In some sense the language is the compiler and the compiler is the language; the language is much like a human language, used for its utility in expressing things (ideas, programs). You can tell if your human language words work by determining if people understand you. If people start being obtuse and refusing to understand you because of an arbitrary grammar rule that isn't really enforced, you'd be right to be upset with the people just as much as the grammar.
It in fact doesn't matter at all what the standard says if GCC and LLVM say something different, because you can't use the standard to generate assembly code.
The standard doesn't have anything to say about UB, so it's the compiler's responsibility to do the most reasonable, non-shocking thing with it possible: if I'm a GCC developer and you ran GCC on one of these fairly mundane examples and it compiled without error then ran rm -rf / or stole your private RSA keys and posted them on 4chan and I said "well, you can't be mad because it's undefined, it's the standard's fault" you'd probably punch me in the face after some quick damage control.
If it deletes an if loop or terminates a spinlock early that's potentially even worse than those two examples.
It in fact doesn't matter at all what the standard says if GCC and LLVM say something different, because you can't use the standard to generate assembly code.
The standard doesn't have anything to say about UB, so it's the compiler's responsibility to do the most reasonable, non-shocking thing with it possible: if I'm a GCC developer and you ran GCC on one of these fairly mundane examples and it compiled without error then ran rm -rf / or stole your private RSA keys and posted them on 4chan and I said "well, you can't be mad because it's undefined, it's the standard's fault" you'd probably punch me in the face after some quick damage control.
If it deletes an if loop or terminates a spinlock early that's potentially even worse than those two examples.