the second most common cases of /slightly/
larger increments and small loops.
Pairs of 32-bit and 64-bit integers are unoptimized since they are
uncommon.
When #2722 implemented this initially and #4271 switched over to signed
int16_t there was assumptions made that int16_t was a reasonable
trade-off in encoding size versus needing to deal with 8-bit values
being too small in some cases.
In the common case we are almost always encoding 8-bit values because
instructions are typically linear (and less than 15-bytes in size), but
16-bit was chosen because optimizing JIT and multiple instructions that
don't cause exceptions can add up to larger than 8-bit.
Instead of hardcoding 16-bit values, implement a variable length integer
class where ~96.8% of values are 8-bit encoded, and the remaining 3.19% are encoded using 16-bit.
Due to some constraints that #4271 put in place, we can basically
guarantee currently that branch targets are within 16-bit. The VL class
does support 32-bit and 64-bit as well so if we change behaviour then
nothing needs to change.
Some stats when running Sonic Mania with multiblock enabled.
Encoded integers: 3,504,907
Encoded 8-bit: 3,393,095 (96.8%)
Encoded 16-bit: 111,812 (3.19%)
Encoded 32/64-bit: 0
Encoded Size: 3,615,181 bytes (3.44MiB)
Fixed encoded size: 7,007,604 bytes (6.68MiB)
Definitely worth using and saves the headache of large RIP/PC offsets
causing problems.
Ryan Houdek