[all-commits] [llvm/llvm-project] 3bcccb: [Reassociate] Drop weight reduction to fix issue 9...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3bcccb6af685c3132a9ee578b9e11b2503c35a5c
      https://github.com/llvm/llvm-project/commit/3bcccb6af685c3132a9ee578b9e11b2503c35a5c
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-05-29 (Wed, 29 May 2024)

  Changed paths:
    M llvm/lib/Transforms/Scalar/Reassociate.cpp
    M llvm/test/Transforms/Reassociate/repeats.ll

  Log Message:
  -----------
  [Reassociate] Drop weight reduction to fix issue 91417 (#91469)

See the following case: https://alive2.llvm.org/ce/z/A-fBki
```
define i3 @src(i3 %0) {
  %2 = mul i3 %0, %0
  %3 = mul i3 %2, %0
  %4 = mul i3 %3, %0
  %5 = mul nsw i3 %4, %0
  ret i3 %5
}

define i3 @tgt(i3 %0) {
  %2 = mul i3 %0, %0
  %5 = mul nsw i3 %2, %0
  ret i3 %5
}
```


https://github.com/llvm/llvm-project/commit/d7aeefebd6b049f017711cd7c6ef5f217a17b673
introduced weight reduction during weights combination of the same
operand. As the weight of `%0` changes from 5 to 3, the nsw flag in `%5`
should be dropped.

However, the nsw flag isn't cleared by `RewriteExprTree` since `%5 = mul
nsw i3 %0, %4` is not included in the range of `[ExpressionChangedStart,
ExpressionChangedEnd)`.
```
Calculated Rank[] = 3
Combine negations for:   %2 = mul i3 %0, %0
Calculated Rank[] = 4
Combine negations for:   %3 = mul i3 %0, %2
Calculated Rank[] = 5
Combine negations for:   %4 = mul i3 %0, %3
Calculated Rank[] = 6
Combine negations for:   %5 = mul nsw i3 %0, %4
LINEARIZE:   %5 = mul nsw i3 %0, %4
OPERAND: i3 %0 (1)
ADD USES LEAF: i3 %0 (1)
OPERAND:   %4 = mul i3 %0, %3 (1)
DIRECT ADD:   %4 = mul i3 %0, %3 (1)
OPERAND: i3 %0 (1)
OPERAND:   %3 = mul i3 %0, %2 (1)
DIRECT ADD:   %3 = mul i3 %0, %2 (1)
OPERAND: i3 %0 (1)
OPERAND:   %2 = mul i3 %0, %0 (1)
DIRECT ADD:   %2 = mul i3 %0, %0 (1)
OPERAND: i3 %0 (1)
OPERAND: i3 %0 (1)
RAIn:   mul i3  [ %0, #3] [ %0, #3] [ %0, #3] 
RAOut:  mul i3  [ %0, #3] [ %0, #3] [ %0, #3] 
RAOut after CSE reorder:        mul i3  [ %0, #3] [ %0, #3] [ %0, #3] 
RA:   %5 = mul nsw i3 %0, %4
TO:   %5 = mul nsw i3 %4, %0
RA:   %4 = mul i3 %0, %3
TO:   %4 = mul i3 %0, %0
```

The best way to fix this is to inform `RewriteExprTree` to clear flags
of the whole expr tree when weight reduction happens.

But I find that weight reduction based on Carmichael number never
happens in practice.
See the coverage result
https://dtcxzyw.github.io/llvm-opt-benchmark/coverage/home/dtcxzyw/llvm-project/llvm/lib/Transforms/Scalar/Reassociate.cpp.html#L323

I think it would be better to drop `IncorporateWeight`.

Fixes #91417



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