Hey all, Now that we have sign/zero extensions, have people given thought to the elimination of these? There's a paper I downloaded a few years ago called "Effective Sign Extension Elimination" by M. Kawahito, H. Komatsu, and T. Nakatani. I only have a dead tree copy of it, though. But I can share it as much as I can. -bw
a quick google finds the paper here: http://www.trl.ibm.com/projects/jit/paper/sxt.pdf Bill Wendling wrote:> Hey all, > > Now that we have sign/zero extensions, have people given thought to > the elimination of these? There's a paper I downloaded a few years > ago called "Effective Sign Extension Elimination" by M. Kawahito, H. > Komatsu, and T. Nakatani. > > I only have a dead tree copy of it, though. But I can share it as > much as I can. > > -bw > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
On Thu, 4 Jan 2007, Bill Wendling wrote:> Now that we have sign/zero extensions, have people given thought to > the elimination of these? There's a paper I downloaded a few years > ago called "Effective Sign Extension Elimination" by M. Kawahito, H. > Komatsu, and T. Nakatani.Sign extension elimination is most useful in the backend, when targeting processors like PowerPC that have a single register width (e.g. 32-bits). We already do quite a bit of sign extension removal in the dag combiner, but this is limited to a single basic block at a time. In the future, I'd like to extend the selectiondag to work on entire functions at a time, which would increase the scope of this optimization, but this isn't likely to happen in the near future. -Chris -- http://nondot.org/sabre/ http://llvm.org/
C ABI requires function parameters and return value to be promoted to 32-bit. Perhaps we can eliminate these extensions at IPO time if both caller and callee agrees to break the ABI convention? This can have minor performance impact on targets with sub-registers (e.g. x86). Evan On Jan 4, 2007, at 2:02 PM, Chris Lattner wrote:> On Thu, 4 Jan 2007, Bill Wendling wrote: >> Now that we have sign/zero extensions, have people given thought to >> the elimination of these? There's a paper I downloaded a few years >> ago called "Effective Sign Extension Elimination" by M. Kawahito, H. >> Komatsu, and T. Nakatani. > > Sign extension elimination is most useful in the backend, when > targeting > processors like PowerPC that have a single register width (e.g. 32- > bits). > We already do quite a bit of sign extension removal in the dag > combiner, > but this is limited to a single basic block at a time. In the > future, I'd > like to extend the selectiondag to work on entire functions at a time, > which would increase the scope of this optimization, but this isn't > likely > to happen in the near future. > > -Chris > > -- > http://nondot.org/sabre/ > http://llvm.org/ > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
On Jan 4, 2007, at 2:02 PM, Chris Lattner wrote:> On Thu, 4 Jan 2007, Bill Wendling wrote: >> Now that we have sign/zero extensions, have people given thought to >> the elimination of these? There's a paper I downloaded a few years >> ago called "Effective Sign Extension Elimination" by M. Kawahito, H. >> Komatsu, and T. Nakatani. > > Sign extension elimination is most useful in the backend, when > targeting > processors like PowerPC that have a single register width (e.g. 32- > bits). > We already do quite a bit of sign extension removal in the dag > combiner, > but this is limited to a single basic block at a time. In the > future, I'd > like to extend the selectiondag to work on entire functions at a time, > which would increase the scope of this optimization, but this isn't > likely > to happen in the near future. >Okay. That sounds like a good idea. Perhaps this paper/idea can be revisited when the SelectionDAG works on a function at a time. (From a cursory reading of it, and from what I remember, it's a global optimization.) -bw