[PATCH] D20058: [SCEV] No-wrap flags are not propagated when folding "{S, +, X}+T ==> {S+T, +, X}"

[Oleg Ranevskyy via llvm-commits]

iid_iunknown created this revision.
iid_iunknown added a reviewer: sanjoy.
iid_iunknown added a subscriber: llvm-commits.
iid_iunknown set the repository for this revision to rL LLVM.
Herald added subscribers: mzolotukhin, aemerson.

**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.

Repository:
  rL LLVM

http://reviews.llvm.org/D20058

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/LoopAccessAnalysis/number-of-memchecks.ll
  test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
  test/Analysis/ScalarEvolution/flags-from-poison.ll
  test/Analysis/ScalarEvolution/nsw-offset-assume.ll
  test/Analysis/ScalarEvolution/nsw-offset.ll
  test/Analysis/ScalarEvolution/nsw.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D20058.56520.patch
Type: text/x-patch
Size: 8362 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160508/026c2e12/attachment.bin>