[llvm-bugs] [Bug 52104] New: LoopIdiom miscompile

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52104

            Bug ID: 52104
           Summary: LoopIdiom miscompile
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: courbet at google.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

LoopIdiom turns a loop of stores to a zero-size `llvm.memset`:


```
%struct.S = type { [4 x i32], i32 }

define void @f(%struct.S* nocapture nonnull align 4 dereferenceable(20) %0)
local_unnamed_addr #0 align 32 {
  %2 = getelementptr inbounds %struct.S, %struct.S* %0, i64 0, i32 1
  %3 = load i32, i32* %2, align 4
  br label %4

4:                                                ; preds = %4, %1
  %5 = phi i32 [ %9, %4 ], [ %3, %1 ]
  %6 = and i32 %5, 3
  %7 = zext i32 %6 to i64
  %8 = getelementptr inbounds %struct.S, %struct.S* %0, i64 0, i32 0, i64 %7
  store i32 0, i32* %8, align 4
  %9 = add nsw i32 %5, 1
  %10 = and i32 %9, 3
  %11 = icmp eq i32 %10, 0
  br i1 %11, label %12, label %4

12:                                               ; preds = %4
  %13 = phi i32 [ %9, %4 ]
  store i32 %13, i32* %2, align 4
  ret void
}
```

->

```
define void @f(%struct.S* nocapture nonnull align 4 dereferenceable(20) %0)
local_unnamed_addr align 32 {
  %2 = getelementptr inbounds %struct.S, %struct.S* %0, i64 0, i32 1
  %3 = load i32, i32* %2, align 4
  %4 = trunc i32 %3 to i2
  %5 = zext i2 %4 to i64
  %scevgep = getelementptr %struct.S, %struct.S* %0, i64 0, i32 0, i64 %5
  %scevgep1 = bitcast i32* %scevgep to i8*
  call void @llvm.memset.p0i8.i64(i8* align 4 %scevgep1, i8 0, i64 0, i1 false)
  br label %6

6:                                                ; preds = %6, %1
  %7 = phi i32 [ %11, %6 ], [ %3, %1 ]
  %8 = and i32 %7, 3
  %9 = zext i32 %8 to i64
  %10 = getelementptr inbounds %struct.S, %struct.S* %0, i64 0, i32 0, i64 %9
  %11 = add nsw i32 %7, 1
  %12 = and i32 %11, 3
  %13 = icmp eq i32 %12, 0
  br i1 %13, label %14, label %6

14:                                               ; preds = %6
  %15 = phi i32 [ %11, %6 ]
  store i32 %15, i32* %2, align 4
  ret void
}
```

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