[LLVMbugs] [Bug 18680] New: bitfield load/stores not merged

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18680

            Bug ID: 18680
           Summary: bitfield load/stores not merged
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: hfinkel at anl.gov
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 11984
  --> http://llvm.org/bugs/attachment.cgi?id=11984&action=edit
original IR

I noticed, in the context of debugging another problem, that Clang/LLVM
generates what seems like poor code for the bitfield copies in
llvm::APFloat::assign(llvm::APFloat const&):

APFloat::assign(const APFloat &rhs)
{
...
  sign = rhs.sign;
  category = rhs.category;
...

which generates (at least for ppc64):

  %sign = getelementptr inbounds %"class.llvm::APFloat"* %rhs, i64 0, i32 3
  %bf.load = load i8* %sign, align 2
  %sign3 = getelementptr inbounds %"class.llvm::APFloat"* %this, i64 0, i32 3
  %bf.load4 = load i8* %sign3, align 2
  %bf.shl = and i8 %bf.load, 16
  %bf.clear5 = and i8 %bf.load4, -17
  %bf.set = or i8 %bf.clear5, %bf.shl
  store i8 %bf.set, i8* %sign3, align 2
  %bf.load6 = load i8* %sign, align 2
  %bf.lshr7 = and i8 %bf.load6, -32
  %bf.clear13 = and i8 %bf.set, 31
  %bf.set14 = or i8 %bf.clear13, %bf.lshr7
  store i8 %bf.set14, i8* %sign3, align 2

which has an extra load/store pair. We should produce something more like:

  %sign = getelementptr inbounds %"class.llvm::APFloat"* %rhs, i64 0, i32 3
  %bf.load = load i8* %sign, align 2
  %sign3 = getelementptr inbounds %"class.llvm::APFloat"* %this, i64 0, i32 3
  %bf.load4 = load i8* %sign3, align 2
  %bf.lshr7 = and i8 %bf.load4, -32
  %bf.clear13 = and i8 %bf.load, 31
  %bf.set14 = or i8 %bf.clear13, %bf.lshr7
  store i8 %bf.set14, i8* %sign3, align 2

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