[llvm-dev] returns_twice (original) (raw)

[llvm-dev] returns_twice / noreturn

Alexandre Isoard via llvm-dev llvm-dev at lists.llvm.org
Fri Nov 3 17:54:29 PDT 2017

On Fri, Nov 3, 2017 at 5:39 PM, Hal Finkel <hfinkel at anl.gov> wrote:

On 11/03/2017 07:20 PM, Alexandre Isoard via llvm-dev wrote: Hello, I am not sure about the semantic (if any) of returnstwice and noreturn attributes. int fork() attribute((returnstwice)); void join(int) attribute((noreturn)); int f(int n) { int t = fork(); n++; if (t != 0) join(t); return n; } Where the n++ has been moved after the if, is that legal?

Why wouldn't it be?

Because fork() could return 0, then n get incremented (first time), we go into join(t) which do not return... but jump back into fork() which returns again, but 1 this time, then n get incremented (second time), and we return n+2.

While if we move the n++ outside of that "region", we change that semantic? Basically, returns_twice and noreturn have SSA-reaching side-effects.

They have semantics. returnstwice, however, really means, "it may return more than once". noreturn is interpreted as the name implies. Thus the unreachable after the call.

That means we can encode a loop this way? :-)

-Hal -- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory

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