[LLVMdev] replace hardcoded function names by intrinsics

[Hal Finkel]

On Fri, 02 Mar 2012 16:05:17 +0100
Duncan Sands <baldrick at free.fr> wrote:

> Hi,
> 
> >> in the llvm code there are several places with hardcoded function
> >> names for e.g. sin, sinf, sqrt, sqrtf etc., namely
> >> ConstantFolding.cpp
> >> InlineCost.cpp
> >> SelectionDAGBuilder.cpp
> >> IntrinsicLowering.cpp
> >> TargetLowering.cpp
> >>
> >> my question is: wouldn't it be beneficial to use intrinsics for
> >> this? for example a c/c++
> >> frontend (clang) could translate the function calls to intrinsics
> >> and then in a very late
> >> step (IntrinsicLowering.cpp?) translate it back to function calls.
> >> an opencl frontend then could use the intrinsics on vector types
> >> and ConstantFolding.cpp
> >> would work on sin/cos of vector types. currently the intrinsics for
> >> sin/cos are missing in
> >> ConstantFolding.
> >> To summarize, using only intrinsics would reduce complexity and
> >> increase flexibility as
> >> vector types are supported.
> >
> > I also think that this is a good idea.
> 
> intrinsics don't have the same semantics as the library functions.
> For example they don't set errno and in general they are less
> accurate.  Thus you can't turn every use of eg sqrt into an
> intrinsic.  However you will still want to constant fold instances of
> sqrt that weren't turned into an intrinsic, and thus all those names
> will still need to exist in constant fold etc, so this change
> wouldn't buy you much.

In some cases, this will depend on how these things are lowered, if
bounds can be put on the input ranges, etc. Otherwise, I think this is
a "fast math" kind of optimization. Do you disagree?

Would it be useful, for this purpose, to have an
(inter-procedural) analysis pass, or some annotation-driven mechanism,
or both, to mark errno as "dead" so we don't have to worry about this
kind of thing if it is not necessary?

 -Hal

> 
> Ciao, Duncan.
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory