[Bug c/94392] [10 Regression] Infinite loops are optimized away for C99

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94392

--- Comment #4 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 30 Mar 2020, joseph at codesourcery dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94392
> 
> --- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
> I'm not sure the existing infinite loop removal is valid for any C 
> standard version.  The C (C11 and later) rule against infinite loops only 
> applies when the loop is written as an iteration statement whose 
> controlling expression is not constant (not if it's written with goto, or 
> has a constant controlling expression); it's not the more general rule 
> about forward progress that C++ has.

For the middle-end feature we need to define what loops we consider
finite in middle-end terms which currently is that the loop has
at least one "normal" (non-EH, non-abnormal) exit.  In that case
the middle-end assumes the loop will eventually terminate.  That
in particular means if the middle-end removes all exits as unreachable
then the loop will no longer appear finite.  I think the middle-end
never introduces additional normal exits so any loop that did not
contain an exit at the source AST level should never appear finite.

So iff the C rule forbids while (a) {} to be infinite but does
_not_ forbid while (1) {} to be infinite this is in-line with
the middle-end implementation.  I _think_ the C++ standard disallows
while (1) {}, too.