[LLVMdev] Implementation of builtins/intrinsics

[Ruben Van Boxem]

Hi,

Lately, I have been wondering how a compiler like GCC or Clang implements
builtin functions like sqrt, sin, and also more bit-specific things like
bcmp and ffs. Originially, these were all deferred to the C library where
they might still have a "backup implementation", but I don't understand why
a compiler would choose the non-builtin version, as it should know its
version is faster (or that's the way it should be at least).

 - How does LLVM cope with builtin's that are target specific? Does the
Target backend know of all these specialized instructions?
 - Does LLVM have a representation for all C library functions that have or
might have a future hardware implementation? Or is this mostly handled by
the whole toolchain (Clang adds target-specific info into the LLVM bitcode
which is forced to produce the asm instruction if the target supports it)?
 - Is it possible to code a sqrt or sin in LLVM bitcode that will use the
hardware instruction if it's compiled for a certain target?

I know these are probably questions born out of misunderstanding, but
that's why I'm asking. It's important to know what LLVM could provide if I
say were to always bypass the system C library for these "builtins".

Thanks!

Ruben
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