Instead of shifting left by 1, we can first shift right by 2 and then
left by 3. This is both faster and smaller, because we get the right
shift for free with the masking and the left shift for free with the
address calculation. It also happens to match the pseudocode more
closely, which is always nice for readability.
This slightly improves instruction-level parallelism in Jit64's slow
dispatcher by shifting the PC left instead of the MSR.
In the past, this also enabled an optimization in JitArm64's fast path
where we could use LDP to load normalEntry and msrBits in one
instruction, but this was superseded by fd9c970.
Just to make the code easier to understand at a glance. I especially
found it a bit annoying to reason about whether callee-saved registers
like W28 were being used because we needed a callee-saved register or
just for no reason in particular.
X8 and up is what compilers normally use when they're not register
starved.
AArch64's handling of NaNs in arithmetic instructions matches PowerPC's
as long as no more than one of the operands is NaN. If we know that all
inputs except the last input are non-NaN, we can therefore skip checking
the last input. This is an optimization that in principle only works for
non-SIMD operations, but ps_sumX effectively is non-SIMD as far as the
arithmetic part of it is concerned, so we can use it there too.
FL_EVIL is only used for blocking instructions from being reordered.
There are three types of instructions which have FL_EVIL set:
1. CR operations. The previous commits improved our CR analysis
and removed FL_EVIL from these instructions.
2. Load/store operations. These are always blocked from reordering
due to always having canCauseException set.
3. isync. I don't know if we actually need to prevent reordering
around this one, since as far as I know we only do reorderings
that are guaranteed to not change the behavior of the program.
But just in case, I've renamed FL_EVIL to FL_NO_REORDER instead of
removing it entirely, so that it can be used for this instruction.
Other than the CR instructions, which we now analyze properly,
all the covered instructions are not integer operations and also
have either FL_ENDBLOCK or FL_EVIL set, so there are two other
checks in CanSwapAdjacentOps that will reject them.
This brings the analysis done for condition registers
more in line with the analysis done for GPRs and FPRs.
This gets rid of the old wantsCR member, which wasn't actually
used anyway. In case someone wants it again in the future, they
can compute the bitwise inverse of crDiscardable.
Instead of materializing the quiet bit in a register and ORing the NaN
with it, we can perform an arithmetic operation on the NaN. This is a
cycle or two slower on some CPUs in cases where generating the quiet bit
pipelined well, but this is farcode that rarely runs, so instruction
fetch latency is the bigger concern. And for non-SIMD cases, we also
save a register.
By using MOVI2R+MOVI2R+CSEL in the zero case instead of doing bitwise
operations on the output of the other MOVI2R+MOVI2R+CSEL, we avoid using
BFI, an instruction that takes two cycles on most CPUs. The instruction
count is the same and the pipelining should be at least equally good.
This is a little trick I came up with that lets us restructure our float
classification code so we can exit earlier when the float is normal,
which is the case more often than not.
First we shift left by 1 to get rid of the sign bit, and then we count
the number of leading sign bits. If the result is less than 10 (for
doubles) or 7 (for floats), the float is normal. This is because, if the
float isn't normal, the exponent is either all zeroes or all ones.
Not sure if this was causing correctness issues – it depends on whether
the HLE code was actually reading the discarded registers – but it was
at least causing annoying assert messages in one piece of homebrew.
