Re: Infering that the condition of a for loop is initially true?

[Marc Glisse <marc dot glisse at inria dot fr>]

On Thu, 14 Sep 2017, Niels Möller wrote:

> This is more of a question than a bug report, so I'm trying to send it
> to the list rather than filing a bugzilla issue.
>
> I think it's quite common to write for- and while-loops where the
> condition is always initially true. A simple example might be
>
> double average (const double *a, size_t n)
> {
>  double sum;
>  size_t i;
>
>  assert (n > 0);
>  for (i = 0, sum = 0; i < n; i++)
>    sum += a[i];
>  return sum / n;
> }
>
> The programmer could do the microptimization to rewrite it as a
> do-while-loop instead. It would be nice if gcc could infer that the
> condition is initially true, and convert to a do-while loop
> automatically.
>
> Converting to a do-while-loop should produce slightly better code,
> omitting the typical jump to enter the loop at the end where the
> condition is checked. It would also make analysis of where variables are
> written more accurate, which is my main concern at the moment.

Hello,

assert is not what you want, since it completely disappears with -DNDEBUG. 
clang has __builtin_assume, with gcc you want a test and 
__builtin_unreachable. Replacing your assert with
if(n==0)__builtin_unreachable();
gcc does skip the first test of the loop, as can be seen in the dump
produced with -fdump-tree-optimized.

--
Marc Glisse