[llvm-bugs] [Bug 34471] New: small improvement to switch compilation

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34471

            Bug ID: 34471
           Summary: small improvement to switch compilation
           Product: clang
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Dave at Yost.com
                CC: llvm-bugs at lists.llvm.org

This code

  int switcher(int x) {
    switch(x) {
    case 17: return 17;
    case 42: return 42;
    default: break;
    }
    return 0;
  }

generates this:

    switch i32 %4, label %7 [
      i32 17, label %5
      i32 42, label %6
    ]

  ; <label>:5:                                      ; preds = %1
    store i32 17, i32* %2, align 4
    br label %9

  ; <label>:6:                                      ; preds = %1
    store i32 42, i32* %2, align 4
    br label %9

I suspect that most targets would generate better code from equivalent llvm
output in which both cases jump to a common label that returns x instead of
jumping to separate labels that eventually return the constants 17 and 42.

Certainly the Mill compiler would generate better code. :)
See https://www.youtube.com/watch?time_continue=2191&v=FeYsKF3a14k

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