[LLVMbugs] [Bug 8973] New: remove llvm_unreachable

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8973

           Summary: remove llvm_unreachable
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Support Libraries
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: clattner at apple.com
                CC: llvmbugs at cs.uiuc.edu


llvm_reachable does two things: it prints an error message, and is marked
noreturn to silence compiler warnings.  It was added as a replacement for
"assert(0 &&" for the second feature, but it has since pervaded the codebase.

It would be really nice to eradicate almost all (or all) uses of this function,
replacing them with proper assertions.  We have lots of stuff like this:

int foo() {

 switch (x) {
  case 1: ..
  case 2: ..
  case 3: return 27;
  default: llvm_unreachable("whatever"); 
 }
}

This sort of thing should be turned into:

int foo() {

 switch (x) {
  default: assert(0 && "whatever");
  case 1: ..
  case 2: ..
  case 3: return 27;
 }
}

There are other similar patterns to remove uses of llvm_unreachable.  One
benefit of this is to get rid of all the ErrorHandling.h includes.

In contrast to llvm_unreachable, report_fatal_error *is* useful.  Those handle
possible, but erroneous cases in the backend, such as inline asm constraint
failures.

-Chris

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