[LLVMdev] Optimization of sqrt() with invalid argument

[Hal Finkel]

----- Original Message -----
> From: "Bill Schmidt" <wschmidt at linux.vnet.ibm.com>
> To: llvmdev at cs.uiuc.edu
> Cc: "Will Schmidt" <will_schmidt at vnet.ibm.com>, "Hal Finkel" <hfinkel at anl.gov>, spatel+llvm at rotateright.com
> Sent: Thursday, September 25, 2014 2:50:48 PM
> Subject: Optimization of sqrt() with invalid argument
> 
> My colleague Will Schmidt recently discovered
> (http://llvm.org/bugs/show_bug.cgi?id=21048) that the LLVM optimizers
> are converting sqrt(n), where n is negative, into 0.0.  Now, as
> Sanjay
> pointed out, the value of sqrt(n) for negative n other than -0.0 is
> undefined according to the C99 standard, so this is allowable
> behavior.
> But I think that it's not necessarily good behavior, nonetheless.
> 
> The problem is that an unoptimized call to sqrt(n) in libm will
> produce
> a NaN, at least in the implementations I have access to.  So this
> particular choice changes the behavior of optimized versus
> unoptimized
> code, which of course is allowable, but probably to be avoided when
> possible.
> 
> However, there's a de facto standard of producing NaN for these
> cases.
> The gcc and xlc compilers will optimize the call into a constant NaN,
> given the right options (-O2 -ffast-math for gcc, -O2 -qignerrno for
> xlc).  All of gcc, icc, and xlc produce NaN via the library call with
> unoptimized code.  Choosing 0.0 for Clang/LLVM introduces an
> unnecessary
> incompatibility.
> 
> This is important in practice because of the xalanc benchmark in SPEC
> CPU2006.  This code initializes a variable to a NaN value by
> assigning
> it the value sqrt(-2.01) -- don't ask me why.  This variable is used
> to
> terminate a loop, and when Clang/LLVM produces 0.0 instead of NaN,
> the
> loop spins forever.
> 
> Are there any objections to changing the LLVM optimization behavior
> to
> produce a constant NaN instead of 0.0 in this case?

I think that changing this makes sense. Returning NaN is constant with the POSIX sqrt() specification (http://pubs.opengroup.org/onlinepubs/009695399/functions/sqrt.html).

 -Hal

> 
> Thanks,
> Bill
> 
> 

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