John Fine wrote:
But, if the machine has a 32bit by 32bit multiply that produces the identical 64bit result, the optimizer is free to use it.
Here's the catch: Optimizers aren't part of the standard. If the standard(s) were to allow for much more explicit expressions, many optimizers would do a much better job. Writing optimizers around standards is what since the 90ies characterizes ``good'' compilers. Here's where the kewl stuff happens. A strictly standard conforming C/C++ frontend would not (actually see the need to) propagate that information to the middle/back ends. Given a multiplication expression of two 32 bit integers that has (one of) its arguments cast to a 64 bit type, all that the middle end receives from the front end is the information that a 64<-64x64 integer multipication is to be performed. Without _extra_ information attached to the tree, there is no way other than guessing that the middle/back ends can infer from that tree that it was actually meant as 64<-32x32 or indeed 64<-64x64 as modulo 2^64 multiplication. Cheers, Christian
