[LLVMdev] Min and max

[Nicolas Capens]

Hi all,

 

Marc pointed out to me that this might be related to my question about
floating-point equality, however I still think there's a valid optimization
opportunity here.

 

The Intel documents specify that the maxss instruction returns the second
operand if either operand is a NaN. I believe this is exactly the intended
behavior of (x > y) ? x : y where > is an ordered compare. If either x or y
is NaN the > returns false so y is returned, the second argument.

 

I wrote a little test app where I compared the results of (x > y) ? x : y
with some inline assembly using maxss for all combinations of 0.0, 1.0 and
NaN as inputs for x and y, and they were identical.

 

Cheers,

 

Nicolas

 

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Nicolas Capens
Sent: Tuesday, 27 May, 2008 11:50
To: 'LLVM Developers Mailing List'
Subject: [LLVMdev] Min and max

 

Hi all,

 

I'm trying to implement a floating-point 'min' and 'max' operation using
select. For 'min' I get the expected x86 assembly minss instruction, but for
'max' I get a branch instead of maxss.

 

The corresponding C syntax code looks like this:

 

float z = (x > y) ? x : y;

 

Any clues?

 

Could someone maybe explain to me the basics of LLVM's target specific
optimizations and code generation? I'd love to analyze things like this
myself but I don't know where to start.

 

Thanks,

 

Nicolas Capens

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20080528/ba8c4b21/attachment.html>