[LLVMbugs] [Bug 2874] New: missed optimization opportunity: imull -> shl+add

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Thu Oct 9 16:17:26 PDT 2008


           Summary: missed optimization opportunity: imull -> shl+add
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nlewycky at google.com
                CC: llvmbugs at cs.uiuc.edu

LLVM will optimize (x << 8) + x into x*257, but on some CPUs the imull is
actually slower than the shift and add. The backend should take a moment to
unfurl those multiplies when possible.

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