[all-commits] [llvm/llvm-project] 1e072a: [CGP] [CodeGenPrepare] Folding `urem` with loop in...

[goldsteinn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1e072ae289d77c3c9704a9ae832c076a303c435b
      https://github.com/llvm/llvm-project/commit/1e072ae289d77c3c9704a9ae832c076a303c435b
  Author: goldsteinn <35538541+goldsteinn at users.noreply.github.com>
  Date:   2024-10-31 (Thu, 31 Oct 2024)

  Changed paths:
    M llvm/lib/CodeGen/CodeGenPrepare.cpp
    M llvm/test/Transforms/CodeGenPrepare/X86/fold-loop-of-urem.ll

  Log Message:
  -----------
  [CGP] [CodeGenPrepare] Folding `urem` with loop invariant value plus offset (#104724)

This extends the existing fold:

```
for(i = Start; i < End; ++i)
   Rem = (i nuw+- IncrLoopInvariant) u% RemAmtLoopInvariant;
```
 ->
```
Rem = (Start nuw+- IncrLoopInvariant) % RemAmtLoopInvariant;
for(i = Start; i < End; ++i, ++rem)
   Rem = rem == RemAmtLoopInvariant ? 0 : Rem;
```

To work with a non-zero `IncrLoopInvariant`.

This is a common usage in cases such as:

```
for(i = 0; i < N; ++i)
    if ((i + 1) % X) == 0)
        do_something_occasionally_but_not_first_iter();
```

Alive2 w/ i4/unrolled 6x (needs to be ran locally due to timeout):
https://alive2.llvm.org/ce/z/6tgyN3

Exhaust proof over all uint8_t combinations in C++:
https://godbolt.org/z/WYa561388



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