[Llvm-bgq-discuss] bgclang r185769-20130706 on vesta/mira

[Hal Finkel]

Erik,

I've updated the installs on vesta/mira to fix this issue as well. First, the underlying problem was that structures that contain a vector4double a member need to have at least 32-byte alignment on the stack. LLVM had been getting this right for most things, but not when passing the structure *by value* to a callee (which requires a separate copy be made for use by the callee).

Also, I'd like to make the following point (for everyone): Because of the way that the PowerPC ABI is defined, passing an aggregate, no matter how small or simple internally, is never exactly the same as passing raw data types. At best, it costs you some extra stack space. But...

struct wrapper {
  vector4double x;
};

foo(struct wrapper x) { ... }
bar(vector4double y) { ... }

When you call bar(y) then y should be passed directly in one of the QPX vector registers. When you call foo(x), then x is passed in a collection of 64-bit general-purpose registers (as are all aggregates), and/or on the stack (depending on how many other function parameters there are and in what order they appear). In C++, it is almost always better to pass by const reference (unless you really do need a copy to modify):

foo(const wrapper &x) { ... }

which should be just as easy for the compiler to inline, but is much more efficient if you end up with non-inlined calls.

 -Hal

P.S. For those with your own installs, there is a r185769-20130706-v3 archive on http://trac.alcf.anl.gov/projects/llvm-bgq which includes this fix.

----- Original Message -----
> On 2013-07-11, at 15:50 , Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > ----- Original Message -----
> >> Hal
> >> 
> >> Is this content of this archive installed on Vesta? If not, how do
> >> I
> >> use the content of this archive?
> > 
> > Yes, it is installed on vesta (and mira). The archive is just for
> > other people who have installs on their own.
> 
> 
> Here is my next issue:
> 
> The directory is ~eschnett/src/vecmathlib-qpx4, the source file is
> test.cc. I build with
> 
> /home/projects/llvm/bin/bgclang++
> -I/soft/compilers/ibmcmp-may2013/xlmass/bg/7.3/include -o test
> test.cc -L/soft/compilers/ibmcmp-may2013/xlmass/bg/7.3/bglib64
> -lmass_simd
> 

[snip]

> 
> Current output is in the file 107701.output. This indicates that my
> function any(boolvec)->bool encounters an error. It is here applied
> to a boolean vector with all elements set to false, yet the function
> 
>    bool any() const
>     {
>       return (*this)[0] || (*this)[1] || (*this)[2] || (*this)[3];
>     }
> 
> returns true.
> 
> -erik
> 
> --
> Erik Schnetter <schnetter at cct.lsu.edu>
> http://www.perimeterinstitute.ca/personal/eschnetter/
> 
> My email is as private as my paper mail. I therefore support
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> and signing email messages. Get my PGP key from
> http://keys.gnupg.net.
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory