On 08/06/2018 11:15 AM, Martin Sebor wrote: >>> These examples do not aim to be valid C, they just point out limitations >>> of the middle-end design, and a good deal of the problems are due >>> to trying to do things that are not safe within the boundaries given >>> by the middle-end design. >> I really think this is important -- and as such I think we need to move >> away from trying to describe scenarios in C because doing so keeps >> bringing us back to the "C doesn't allow XYZ" kinds of arguments when >> what we're really discussing are GIMPLE semantic issues. >> >> So examples should be GIMPLE. You might start with (possibly invalid) C >> code to generate the GIMPLE, but the actual discussion needs to be >> looking at GIMPLE. We might include the C code in case someone wants to >> look at things in a debugger, but bringing the focus to GIMPLE is really >> important here. > > I don't understand the goal of this exercise. Unless the GIMPLE > code is the result of a valid test case (in some language GCC > supports), what does it matter what it looks like? The basis of > every single transformation done by a compiler is that the source > code is correct. If it isn't then all bets are off. I'm no GIMPLE > expert but even I can come up with any number of GIMPLE expressions > that have undefined behavior. What would that prove? The GIMPLE IL is less restrictive than the original source language. The process of translation into GIMPLE and optimization can create situations in the GIMPLE IL that can't be validly represented in the original source language. Subobject crossing being one such case, there are certainly others. We have to handle these scenarios correctly.
My favorite from a long time ago was the RTL loop optimizer creating a pointer well outside the bounds of an object. That pointer was then used in a reg+d addressing mode where the displacement brought the final effective address back into the bounds of the object. You can't validly do that in C/C++, but it was certainly valid RTL and it was useful to allow creation of such pointers which were outside the bounds of the object. Jeff
