Unnecessary Long computation/loads feeding into ConvL2I

[Edward Lee]

I was looking at the OptoAssembly and final x86 code for the following
code sequence and noticed a bit of unnecessary work in the generated
code:

int hash = (int) (this.identity.containerId ^ this.identity.segmentId);
hash = (int) ((long) hash ^ pageNumber);

The ideal graph looks like..
L2I(XorL(
  I2L(L2I(XorL(containerId, segmentId))),
  pageNumber))

It eventually becomes..
mov    0x10(%esi),%ecx ; containerId.lo
mov    0x14(%esi),%ebx ; containerId.hi
xor    0x8(%esi),%ecx ; XorL segmentId.lo
xor    0xc(%esi),%ebx ; XorL segmentId.hi
mov    %ecx,%ebx ; 2-line upcast I2L
sar    $0x1f,%ebx ; I2L
xor    0x40(%esp),%ecx ; XorL pageNumber.lo
xor    0x44(%esp),%ebx ; XorL pageNumber.hi
mov    %ecx,%ebx ; unnecessary ?? L2I (Opto: MOV    EBX,ECX.lo)

The attached patch splits L2I(XorL(a,b)) into XorI(L2I(a),L2I(b)) and
in this situation, it's simplified to..

XorI(
  L2I(XorL(containerId, segmentId)),
  L2I(pageNumber))

mov    0x10(%esi),%ebp ; containerId.lo
mov    0x14(%esi),%edi ; containerId.hi
xor    0x8(%esi),%ebp ; XorL segmentId.lo
xor    0xc(%esi),%edi ; XorL segmentId.hi
mov    0x40(%esp),%eax ; pageNumber.lo
mov    0x44(%esp),%edx ; pageNumber.hi
mov    %ebp,%ebx ; (Opto: MOV    EBX,EBP.lo)
xor    %eax,%ebx ; XorI pageNumber.lo

Unfortunately, there's still a number of rough spots.. As per comments
in ConvL2INode::Ideal..
  // Disable optimization: LoadL->ConvL2I ==> LoadI.
  // It causes problems (sizes of Load and Store nodes do not match)
  // in objects initialization code and Escape Analysis.

This forces the loads to stay as long loads, and this causes
unnecessary register pressure. In the "after" x86 code, pageNumber is
explicitly moved into eax and edx even though xor could have just used
0x40(%esp) directly. Also, all the high bits of the longs should be
completely ignored.

Additionally, it seems that EBP.lo in the OptoAssembly output gets
treated as a separate entity from EBP. Otherwise, the xor could have
just used %ebp (EBP.lo) directly as in "xor 0x40($esp), %ebp".

Ideally the whole sequence should look like..

mov    0x10(%esi),%ebp ; containerId.lo
xor    0x8(%esi),%ebp ; XorI segmentId.lo
xor    0x40(%esp),%ebp ; XorI pageNumber.lo

Ed
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