See DarkShikari's comment for details on what goes wrong when a compiler tries to be all things to all architectures.

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I should clarify. I agree with what you said below, these days GCC is an excellent compiler for x86. It's just that it's near to impossible to be so excellent for all archs.

But isn't this the whole point of LLVM? Separating the compile process into two intermediate steps, one of compiling a programming given language into an intermediate virtual machine and another of compiling that virtual machine code into a binary for the target architecture. Then each module of the LLVM can be worked on and optimized independently with little duplication of labor.

So the promise of LLVM is not just being able to compile to every architecture, but to compile every language to every architecture.