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Re: [PATCH] add support for strnlen (PR 81384)

On 07/09/2018 08:36 AM, Aldy Hernandez wrote:
>    { dg-do run }
>    { do-options "-O2 -fno-tree-strlen" }  */
> ^^^^ I don't think this is doing anything.
> If you look at the test run you can see that -fno-tree-strlen is never
> passed (I think you actually mean -fno-optimize-strlen for that
> matter).  Also, the builtins.exp harness runs your test for an
> assortment of other flags, not just -O2.
> This test is failing on my range branch for -Og, because
> expand_builtin_strnlen() needs range info:
> +  wide_int min, max;
> +  enum value_range_type rng = get_range_info (bound, &min, &max);
> +  if (rng != VR_RANGE)
> +    return NULL_RTX;
> but interestingly enough, it seems to be calculated in the sprintf
> pass as part of the DOM walk:
>       /* First record ranges generated by this statement.  */
>       evrp_range_analyzer.record_ranges_from_stmt (stmt, false);
> It feels wrong that the sprintf warning pass is generating range info
> that you may later depend on at rtl expansion time (and for a totally
> unrelated thing-- strlen expansion).
We have a fair amount of expansion code these days that will use
globally valid range information if it's been computed.

> I don't know if this is just a quirk of builtins.exp calling your test
> with flags you didn't intend, but the inconsistency could cause
> problems in the future.  Errr, or my present ;-).
> Would it be too much to ask for you to either fix the flags being
> passed down to the test, or better yet, find some non-sprintf
> dependent way of calculating range info earlier?
I think the general issue here is if we do something like
-O <-fblahblahblah> -Wsprintf-blah

Where <blahblahblah> is whatever aggregate of -f options are need to
disable any optimizer that generates range information.

In that case the sprintf warning pass becomes the only pass that
generates ranges.  So whether or not we run the sprintf warning pass
would affect the generated code.  And I think that's really the bigger
issue here -- warnings should affect code generation.

The question is what to do about it.  It probably wouldn't be too
terrible to have clients of the EVRP analyzer to specify if any
discovered ranges should be mirrored into the global range information.

Optimization passes would ask for the mirroring, the sprintf pass or any
other warning pass would not ask for mirroring.



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