While this problem will never be solved in computers with inexact representation of floating-point numbers,
That's simply not true. Compilers have been dealing with this for a long time. For example, good Fortran compilers take great pains not to reorder floating-point computation. There are many solutions available for the x87 problem other than -ffloat-store. For the vast majority of x86 machine today, compiling for sse2 works great.
Usually the user cares more about consistency on one architecture (compiler flags not changing results) than consistency across architectures (bitwise matching results on different processors). The latter is indeed very difficult to achieve but even that is possible with enough work. Maintaining consistency across flags (other than those designed to relax consistency) is not very hard at all.
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