[llvm-bugs] [Bug 49244] New: Suboptimal lowering of `mul x, 0b111....` when using SelectionDAG

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49244

            Bug ID: 49244
           Summary: Suboptimal lowering of `mul x, 0b111....` when using
                    SelectionDAG
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: M68k
          Assignee: unassignedbugs at nondot.org
          Reporter: minyihh at uci.edu
                CC: glaubitz at physik.fu-berlin.de,
                    llvm-bugs at lists.llvm.org, minyihh at uci.edu

As described in test/CodeGen/M68k/ASM/Arith/imul-neg.ll, when multiplying value
`x` with 2^n - 1 integer, the result will be `-x`, which is preferred to be
lowered to `neg x` instruction. However, SelectionDAG ISel will generate `sub
0, x` (FastISel doesn't have this problem though).

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