[cfe-dev] Implementing "external" built-in functions

[Francois Pichet]

Hi, I ran into this exact problem with an out-of-tree backend that will do
stuff like that:

setOperationAction(ISD::ATOMIC_SWAP,  MVT::i32, Expand);


It seems -fno-builtin will only disable builtins marked as "f"

from \include\clang\Basic\Builtins.def
//  f -> this is a libc/libm function without the '__builtin_' prefix. It
can
//       be followed by ':headername:' to state which header this function
//       comes from.

I do not understand the rational for that.
I tried to disable all builtins when -fno-builtin but this caused about a
dozen clang tests to fail
I didn't investigate further.

I currently get around this problem by using this patch that allows
builtins definition if -fno-builtin
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20130603/081354.html





On Fri, Jul 19, 2013 at 12:04 PM, Chris Lattner <clattner at apple.com> wrote:

> Have you tried -fno-builtin?
>
> -Chris
>
> On Jul 19, 2013, at 3:04 AM, Emmanuel Blot <eblot.ml at gmail.com> wrote:
>
> > Hi,
> >
> > clang emits calls to external functions for built-in functions it
> > cannot implement, the way GCC does:
> >
> > From GCC doc about built-in functions for memory accesses:
> > "Not all operations are supported by all target processors. If a
> > particular operation cannot be implemented on the target processor, a
> > warning will be generated and a call an external function will be
> > generated. The external function will carry the same name as the
> > builtin, with an additional suffix `_n' where n is the size of the
> > data type."
> >
> > Is is possible to implement those "external" built-in functions and
> > compile them with clang? The "problem" is that clang recognizes their
> > signature, and emit an error, such as:
> >
> > error: definition of builtin function
> >   '__sync_fetch_and_and_4'
> >
> > I'm unable to find a clang front-end option switch to disable this
> behavior.
> > What is the recommended way to compile such a built-in function?
> >
> > I'm looking for a way to compile those functions in a dedicated
> > library, outside the clang build tree.
> >
> > Thanks,
> > Emmanuel.
> > _______________________________________________
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> > cfe-dev at cs.uiuc.edu
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