[PATCH] D124119: [InstCombine] Combine instructions of type or/and where AND masks can be combined.

[Alexander Kornienko via Phabricator via llvm-commits]

alexfh added a comment.

In D124119#3549716 <https://reviews.llvm.org/D124119#3549716>, @alexfh wrote:

> I was wrong with the initial analysis. The patch doesn't just make the performance worse. It makes clang loop indefinitely on a certain input.
>
>   $ cat q.cc
>   int e;
>   int u;
>   int ag;
>   void f() {
>     int ak = e;
>     int al((unsigned char)(ak >> 23) & 925);
>     if (ak)
>       al = (ak >> 23 & u) | ((unsigned char)(ak >> 23) & 925) | (u >> 23 & 157);
>     ag = al;
>   }
>   
>   $ time ./clang-15-10515 --target=x86_64--linux-gnu -O1  -c q.cc
>   ^C
>   
>   real    0m45.072s
>   user    0m0.025s
>   sys     0m0.099s
>
> This is definitely a problem that has to be fixed. If you don't have an obvious fix in mind, please revert while investigating.

A bit cleaner test case:

  int f(int a, int b) {
    int c = ((unsigned char)(a >> 23) & 925);
    if (a)
      c = (a >> 23 & b) | ((unsigned char)(a >> 23) & 925) | (b >> 23 & 157);
    return c;
  }


Repository:
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  https://reviews.llvm.org/D124119/new/

https://reviews.llvm.org/D124119