On Thu, 26 Sep 2019, Konstantin Shegunov wrote: > Hello, > I have a piece of floating point EFT arithmetic in my code, but I get a > rather strange code generation for it. Here's the actual source code and > flags I've experimented with: https://godbolt.org/z/oJEs4U > > The first pane is what I expect to get as output, that is, all the > arithmetic operations in the assembly. However the strange thing is I have > to compile with -funsafe-math-optimizations to actually produce that. > The second pane is the same thing but with -fno-unsafe-math-optimizations > (default) and surprisingly the optimizer strips the relevant instructions. The 'add' function is inlined into main, and floating-point operations that would compute result.c are eliminated as dead code because result.c is never used. (and because the compiler doesn't attempt to preserve floating-point exceptions that those eliminated instructions might raise) > I would've thought the -fno-associative-math enough to prevent that from > happening but for some reason it's not. > The third pane I added for good measure. It illustrates the same flags as > the first one (the one that generates the correct assembly), but the flags > are applied through an attribute directly to the symbol. Even more > surprisingly this generates yet another completely different piece of > output, which doesn't even seem to do what's supposed to (unsafe means > unsafe in this case, I gather). In this case inlining does not happen due to flags being different between 'main' and 'add'. In the first case inlining also does not happen for the same reason. Alexander
