[llvm-bugs] [Bug 47366] New: DAGCombine of ARMISD::SUBC gets stuck in an infinite loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47366

            Bug ID: 47366
           Summary: DAGCombine of ARMISD::SUBC gets stuck in an infinite
                    loop
           Product: libraries
           Version: 11.0
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: simonas+llvm.org at kazlauskas.me
                CC: llvm-bugs at lists.llvm.org, smithp352 at googlemail.com,
                    Ties.Stuij at arm.com

Given the following LLVM-IR derived by bugpoint and then some cleanup by me:

    target triple = "thumbv6m-none-unknown-eabi"

    define i1 @hangit() unnamed_addr {
    start:
      %0 = tail call { i128, i1 } @llvm.uadd.with.overflow.i128(i128 undef,
i128 undef)
      %1 = extractvalue { i128, i1 } %0, 1
      %2 = select i1 %1, i128 18446744073709551616, i128 0
      %3 = tail call { i128, i1 } @llvm.uadd.with.overflow.i128(i128 undef,
i128 %2)
      %4 = extractvalue { i128, i1 } %3, 1
      ret i1 %4
    }

    declare { i128, i1 } @llvm.uadd.with.overflow.i128(i128, i128)


When built with `llc`, the build will hang. This is confirmed to happen with
both 10.0.1 and very recent master (8e5b1557e5b3)

DAGCombine will get stuck in the following loop:

    Replacing.1 t106: i32,i32 = ARMISD::SUBC t79, Constant:i32<1>
    With: t106: i32,i32 = ARMISD::SUBC t79, Constant:i32<1>
        and 1 other values
    Legalizing: t79: i32,i32 = ARMISD::ADDE Constant:i32<0>, Constant:i32<0>,
t78:1
    Legal node: nothing to do
    Combining: t79: i32,i32 = ARMISD::ADDE Constant:i32<0>, Constant:i32<0>,
t78:1
    Legalizing: t78: i32,i32 = ARMISD::ADDE undef:i32, undef:i32, t82:1
    Legal node: nothing to do
    Combining: t78: i32,i32 = ARMISD::ADDE undef:i32, undef:i32, t82:1
    Legalizing: t106: i32,i32 = ARMISD::SUBC t79, Constant:i32<1>
    Legal node: nothing to do
    Combining: t106: i32,i32 = ARMISD::SUBC t79, Constant:i32<1>
    Replacing.1 t106: i32,i32 = ARMISD::SUBC t79, Constant:i32<1>
    With: t106: i32,i32 = ARMISD::SUBC t79, Constant:i32<1>
        and 1 other values

ad nauseam.

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