[LLVMdev] [patch] Change llvm_unreachable to use __builtin_unreachable() in -asserts

[Jim Grosbach]

Such an awesome change it was worth saying twice! :)

Sounds great, and I completely agree it's a nice enhancement to what we can effectively express to help the compiler optimize more effectively. Thanks for doing this.

-Jim


On Mar 11, 2011, at 6:55 PM, John McCall wrote:

> This patch implements the current consensus of PR8973:
>   http://llvm.org/bugs/show_bug.cgi?id=8973.
> 
> The macro llvm_unreachable is used in LLVM to indicate that
> a particular place in a function is not supposed to be reachable
> during execution.  Like an assert macro, it takes a string
> argument.  In +Asserts builds, this string argument, together with
> some location information, is passed to a function which prints
> the information and calls abort().  In -Asserts builds, this string
> argument is dropped (to minimize code size impact), and
> instead a bunch of zero arguments are passed to the same
> function.
> 
> The problem is that that's still not very good for code size, as it
> leaves a somewhat bulky function call in the emitted code.  It
> also doesn't let give the compiler any opportunity to optimize
> based on our assertion that the code is unreachable.  A much
> better alternative is to use an intrinsic, provided by Clang and
> GCC 4.5, called __builtin_unreachable;  it has the semantics
> of being undefined behavior if reached, much like LLVM's own
> "unreachable" instruction, which incidentally is what Clang
> generates for it.
> 
> This patch keeps the old behavior of llvm_unreachable in
>  +Asserts (!defined(NDEBUG)) builds, but changes the behavior
> in -Asserts builds to call __builtin_unreachable() (in GCC 4.5
> and Clang) or abort() (in everything else).
> 
> This is effectively a change in the practical semantics of
> llvm_unreachable:  if the call is actually reachable, then you
> will get some really crazy behavior in -Asserts builds.  If you've
> been using this macro in places that can logically be reached —
> e.g., after you've tested for all the instructions you've actually
> implemented in your backend — then you've been violating the
> spirit of this macro, as communicated by its name, and you
> should change your code to handle unexpected patterns
> more responsibly.
> 
> John.
> 
> <unreachable.patch.txt>
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