[llvm-bugs] [Bug 43111] New: Avoid reloads after vectorizer's runtime alias check

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43111

            Bug ID: 43111
           Summary: Avoid reloads after vectorizer's runtime alias check
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

Consider loop:
int foo(int *p, int *q) {
    int s = 0;
    for (int i = 0; i < 256; ++i) {
        int k = p[i];
        q[i] = 2*k;
        s+= k;
        int o = p[i];
        s+= o;
    }
    return s;
}

Flags:  clang -fno-unroll-loops -O3 loop.c -mavx2

In entry block there is a runtime check to check whether pointers do not alias.
entry:
  %scevgep = getelementptr i32, i32* %q, i64 256
  %scevgep23 = getelementptr i32, i32* %p, i64 256
  %bound0 = icmp ugt i32* %scevgep23, %q
  %bound1 = icmp ugt i32* %scevgep, %p
  %found.conflict = and i1 %bound0, %bound1
  br i1 %found.conflict, label %for.body, label %vector.body


Then, in loop body, we know there is no aliasing.

vector.body:                                      ; preds = %entry,
%vector.body
  %index = phi i64 [ %index.next, %vector.body ], [ 0, %entry ]
  %vec.phi = phi <8 x i32> [ %7, %vector.body ], [ zeroinitializer, %entry ]
  %0 = getelementptr inbounds i32, i32* %p, i64 %index
  %1 = bitcast i32* %0 to <8 x i32>*
  %wide.load = load <8 x i32>, <8 x i32>* %1, align 4, !tbaa !2, !alias.scope
!6
  %2 = shl nsw <8 x i32> %wide.load, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1,
i32 1, i32 1>
  %3 = getelementptr inbounds i32, i32* %q, i64 %index
  %4 = bitcast i32* %3 to <8 x i32>*
  store <8 x i32> %2, <8 x i32>* %4, align 4, !tbaa !2, !alias.scope !9,
!noalias !6
  %5 = add nsw <8 x i32> %wide.load, %vec.phi
  %6 = bitcast i32* %0 to <8 x i32>*
  %wide.load25 = load <8 x i32>, <8 x i32>* %6, align 4, !tbaa !2, !alias.scope
!6
...

But we still reload p[i] '%wide.load25'..

Expected IR:
vector.body:                                      ; preds = %entry,
%vector.body
  %index = phi i64 [ %index.next, %vector.body ], [ 0, %entry ]
  %vec.phi = phi <8 x i32> [ %5, %vector.body ], [ zeroinitializer, %entry ]
  %0 = getelementptr inbounds i32, i32* %p, i64 %index
  %1 = bitcast i32* %0 to <8 x i32>*
  %wide.load = load <8 x i32>, <8 x i32>* %1, align 4, !tbaa !2, !alias.scope
!6
  %2 = shl <8 x i32> %wide.load, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32
1, i32 1>
  %3 = getelementptr inbounds i32, i32* %q, i64 %index
  %4 = bitcast i32* %3 to <8 x i32>*
  store <8 x i32> %2, <8 x i32>* %4, align 4, !tbaa !2, !alias.scope !9,
!noalias !6
...

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