Re: [Emacs-diffs] trunk r114593: * lisp.h (eassert): Don't use 'assume'.

[Daniel Colascione]

On 10/11/13 1:59 AM, Stephen J. Turnbull wrote:
> apparently optimizers are sufficiently stupid
> as to compile worse code when assume is present (at least that's what
> Paul seemed to claim).

I don't think that's actually the case. We implement assume using GCC's 
__builtin_unreachable: if (!(assumed_condition)) { 
__builtin_unreachable(); }.  If assumed_condition has side effects, then 
normal C language semantics requires that the compiler emit code to 
implement these side effects. The code required to do so might make the 
surrounding code "worse" than it would be otherwise, but the problem is 
with the side effects, not the optimizer hint per se. I haven't seen any 
instance of assume itself reducing the quality of produced code.

We don't have this problem with plain assertions, since in non-checked 
builds, we emit 0&&(assertion) so that the compiler need not ever 
actually evaluate assertion. (Also, it's for this reason that MSVC's 
__assume construct is cleaner than __builtin_unreachable.)