I'm against compiling this particular example to return a constant. I presumably wrote the awkward and unnatural construction return *x because I want to to force a load from x at return. If I wanted to return a constant, I'd have written it differently! I'm odd though in that I occasionally do optimizations to the level where I intentionally need to force a reload to get the assembly that I want.
Philosophically, I think our difference might be that you want to conclude that one answer to this question directly implies that the "compiler can't omit any load", while I'd probably argue that it's actually OK for the compiler to treat cases differently based on the apparent intent of the programmer. Or maybe it's OK to treat things differently if f() can be analyzed by the compiler than if it's in a shared library.
It would be interesting to see whether your prediction holds: do a majority actually want to return a constant here? My instinct is that C programmers who complain about the compiler treatment of UB behavior will agree with me, but that C++ programmers dependent on optimizations of third party templates might be more likely to agree with you.
Oh, so you are on "that's what I want!" camp. But I am pretty sure you are in minority, or at the very least economic minority. Slowdown implied by this semantics is large, and easily costs millions of dollars.
> while I'd probably argue that it's actually OK for the compiler to treat cases differently based on the apparent intent of the programmer.
This is actually what I am looking for, i.e. answer to "then what do you mean?". Standard should define how to divine the apparent intent of the programmer, so that compilers can do divination consistently. So far, proposals have been lacking in detailed instruction of how to do this divination.
Bugs based on optimizations that compilers make based on assumptions enabled by undefined behavior (like the null check issue from 2009 in the Linux kernel) actually don't cost very much. They get a disproportionate amount of scrutiny relative to their importance.
I'm against compiling this particular example to return a constant. I presumably wrote the awkward and unnatural construction return *x because I want to to force a load from x at return. If I wanted to return a constant, I'd have written it differently! I'm odd though in that I occasionally do optimizations to the level where I intentionally need to force a reload to get the assembly that I want.
Philosophically, I think our difference might be that you want to conclude that one answer to this question directly implies that the "compiler can't omit any load", while I'd probably argue that it's actually OK for the compiler to treat cases differently based on the apparent intent of the programmer. Or maybe it's OK to treat things differently if f() can be analyzed by the compiler than if it's in a shared library.
It would be interesting to see whether your prediction holds: do a majority actually want to return a constant here? My instinct is that C programmers who complain about the compiler treatment of UB behavior will agree with me, but that C++ programmers dependent on optimizations of third party templates might be more likely to agree with you.