[LLVMdev] Implementation of builtins/intrinsics

[Ruben Van Boxem]

2012/1/20 Jean-Daniel Dupas <devlists at shadowlab.org>

>
> Le 20 janv. 2012 à 14:38, Ruben Van Boxem a écrit :
>
> > 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?
>
>
> See the LLVM language reference for the list of supported intrinsics:
> http://llvm.org/docs/LangRef.html#int_libc


Thanks for that. I still wonder what happens to stuff like log1p, which
translates to a machine instruction on x86. Can the LLVM backend discover
that this operation is called for, and use the native instruction in this
case?

Thanks,

Ruben


>
> -- Jean-Daniel
>
>
>
>
>
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