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Re: [PATCH] Prevent loops from being optimized away
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Fri, 01 Apr 2016 19:34:48 +0200
- Subject: Re: [PATCH] Prevent loops from being optimized away
- Authentication-results: sourceware.org; auth=none
- References: <9fc51a58cc354a934ff5a5e2b232dae520090e2b dot 1459485314 dot git dot segher at kernel dot crashing dot org> <CAFiYyc2qMkR+5nezXENWJqL71U8m3HJBoq7C1EuPoxK-a5n4BQ at mail dot gmail dot com> <20160401151810 dot GA11748 at gate dot crashing dot org>
On April 1, 2016 5:18:10 PM GMT+02:00, Segher Boessenkool <segher@kernel.crashing.org> wrote:
>On Fri, Apr 01, 2016 at 09:36:49AM +0200, Richard Biener wrote:
>> On Fri, Apr 1, 2016 at 6:54 AM, Segher Boessenkool
>> <segher@kernel.crashing.org> wrote:
>> > Sometimes people write loops that they do not want optimized away,
>even
>> > when the compiler can replace those loops by a simple expression
>(or
>> > nothing). For such people, this patch adds a compiler option.
>> >
>> > Bootstrapped on powerpc64-linux; regression check still in progress
>> > (with Init(1) to actually test anything).
>>
>> -fno-tree-scev-cprop? -O0?
>
>There are other cases where GCC can delete loops, for example cddce1.
>
>> A new compiler option for this is complete overkill (and it's
>implementation
>> is gross ;)). Semantics are also unclear, your patch would only make
>sure
>> to preserve an empty loop with the asm in the latch, it wouldn't
>disallow
>> replacing the overall effect with a computation.
>
>That's right, and the loop can even still be unrolled, even fully
>unrolled (which is good, not only should we not desert the loop but
>we also shouldn't run around so much).
>
>> Your patch would also miss a few testcases.
>
>It already makes ~2000 (mainly vectorisation) testcases fail, is that
>not enough coverage? :-)
For an April 1st joke yes, otherwise no :)
Richard.
>Cheers,
>
>
>Segher