[llvm-commits] [llvm] r169049 - /llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

[Stephen Canon]

On Dec 1, 2012, at 4:38 PM, Owen Anderson <resistor at mac.com> wrote:

> On Dec 1, 2012, at 5:13 AM, Duncan Sands <baldrick at free.fr> wrote:
> 
>> it would be great to have a library that evaluates constant expressions
>> like log2(something) perfectly, rather than having to do this dance around
>> whether the host evaluates functions accurately enough.  I think GCC uses
>> MPFR for this.  In the meantime I suppose we are obliged, as you say, to
>> restrict to cases we can evaluate exactly.
> 
> There's an interesting question in there about whether one should implement "perfect" evaluation, or should try to match the semantics of the target.  I've heard of some vendor compilers that went so far as it embed a simulator for the target platform which they used to do truly accurate target-platform-accurate constant folding.

The general transformation log2(c * x) --> log2(c) + log2(x) can lead to catastrophic cancellation and change answers quite drastically (specifically, this happens if c*x ~ 1), even if both log2(c) and c*x are evaluated exactly (as is the case when c is a power of two).  Because of this, it should only be performed in fast-math modes where trying to match the semantics of the target should be much less of an issue.

- Steve