[LLVMbugs] [Bug 11973] New: BB vectorizer does not fuse shifts with multiplies

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=11973

             Bug #: 11973
           Summary: BB vectorizer does not fuse shifts with multiplies
           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 8037
  --> http://llvm.org/bugs/attachment.cgi?id=8037
testcase

Looking at the dumps of 

clang -O2 test.c -mllvm -vectorize \
-mllvm -debug-only=bb-vectorize \
-mllvm -bb-vectorize-debug-pair-selection \
-mllvm -bb-vectorize-debug-candidate-selection

when test.c does not contain coefficients that can be transformed into shifts,
the vectorizer matches different pairable instructions than when it contains a
multiple of 2.

For instance, when I replace the first write to w of the attached testcase
with:
      *w++ = 2*a - 5*b + 7*c;
The debugging dumps look like this:
BBV: looking at   %mul = shl i8 %0, 1 <->   %3 = mul i8 %1, -5

Whereas if that first write to w is like this:
     *w++ = 3*a - 5*b + 7*c;
we have the following candidate pair:
BBV: looking at   %mul = mul i8 %0, 3 <->   %3 = mul i8 %1, -5
BBV: candidate pair   %mul = mul i8 %0, 3 <->   %3 = mul i8 %1, -5

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