Re: [COMMITTED] PR tree-optimization/106114 - Don't use gori dependencies to optimize.

[Richard Biener via Gcc-patches]

> Am 30.06.2022 um 03:43 schrieb Andrew MacLeod via Gcc-patches 
> <gcc-patches@gcc.gnu.org>:
> 
> The routine which tried to fold and's and or's using relations was using the 
> dependency cache as a shortcut to determine if there were 2 ssa names on the 
> feeding expressions, and assuming that was correct.
> 
> ie
> 
>   _16 = a.0_1 < -117;
>   _17 = a.0_1 >= -83;
>   _18 = _16 | _17;
> 
> the dependency cache indicates that a.0_1 is "ssa1" dependency for _16 and 
> also for _17.  we dont have to scan the statement, so temporal out of date 
> info is very quick.
> 
> Its also not meant to reflect that actual statement.. ie, it can get out of 
> date.  Not is a way that makes anything incorrect, but in a way that may 
> possibly result in a  either a missed opportunity or slightly more work when 
> statements are being rewritten on the fly..  ie  DOM rewrites that to:
> 
>   _16 = a.1_15 < -117;
>   _17 = a.1_15 >= -83;
>   _18 = _16 | _17;
> 
> When fold_using_range is later invoked, a.1_15 is added a dependency to _16 
> and _17,  not attempting to understand that its a replacement, we simply now 
> think that both a.0_1 and a.1_15 are dependencies.  so if either one becomes 
> out of date, then ranger will recalculate _16 and/or _17
> 
> fold_using_range::relation_fold_and_or was using thet dependency cache as if 
> it represent the operands of the statement accurately... so after the DOM 
> rewrite, it thought that there were 2 operands on the _16 and _17 expression, 
> the 2 dependencies in the cache, misconstruing it as
> 
>   _16 = a.0_1_ < a.1_15;
>   _17 = a.0_1 >= a.1_15;
>   _18 = _16 | _17;
> 
> Thus it thought is could fold it away.
> 
> 
> The dependency cache shortcut should NOT be used for optimizations.   THis 
> patch correct the problem, and simply looks at the 2 operands of the feeding 
> instructions.
> 
> bootstrapped on build-x86_64-pc-linux-gnu with no regressions. Pushed.
> 
> This is less likely to occur in GCC12 since there is less IL change on the 
> fly, but it should be safe to make this change just in case.  OK for GCC12?

Ok for gcc12

Richard 


> Andrew
> 
> PS. and yes, it fixes the other 2 testcases as well.
> <0001-Don-t-use-gori-depedencies-to-optimize.patch>