[LLVMbugs] [Bug 11485] New: Division by power of two expansion blocked by other flag

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11485

             Bug #: 11485
           Summary: Division by power of two expansion blocked by other
                    flag
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: javier.e.martinez at intel.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 7675
  --> http://llvm.org/bugs/attachment.cgi?id=7675
Fix for expanding integer division by 2

Assume that a target can do integer division cheaply but not as cheap as
shifts. This target would call setIntDivIsCheap(true) and
setPow2DivIsCheap(false). The issue is that the DAGCombiner visitSDIV punts if
isIntDivCheap is true. The fix is to check for isPow2DivCheap instead. Below is
the current code and the fix. A patch is also attached.

[CURRENT]
  // fold (sdiv X, pow2) -> simple ops after legalize
  if (N1C && !N1C->isNullValue() && !TLI.isIntDivCheap() &&
      (N1C->getAPIntValue().isPowerOf2() ||
       (-N1C->getAPIntValue()).isPowerOf2())) {
    // If dividing by powers of two is cheap, then don't perform the following
    // fold.
    if (TLI.isPow2DivCheap())
      return SDValue();

    unsigned lg2 = N1C->getAPIntValue().countTrailingZeros();

[FIX]
  // fold (sdiv X, pow2) -> simple ops after legalize
  if (N1C && !N1C->isNullValue() && !TLI.isPow2DivCheap() &&
      (N1C->getAPIntValue().isPowerOf2() ||
       (-N1C->getAPIntValue()).isPowerOf2())) {

    unsigned lg2 = N1C->getAPIntValue().countTrailingZeros();

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