[SCEV] No-wrap flags are not propagated when folding "{S,+,X}+T ==> {S+T,+,X}"

Summary:
**Description**

This makes `WidenIV::widenIVUse` (IndVarSimplify.cpp) fail to widen narrow IV uses in some cases. The latter affects IndVarSimplify which may not eliminate narrow IV's when there actually exists such a possibility, thereby producing ineffective code.

When `WidenIV::widenIVUse` gets a NarrowUse such as `{(-2 + %inc.lcssa),+,1}<nsw><%for.body3>`, it first tries to get a wide recurrence for it via the `getWideRecurrence` call.
`getWideRecurrence` returns recurrence like this: `{(sext i32 (-2 + %inc.lcssa) to i64),+,1}<nsw><%for.body3>`.

Then a wide use operation is generated by `cloneIVUser`. The generated wide use is evaluated to `{(-2 + (sext i32 %inc.lcssa to i64))<nsw>,+,1}<nsw><%for.body3>`, which is different from the `getWideRecurrence` result. `cloneIVUser` sees the difference and returns nullptr.

This patch also fixes the broken LLVM tests by adding missing <nsw> entries introduced by the correction.

**Minimal reproducer:**
```
int foo(int a, int b, int c);
int baz();

void bar()
{
   int arr[20];
   int i = 0;

   for (i = 0; i < 4; ++i)
     arr[i] = baz();

   for (; i < 20; ++i)
     arr[i] = foo(arr[i - 4], arr[i - 3], arr[i - 2]);
}
```

**Clang command line:**
```
clang++ -mllvm -debug -S -emit-llvm -O3 --target=aarch64-linux-elf test.cpp -o test.ir
```

**Expected result:**
The ` -mllvm -debug` log shows that all the IV's for the second `for` loop have been eliminated.

Reviewers: sanjoy

Subscribers: atrick, asl, aemerson, mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D20058

llvm-svn: 270695

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index bfd333d911c..10455a4224c 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2306,7 +2306,10 @@ const SCEV *ScalarEvolution::getAddExpr(SmallVectorImpl<const SCEV *> &Ops,
 
       SmallVector<const SCEV *, 4> AddRecOps(AddRec->op_begin(),
                                              AddRec->op_end());
-      AddRecOps[0] = getAddExpr(LIOps);
+      // This follows from the fact that the no-wrap flags on the outer add
+      // expression are applicable on the 0th iteration, when the add recurrence
+      // will be equal to its start value.
+      AddRecOps[0] = getAddExpr(LIOps, Flags);
 
       // Build the new addrec. Propagate the NUW and NSW flags if both the
       // outer add and the inner addrec are guaranteed to have no overflow.