[llvm-dev] Implementing a proposed InstCombine optimization

[Daniel Sanders via llvm-dev]

> I am not entirely sure this is safe. Transforming this to an fsub could change the value stored on platforms that implement negates using arithmetic instead of with bitmath (such as ours)

I think it's probably safe for IEEE754-2008 conformant platforms because negation was clarified to be a non-arithmetic bit flip that cannot cause exceptions in that specification. However, I'm sure it's unsafe for some IEEE754-1985 platforms because it introduces exceptions when given a NaN.

On MIPS, the semantics for negation depend on a configuration bit (ABS2008) but in practice the majority of MIPS environments use arithmetic negation and trigger exceptions when negating a NaN. That said, the most recently published MIPS specifications require non-arithmetic negation and drop support for the IEEE754-1985 standard.

> This could introduce new FP exceptions. It's also likely to be much worse on platforms with no FPU like early MIPS.

Quite a few modern implementations too. MIPS is often used in domains where having an FPU would be wasteful.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Alex Rosenberg via llvm-dev
Sent: 09 April 2016 02:44
To: escha at apple.com
Cc: Carlos Liam; llvm-dev
Subject: Re: [llvm-dev] Implementing a proposed InstCombine optimization

This doesn't seem like a good idea to me. There are many architectures where those bitcasts are free operations and the xor will be executed in a shorter pipe than any FP op would. Cell SPU, for example.

This could introduce new FP exceptions. It's also likely to be much worse on platforms with no FPU like early MIPS.

Alex

On Apr 7, 2016, at 9:43 AM, via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
I am not entirely sure this is safe. Transforming this to an fsub could change the value stored on platforms that implement negates using arithmetic instead of with bitmath (such as ours) and either canonicalize NaNs or don’t support denormals. This is actually important because this kind of bitmath on floats is very commonly used as part of algorithms for complex math functions that need to get precise bit patterns from the source (similarly for the transformation of masking off the sign bit -> fabs). It’s also important because if the float happens to “really” be an integer, it’s highly likely we’ll end up zero-flushing it and losing the data.

Example:

a = load float
b = bitcast a to int
c = xor b, signbit
d = bitcast c to float
store d

Personally I would feel this is safe if and only if the float is coming from an arithmetic operation — in that case, we know that doing another arithmetic operation on it should be safe, since it’s already canonalized and can’t be a denorm [if the platform doesn’t support them].

I say this coming only a few weeks after our team spent literally dozens of human-hours tracking down an extremely obscure bug involving a GL conformance test in which ints were casted to floats, manipulated with float instructions, then sent back to int, resulting in the ints being flushed to zero and the test failing.

—escha

On Apr 7, 2016, at 9:09 AM, Sanjay Patel via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi Carlos -
That sounds like a good patch.
Warning - following the link below may remove some of the educational joy for the immediate task at hand:
http://reviews.llvm.org/D13076
...but I wouldn't worry too much, there's plenty more opportunity where that came from. :)

Feel free to post follow-up questions here or via a patch review on Phabricator:
http://llvm.org/docs/Phabricator.html


On Thu, Apr 7, 2016 at 7:17 AM, Carlos Liam via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi,

I'm interested in implementing an InstCombine optimization that I discovered and verified with Alive-NJ (with the help of the authors of Alive-NJ). The optimization is described in Alive-NJ format as follows:

Name: xor->fsub
Pre: isSignBit(C)
%x = bitcast %A
%y = xor %x, C
%z = bitcast %y
=>
%z = fsub -0.0, %A

Effectively the optimization targets code that casts a float to an int with the same width, XORs the sign bit, and casts back to float, and replaces it with a subtraction from -0.0.

I am not very familiar with C++ or the LLVM codebase so I would greatly appreciate some help in writing a patch adding this optimization.

Thanks in advance.

 - CL

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160411/6cb54f37/attachment.html>