[llvm-bugs] [Bug 27116] New: RDDSP operands are added too late

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27116

            Bug ID: 27116
           Summary: RDDSP operands are added too late
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: MIPS
          Assignee: unassignedbugs at nondot.org
          Reporter: matze at braunis.de
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The RDDSPs operands are currently added in
MipsSEDAGToDAGISel::processFunctionAfterISel(), if they are the only user of a
physreg then these physreg will appear dead during the earlier InstrEmitter
phase of selection dag, so some defs will be incorrectly marked as dead.

This is currently not a problem because the LiveVariables pass happens to reset
all dead flags, however long term we plan to remove the whole LiveVariables
pass.

(The problem is visible in some functions in  test/CodeGen/Mips/dsp-r1.ll when
dead flag removal is disabled in the LiveVariables pass.)

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