[LLVMbugs] [Bug 18478] New: Overzealous strength reduction on _mm_mullo_epi16 producing inferior code

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Jan 14 13:11:42 PST 2014


            Bug ID: 18478
           Summary: Overzealous strength reduction on _mm_mullo_epi16
                    producing inferior code
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: fabiang at radgametools.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 11871
  --> http://llvm.org/bugs/attachment.cgi?id=11871&action=edit
Small repro with generated code

Both of the functions in this file should be equivalent; the former
("good_...") does generate the expected code, but the latter ("bad_...") ends
up "de-SIMDifying" the function almost completely.

The only difference between the two is that the former declares the magic value
at global scope whereas the latter uses the _mm_setr_epi16 intrinsic.

This was tested using the official Clang 3.4 release binaries, not trunk, but
the "Version" field doesn't have 3.4 yet.

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