[llvm-bugs] [Bug 44973] New: [DAG] Missing constant fold in avg pattern

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44973

            Bug ID: 44973
           Summary: [DAG] Missing constant fold in avg pattern
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: lebedev.ri at gmail.com, llvm-bugs at lists.llvm.org,
                    nikita.ppv at gmail.com, spatel+llvm at rotateright.com

So, I was trying to improve the codegen in llvm\test\CodeGen\X86\avg.ll for the
'not_avg_v16i8_wide_constants' test case as its incredibly inefficient wrt
demandedbits and shuffle combining:

Then I discovered this: https://gcc.godbolt.org/z/WbMDbA

define void @not_avg_v16i8_wide_constants(<16 x i8>* %a, <16 x i8>* %b, <16 x
i8>* %c) {
  %1 = load <16 x i8>, <16 x i8>* %a
  %2 = load <16 x i8>, <16 x i8>* %b
  %3 = zext <16 x i8> %1 to <16 x i128>
  %4 = zext <16 x i8> %2 to <16 x i128>
  %5 = add nuw nsw <16 x i128> %3, <i128 -1, i128 -1, i128 -1, i128 -1, i128
-1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1, i128 -1,
i128 -1, i128 -1, i128 -1>
  %6 = add nuw nsw <16 x i128> %5, %4
  %7 = lshr <16 x i128> %6, <i128 1, i128 1, i128 1, i128 1, i128 1, i128 1,
i128 1, i128 1, i128 1, i128 1, i128 1, i128 1, i128 1, i128 1, i128 1, i128 1>
  %8 = trunc <16 x i128> %7 to <16 x i8>
  store <16 x i8> %8, <16 x i8>* %c, align 4
  ret void
}

-mcpu=btver2 -instsimplify

define void @not_avg_v16i8_wide_constants(<16 x i8>* %a, <16 x i8>* %b, <16 x
i8>* %c) {
  store <16 x i8> <i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8
-1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1, i8 -1>, <16 x i8>* %c, align 4
  ret void
}

So, it looks like we're missing some constant folding / value tracking in
DAG....

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