[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, 2 insertions, 2 deletions

diff --git a/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll b/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
index 2053e49826b..0a0194f8abb 100644
--- a/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
+++ b/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
@@ -15,7 +15,7 @@ target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
 target triple = "aarch64--linux-gnueabi"
 
 ; CHECK: function 'f':
-; CHECK: (Low: (20000 + %a) High: (60000 + %a))
+; CHECK: (Low: (20000 + %a) High: (60000 + %a)<nsw>)
 
 @B = common global i32* null, align 8
 @A = common global i32* null, align 8
@@ -58,7 +58,7 @@ for.end:                                          ; preds = %for.body
 ; Here it is not obvious what the limits are, since 'step' could be negative.
 
 ; CHECK: Low: (-1 + (-1 * ((-60001 + (-1 * %a)) umax (-60001 + (40000 * %step) + (-1 * %a)))))
-; CHECK: High: ((60000 + %a) umax (60000 + (-40000 * %step) + %a))
+; CHECK: High: ((60000 + %a)<nsw> umax (60000 + (-40000 * %step) + %a))
 
 define void @g(i64 %step) {
 entry: