[cfe-dev] Reaching the end of a value-returning function in C++

[Matthieu Monrocq]

On Tue, Oct 16, 2012 at 6:04 PM, John McCall <rjmccall at apple.com> wrote:

> On Oct 15, 2012, at 10:26 PM, Argyrios Kyrtzidis wrote:
>
> On Oct 15, 2012, at 9:45 PM, John McCall <rjmccall at apple.com> wrote:
>
> On Oct 15, 2012, at 9:34 PM, Richard Smith wrote:
>
> On Mon, Oct 15, 2012 at 11:39 AM, Argyrios Kyrtzidis <kyrtzidis at apple.com>wrote:
>
>> Unless I'm missing something, this will benefit functions that are not
>> checked with -Wreturn-type and are supposed to be unreachable in some path
>> but are not marked as such.
>>
> I'd prefer that these functions are actually marked as 'unreachable' in
>> source code, instead of depending on the compiler implicitly assuming that
>> in order to get such an optimization.
>>
>
> I agree, but if they're not marked 'unreachable' in the source code, what
> IR would you want to produce for code paths which fall off the end?
> @llvm.trap() at -O0 and unreachable otherwise seems reasonable to me; would
> you prefer something else? (Perhaps always emitting a call to @llvm.trap?)
>
>
> FWIW, I endorse using 'unreachable' here outside of -O0.
>
>
> Compared to 'unreachable', I prefer always emitting a call to @llvm.trap.
>
> Please keep in mind that there's debugging and investigation of crash
> reports from -Os/O2 code as well..
> I didn't yet see an argument that there's enough optimization opportunity
> in practical terms to justify the havoc that 'unreachable' will cause with
> a buggy function.
> Valid code is, in reality, going to use 'unreachable' marks and 'noreturn'
> functions, so all we are going to achieve is "speed up" buggy code,
> relinquishing any hope of finding the bug or figuring out what is going on
> in general.
>
>
> Is there a case where we wouldn't actually warn before doing this?  Buggy
> C++ system headers?
>
> John.
>
> Actually Richard did fix a couple issue in libc++ headers (missing "return
*this" in swaps/assignment operators for what I could see), so yes, c++
system headers (and possibly c platform headers ?)

 -- Matthieu
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