"Matt Lee" <reachmatt.lee@xxxxxxxxx> writes:
> Here is what I have,
>
> case AND:
> case IOR:
> case XOR:
> *total = COSTS_N_INSNS (1);
> return true;
> case ASHIFT:
> case ASHIFTRT:
> case LSHIFTRT:
> if (GET_CODE (XEXP (x, 1)) == CONST_INT)
> *total = COSTS_N_INSNS (INTVAL (XEXP (x, 1)));
> return true;
>
>
> This looks quite OK to me. I tried debugging if rtx_costs was doing
> something wrong. I do see rtx_cost being invoked for the lshiftrt
> expressions in question, but never for the "and". Seems like GCC had
> pre-decided to only use lshiftrt even though it is expensive.
Your computations look wrong to me. You are saying that an AND always
costs 1 instruction. But you need to actually look at the operands.
Specifically, the compiler is going to compare these two:
(and (reg ...) (const_int ...)
(and (ashiftrt (reg ...) (const_int ...)) (const_int 1))
> Any other ideas? Btw, I couldn't find prefer_and_bit_test in dojump.c.
I'm looking at the current version of gcc. prefer_and_bit_test was
added 2004-03-20, so I guess that just missed 3.4. I don't have 3.4
sources handy. In general the decision is made in the do_jump
function.
Ian
