Re: Possible GCC optimization bug

Arsen Arsenović via Gcc-help <[email protected]> Thu, 12 Feb 2026 18:59:14 +0100

Newsgroups	gmane.comp.gcc.help
Message-ID	<[email protected]>

Anubis 1101 <[email protected]> writes:

> The "10 < 10" comes from the subscript operator overload, where it
> checks the index against the Capacity, which is known at compile
> time. The "if(i < Capacity)" statement passes, and then prints out
> both i and Capacity, which are both 10.

Yes, and 'i' is "known" at compile-time to be < 10 due to the mechanism
I described earlier.  This is due to inlining.  Take the following
simplified example:

  int i = /* doesn't matter */;
  int a[10] = /* doesn't matter */;
  print(a[i]);
  /* here, the compiler knows i < 10 due to a[i].  */

This is essentially what happens in your example also.  The compiler did
a lot of inlining to turn multiple functions into one single function
that has pretty much the same pattern as the above in it.  It is
permitted to do that.  It can, in fact, copy length and operator[] into
other functions that use them as many times as it likes, as long as it
still behaves the same as the abstract machine of the language requires.

In fact, 'if (i < Capacity)' probably never happens.  The compiler
deduces that it is true due to the array accesses.
-- 
Arsen Arsenović

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