[llvm-bugs] [Bug 39317] New: BPF target: r11 leaks to generated code on array access in a loop using volatile index

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39317

            Bug ID: 39317
           Summary: BPF target: r11 leaks to generated code on array
                    access in a loop using volatile index
           Product: new-bugs
           Version: 6.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: elazarg at gmail.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 21010
  --> https://bugs.llvm.org/attachment.cgi?id=21010&action=edit
zip file with bitcode, llvm-disassembly, bpf-disassembly, object file and
source C file

Running on a source code very similar to bug #39316, llc emits invalid code
that uses the nonexistent register r1. This register seems to be used for
internal manipulations in the backend, but should not be exposed in the BPF
code.

(The usage also does not make sense because it reads from an uninitialized
register, and then writes to an unused register)

To reproduce, run 

$ llc-6.0 -march=bpf -filetype=obj bug_r11.bc

To see the bug, run 

$ llvm-objdump-6.0 -S bug_r11.o
...
      11:       bf b1 00 00 00 00 00 00         r1 = r11
...
LBB0_3:
      21:       bf 1b 00 00 00 00 00 00         r11 = r1
...

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