llvm dev - Sep 2005 - [LLVMdev] Promotion of integral values

There's some optimization that turns expressions like this:

   short a = ...
   short b    int a' = cast short a to int
   int b' = cast short b to int
   int c = a' + b'

into this:

   short a = ...
   short b = ...
   uint a' = cast short a to uint
   uint b' = cast short b to uint
   uint c = a' + b'
   int c = cast uint c to int

Does anyone know which optimization is doing this?  I'd like to be able 
to turn it off.

Rob

Robert L. Bocchino Jr.
Ph.D. Student, Computer Science
University of Illinois, Urbana-Champaign
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On Wed, 14 Sep 2005, Robert L. Bocchino Jr. wrote:
> There's some optimization that turns expressions like this:
>
>  short a = ...
>  short b >  int a' = cast short a to int
>  int b' = cast short b to int
>  int c = a' + b'
>
> into this:
>
>  short a = ...
>  short b = ...
>  uint a' = cast short a to uint
>  uint b' = cast short b to uint
>  uint c = a' + b'
>  int c = cast uint c to int
>
> Does anyone know which optimization is doing this?  I'd like to be able
to
> turn it off.
Hard to say without more information.  This is a semantically valid xform, 
so it's not "wrong" that it's doing it.  To find out what is
doing it,
pass -debug-pass=Arguments to gccas or gccld (whichever is doing it). 
This will give you a list of passes you can give to opt to get the same 
effect.

You can use this list of passes to do a binary search for the pass that is 
doing the xform.  If I had to guess, I'd say instcombine, levelraise or
indvars.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

Were there ever any plans to make a staged emulator that only JIT's hot
functions (interpreting the lesser used ones)?  Emulators such as this tend
to perform better when there are a lot of infrequently used functions.