[LLVMbugs] [Bug 17285] New: Regression due to increased register pressure after r172868 revision.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=17285

            Bug ID: 17285
           Summary: Regression due to increased register pressure after
                    r172868 revision.
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: llc
          Assignee: unassignedbugs at nondot.org
          Reporter: ili.filippov at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 11236
  --> http://llvm.org/bugs/attachment.cgi?id=11236&action=edit
Reproducer

Revision r172868: "On Sandybridge loading unaligned 256bits using two XMM loads
(vmovups and vinsertf128) is faster than using a single vmovups instruction."
increases register pressure, which becomes critical in case of multiple loads.
This shows up on many ISPC tests with total test slow down up to 1.5x (due to
increased number of spill/fills).

An attached example is extracted from one of these tests and demonstrated the
problem.

I don't know how much faster loading through XMM on Sandybridge is, but in case
of multiple loads it's surely isn't.

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