[llvm] r251097 - [SCEV] Fix a latent bug in `getPreStartForExtend`

[Sanjoy Das via llvm-commits]

Author: sanjoy
Date: Fri Oct 23 01:33:47 2015
New Revision: 251097

URL: http://llvm.org/viewvc/llvm-project?rev=251097&view=rev
Log:
[SCEV] Fix a latent bug in `getPreStartForExtend`

I could not come up a way to test this -- I think this bug is latent
today, and will not actually result in a miscompile.

In `getPreStartForExtend`, SCEV constructs `PreStart` as a sum of all of
`SA`'s operands except `Op`.  It also uses `SA`'s no-wrap flags, and
this is problematic because removing an element from an add expression
can make it signed-wrap.  E.g. if `SA` was `(127 + 1 + -1)`, then it
could safely be `<nsw>` (since `sext(127) + sext(1) + sext(-1)` ==
`sext(127 + 1 + -1)`), but `(127 + 1)` (== `PreStart` if `Op` is `-1`)
is not `<nsw>`.

Transferring `<nuw>` from `SA` to `PreStart` is safe, as far as I can
tell.

Modified:
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=251097&r1=251096&r2=251097&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Fri Oct 23 01:33:47 2015
@@ -1268,7 +1268,9 @@ static const SCEV *getPreStartForExtend(
   // `Step`:
 
   // 1. NSW/NUW flags on the step increment.
-  const SCEV *PreStart = SE->getAddExpr(DiffOps, SA->getNoWrapFlags());
+  auto PreStartFlags =
+    ScalarEvolution::maskFlags(SA->getNoWrapFlags(), SCEV::FlagNUW);
+  const SCEV *PreStart = SE->getAddExpr(DiffOps, PreStartFlags);
   const SCEVAddRecExpr *PreAR = dyn_cast<SCEVAddRecExpr>(
       SE->getAddRecExpr(PreStart, Step, L, SCEV::FlagAnyWrap));