Andrew Pinski wrote: It does buy you something for code like:
if (a - 10 < 20) Well that particular example is far fetched in code that people expect to run efficiently, but with a bit of effort you could come up with a more realistic example. Compilers are always full of such optimizations which are simple to do and get put in without any sound basis for determining that they actually help. Now if they don't change semantics, then unhelpful optimizations (by this I mean optimizations which don't in practice help real applications), are only slightly damaging (they damage maintainability and reliability by creating additional unnecessary special cases). But if they are NOT semantically neutral in terms of expected usage, then the burden should be much higher. In my view, this comparison optimization should not have been put in without justification given that it clearly does affect the semantics of real code. Indeed if you really see code like if (a - 10 < 20) in place of if (a < 30) a very likely explanation is that you are deliberately doing something strange with wrap around, and should leave it alone. You say "I don't want to see this optimization removed", why not? The answer should hopefully be something like because test # 5 in benchmark suite xxx takes a 3% hit or even better awk processing expressions like bla bla takes an n% hit However, I suspect the real answer is "it's a neat optimization, I did it, and it can only help, so why not leave it in" And that's to me not good enough for optimziations that break existing programs. Note that if (a - 10 < 20) in the presence of wrap around semantics means something like if (a < 30 && a > minint + 9) ... and perhaps that was exactly what was meant!
