Re: [PATCH 0/5] target/i386: CCOp cleanups

[Paolo Bonzini]

On Mon, Jul 1, 2024 at 4:51 AM Richard Henderson
<richard.henderson@linaro.org> wrote:
> While debugging #2413, I spent quite a bit of time trying to work
> out if the CCOp value was incorrect.  I think the following is a
> worthwhile cleanup, isolating potential problems to asserts.

Hi Richard,

no objections at all to introducing more asserts. I think keeping the
array is a better underlying implementation for cc_op_live() however.

I'm also not very fond of mixing "sized" and "unsized" CCOps in the
4..7 range, there's no real reason why CC_OP_DYNAMIC and CC_OP_CLR
must be close to CC_OP_EFLAGS and the ADCOX CCOps.  I also think it's
clearer to keep CC_OP_POPCNT[BWLQ] (even though in practice only one
will be used because popcnt needs zero extension anyway).

As an aside, I'm wondering if CC_OP_CLR is particularly important; I
expect "xor reg, reg" to be followed by more ALU operations most of
the time and to not be followed by a jump, so it only saves a spill if
xor reg, reg is followed by a lot or store. If gen_XOR used either
CC_OP_LOGICn or CC_OP_EFLAGS for "xor reg, reg", the values in
decode->cc_op_* (CC_OP_DST=0 for CC_OP_LOGICn; CC_OP_SRC=CC_Z|CC_P for
CC_OP_EFLAGS) would be constant and wouldn't add to register pressure.

Paolo