At least as of a few years ago, the x264 team claimed they didn't use C intrinsics for their inline asm because automatic register allocation wasn't very good:

At least as of a few years ago, the x264 team claimed they didn't use C intrinsics for their inline asm because automatic register allocation wasn't very good: http://x264dev.multimedia.cx/archives/191#comment-1763 http://x264dev.multimedia.cx/archives/191#comment-1763 (exchange between comments 3 and 4). Sure, you can write SIMD in LLVM IR, but to do it efficiently you need some knowledge of the target. Should you use i32, or rework your code to use more i16, or throw in some parallel i64 multiplications? Those aren't really portable choices, unless you have a really good auto-vectorizer, in which case the advantage of writing in asm at all disappears, since you can just rely on auto-vectorized C.