On 9/15/06, Ross Ridge <[EMAIL PROTECTED]> wrote:
Ross Ridge writes: >I don't think this is a good idea. With different compiler options the >same RTL can generate different assembly instructions. Consider the case >of compiling the same function multiple times with different names and >different CPU architectures selected. You'd actually want the linker >to merge the functions that ended up having the same assembly, but not >the ones with the same RTL but different assembly. Daniel Berlin writes: >So basically you are saying if you don't know what you are doing, or >know you don't want to use it, you shouldn't be using it. No, and I can't see how how you've came up with such an abusurd misintepretation of what I said. As I said clearly and explicity, the example I gave was where you'd want to use function merging.
Whatever. Why would you turn on function merging if you are trying to specifically get the compiler to produce different code for your functions than it did before? Not that it matters, since it won't do it anyway since it takes compiler options into account. As an FYI, you already have this situation with linkonce functions.
>The current hash actually takes into account compiler options as a >starting value for the hash, btw!) Well, then that brings up the other problem I have with this, figuring out exactly which options and which parts of the RTL should be hashed seems to be too error prone.
Error prone? We do it already, and rely on it to perform value numbering on RTL. Conservatively coming up with correct answers to determining the value pieces of RTL compute is neither error prone, nor difficult.
I think this is best done by linker which can much more reliably compare the contents of functions to see if they are the same.
No it can't. It has no idea what a function consists of other than a bunch of bytes, in pretty much all cases. It certainly shouldn't be disassembling the functions and comparing them to see if they will always generate the same results and have the same side effects. Even if it could, it would take the same kind of value numbering optimization infrastructure to get good results. Stupid byte comparisons of functions generally won't save you anything truly interesting.
