[polly] r238929 - Lower signed-divisions without rounding to ashr instructions

[David Majnemer]

On Wed, Jun 3, 2015 at 9:13 AM, Johannes Doerfert <
doerfert at cs.uni-saarland.de> wrote:

> Isnt't there a LLVM pass that will do this kind of optimization
> (Instcombine)?
>

InstCombine optimizes signed divides to unsigned divides when doing so is
obvious (both operands have a clear sign bit).  Unsigned divides are then
turned in shift right operations.

q = n / (2**k) is equivalent to (n >= 0 ? n : -n) >> k.

Modeling this in InstCombine would make it harder to analyze so we leave it
to CodeGen to emit.



>
> On 06/03, Tobias Grosser wrote:
> > Author: grosser
> > Date: Wed Jun  3 10:14:58 2015
> > New Revision: 238929
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=238929&view=rev
> > Log:
> > Lower signed-divisions without rounding to ashr instructions
> >
> > Modified:
> >     polly/trunk/lib/CodeGen/IslExprBuilder.cpp
> >     polly/trunk/test/Isl/CodeGen/exprModDiv.ll
> >
> > Modified: polly/trunk/lib/CodeGen/IslExprBuilder.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/polly/trunk/lib/CodeGen/IslExprBuilder.cpp?rev=238929&r1=238928&r2=238929&view=diff
> >
> ==============================================================================
> > --- polly/trunk/lib/CodeGen/IslExprBuilder.cpp (original)
> > +++ polly/trunk/lib/CodeGen/IslExprBuilder.cpp Wed Jun  3 10:14:58 2015
> > @@ -295,6 +295,13 @@ Value *IslExprBuilder::createOpBin(__isl
> >      Res = Builder.CreateNSWMul(LHS, RHS);
> >      break;
> >    case isl_ast_op_div:
> > +    if (auto *Const = dyn_cast<ConstantInt>(RHS)) {
> > +      auto &Val = Const->getValue();
> > +      if (Val.isPowerOf2() && Val.isNonNegative()) {
> > +        Res = Builder.CreateAShr(LHS, Val.ceilLogBase2(),
> "pexp.div.shr");
> > +        break;
> > +      }
> > +    }
> >      Res = Builder.CreateSDiv(LHS, RHS, "pexp.div");
> >      break;
> >    case isl_ast_op_pdiv_q: // Dividend is non-negative
> >
> > Modified: polly/trunk/test/Isl/CodeGen/exprModDiv.ll
> > URL:
> http://llvm.org/viewvc/llvm-project/polly/trunk/test/Isl/CodeGen/exprModDiv.ll?rev=238929&r1=238928&r2=238929&view=diff
> >
> ==============================================================================
> > --- polly/trunk/test/Isl/CodeGen/exprModDiv.ll (original)
> > +++ polly/trunk/test/Isl/CodeGen/exprModDiv.ll Wed Jun  3 10:14:58 2015
> > @@ -46,10 +46,8 @@
> >  ; POW2:  %polly.access.A6 = getelementptr float, float* %A, i64
> %pexp.pdiv_r
> >
> >  ; A[i / 128]
> > -; POW2:  %pexp.div = sdiv i64 %polly.indvar, 128
> > +; POW2:  %pexp.div.shr = ashr i64 %polly.indvar, 7
> >  ; POW2:  %polly.access.B8 = getelementptr float, float* %B, i64
> %pexp.div
> > -;
> > -; FIXME: Make isl mark this as an udiv expression.
> >
> >  ; #define floord(n,d) ((n < 0) ? (n - d + 1) : n) / d
> >  ; A[p + 128 * floord(-p - 1, 128) + 128]
> > @@ -62,8 +60,8 @@
> >  ; POW2:  %polly.access.A10 = getelementptr float, float* %A, i64 %24
> >
> >  ; A[p / 128]
> > -; POW2:  %pexp.div12 = sdiv i64 %p, 128
> > -; POW2:  %polly.access.B13 = getelementptr float, float* %B, i64
> %pexp.div12
> > +; POW2:  %pexp.div.shr12 = ashr i64 %p, 7
> > +; POW2:  %polly.access.B13 = getelementptr float, float* %B, i64
> %pexp.div.shr12
> >
> >  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> >
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
> --
>
> Johannes Doerfert
> Researcher / PhD Student
>
> Compiler Design Lab (Prof. Hack)
> Saarland University, Computer Science
> Building E1.3, Room 4.31
>
> Tel. +49 (0)681 302-57521 : doerfert at cs.uni-saarland.de
> Fax. +49 (0)681 302-3065  : http://www.cdl.uni-saarland.de/people/doerfert
>
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>
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