[LLVMbugs] [Bug 17368] New: Unnecessary multiplication distribution of vectors on ARM

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=17368

            Bug ID: 17368
           Summary: Unnecessary multiplication distribution of vectors on
                    ARM
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: weimingz at codeaurora.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Generally, it is desirable to distribute (a + b) * c to a*c + b*c on
ARM with VMLx Forwarding, where a,b and c are vectors.
However, for (a + b)*(a + b), distribution will result in one extra
instruction.
With distrution:
x = a + b (vadd)
y = a * x (vmul)
z = y + b * y (vmla)

Without distribution:
x = a + b (vadd)
z = x * x (vmul)

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