[LLVMbugs] [Bug 11976] New: BB vectorizer does not copy data to increase the number of pairable instructions

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11976

             Bug #: 11976
           Summary: BB vectorizer does not copy data to increase the
                    number of pairable instructions
           Product: new-bugs
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: spop at codeaurora.org
                CC: hfinkel at anl.gov, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 8038
  --> http://llvm.org/bugs/attachment.cgi?id=8038
testcase

While looking at why the attached testcase does not get vectorized
profitably on ARM, I realized that the length of the instruction
chains using the data in vector registers is too short with respect to
what I could be doing on paper with a pencil ;-)

Here is the kernel to be vectorized:

      a = *r++;
      b = *r++;
      c = *r++;

      *w++ = 3*a - 5*b + 7*c;
      *w++ = 11*a - 13*b + 17*c;
      *w++ = 19*a - 23*b - 29*c;

Currently the BB vectorizer pairs the first two multiplies and then
goes back to scalar mode for the rest of the computation, so we have
something like this:

load {a,b}
{x,y} = {3,5} * {a,b}
t = x + y
u = 7 * c
t = t + u
store t
{x,y} = {11,13} * {a,b}
t = x + y
u = 17 * c
t = t + u
store t
[...]

This is suboptimal as we have the overhead of transfer between vector
and scalar registers inside the loop.

A better way to vectorize this loop is to duplicate the data, and this
would increase the number of instructions that can then be paired,
like this:

load {a,b}
({a,a}, {b,b}) = shuffle ({a,b}, copy {a,b})
load c
{c,c} = build_vector (c)
{a1,a2} = {3,11} * {a,a}
{b1,b2} = {5,13} * {b,b}
{c1,c2} = {7,17} * {c,c}
{t1,t2} = {a1,a2} + {b1,b2} + {c1,c2}
store {t1,t2}

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