Implementation of builtins/intrinsics

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1

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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See the LLVM language reference for the list of supported intrinsics: http://llvm.org/docs/LangRef.html#int_libc

-- Jean-Daniel

3

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

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