[all-commits] [llvm/llvm-project] f7ef07: [SCEV] Do not allow refinement in the rewriting of...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f7ef0721d60f85e1f699f8d1b83d4402ae19b122
      https://github.com/llvm/llvm-project/commit/f7ef0721d60f85e1f699f8d1b83d4402ae19b122
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-12-01 (Sun, 01 Dec 2024)

  Changed paths:
    M llvm/include/llvm/Analysis/ScalarEvolution.h
    M llvm/lib/Analysis/ScalarEvolution.cpp
    A llvm/test/Analysis/ScalarEvolution/pr117133.ll
    A llvm/test/Transforms/IndVarSimplify/pr117133.ll

  Log Message:
  -----------
  [SCEV] Do not allow refinement in the rewriting of BEValue (#117152)

See the following case:
```
; bin/opt -passes="print<scalar-evolution>" test.ll --disable-output
define i32 @widget() {
b:
  br label %b1

b1:                                              ; preds = %b5, %b
  %phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ]
  %phi2 = phi i32 [ 1, %b ], [ %add, %b5 ]
  %icmp = icmp eq i32 %phi, 0
  br i1 %icmp, label %b3, label %b8

b3:                                              ; preds = %b1
  %udiv = udiv i32 10, %phi2
  %urem = urem i32 %udiv, 10
  %icmp4 = icmp eq i32 %urem, 0
  br i1 %icmp4, label %b7, label %b5

b5:                                              ; preds = %b3
  %udiv6 = udiv i32 %phi2, 0
  %add = add i32 %phi2, 1
  br label %b1

b7:                                              ; preds = %b3
  ret i32 5

b8:                                              ; preds = %b1
  ret i32 7
}
```
```
%phi2 = phi i32 [ 1, %b ], [ %add, %b5 ] -->  {1,+,1}<nuw><nsw><%b1>
%udiv6 = udiv i32 %phi2, 0 --> ({1,+,1}<nuw><nsw><%b1> /u 0)
%phi = phi i32 [ 0, %b ], [ %udiv6, %b5 ] --> ({0,+,1}<nuw><nsw><%b1> /u 0)
```
`ScalarEvolution::createAddRecFromPHI` gives a wrong SCEV result for
`%phi`:

https://github.com/llvm/llvm-project/blob/d7d6fb1804415b0f3e7f1cc9290bfb3d711cb707/llvm/lib/Analysis/ScalarEvolution.cpp#L5926-L5950
It converts `phi(0, ({1,+,1}<nuw><nsw><%b1> /u 0))` into `phi(0 / 0,
({1,+,1}<nuw><nsw><%b1> /u 0))`. Then it simplifies the expr into
`{0,+,1}<nuw><nsw><%b1> /u 0`.

As we did in
https://github.com/llvm/llvm-project/commit/acd700a24b6f767413db3d525e06d03e4245aa40,
this patch disallows udiv simplification if we cannot prove that the
denominator is a well-defined non-zero value.

Fixes https://github.com/llvm/llvm-project/issues/117133.



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