[LLVMbugs] [Bug 20515] New: assembly files do not preserve order of use lists

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20515

            Bug ID: 20515
           Summary: assembly files do not preserve order of use lists
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM assembly language parser
          Assignee: unassignedbugs at nondot.org
          Reporter: dexonsmith at apple.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Bug 5680 allows use-list order to be preserved when round-tripping to bitcode. 
Now that it's wrapping up, the next job is to add support for assembly, which I
imagine can work quite similarly.  My initial idea is to add an assembly
construct that looks something like this:

    shuffleuselist i8* %a, [4 x i32] [i32 0, i32 3, i32 1, i32 2]
    shuffleuselist i64 0, [4 x i32] [i32 0, i32 3, i32 1, i32 2]

This construct can be at the function or global level.  The first argument is a
Value and the second is a use-list shuffle-vector.  The LLParser would apply
these at the point they're encountered.

I'm (naively?) assuming the same approach will work here -- predict in the
AssemblyWriter how the LLParser will see the use-lists, and write out shuffles
at a point where the full use-list will have been reconstructed.  Once I've had
a chance to read the LLParser (and AsmWriter) to confirm just how naive this
idea is I'll throw a post on llvmdev to bikeshed the instruction format.

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