On 27/09/18 08:16, Richard Sandiford wrote:
On keeping the complexity down:if (side_effects_p (x)) return NULL_RTX; makes this quadratic for chains of unary operations. Is it really needed? The code after it simply recurses on operands and doesn't discard anything itself, so it looks like the VEC_MERGE call to side_effects_p would be enough.
The two calls do not check the same thing. The other one checks the other operand of a vec_merge, and this checks the current operand.
I suppose it's safe to discard a VEC_MERGE when the chosen operand contains side effects, but I'm not so sure when the VEC_MERGE itself is an operand to an operator with side effects. I'm having a hard time inventing a scenario in which a PRE_INC could contain a VEC_MERGE, but maybe a volatile MEM or ASM_OPERANDS could do?
Conversely, I don't see that side-effects deep down in an expression should stop us transforming it as a high level.
Is there an equivalent to side_effects_p that doesn't recurse? Should there be?
Andrew
