[LLVMdev] optimization in presence of floating point

[David Tweed]

Note that there are also some constant optimizations which assume the
floating point rounding mode in order to be correct, so that they may
misapply if the rounding mode is different. Duncan Sands has suggested that
the correct "fix" is to add metadata to each floating point operation
describing the rounding mode and then change processing to be aware of
this. This seems like a good plan, but it's also a very big feature to
implement with appropriate confidence that it's right, so it's not
something I'm planning to dive into any time soon.



On Thu, Apr 18, 2013 at 4:59 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "reed kotler" <rkotler at mips.com>
> > To: LLVMdev at cs.uiuc.edu
> > Sent: Wednesday, April 17, 2013 8:32:23 PM
> > Subject: [LLVMdev] optimization in presence of floating point
> >
> > an interesting problem occurs if you do interval arithmetic.
> >
> > void foo() {
> > float XL, XU, Y, Z;
> >
> > ....
> > call_change_rounding_mode(lower);
> > XL = Y + Z; // lower bound
> > call_change_rounding_mode(upper);
> > XU = Y + Z;
> >
> > }
> >
> > Two issues here:
> > 1) will the compiler mistakenly treat Y+Z as a common subexpression.
> > 2) might it move the call_change_rounding_mode after the assignment
> > to
> > XU since it seens no dependency.
>
> I have come across these issues myself as well. To be fair, Clang does not
> support the FENV_ACCESS pragma, and as I recall, the standard allows an
> implementation-defined default value (which, in our case, cannot be
> changed). Moreover, LLVM internally does not have an option to enable the
> correct conservative side-effect model to make all of this really work
> correctly.
>
>  -Hal
>
> >
> > This is actually to my previous mips16 ir post but is a question of
> > more
> > wide interest.
> >
> >
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-- 
cheers, dave tweed__________________________
high-performance computing and machine vision expert: david.tweed at gmail.com
"while having code so boring anyone can maintain it, use Python." --
attempted insult seen on slashdot
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