svn commit: r313110 - projects/clang400-import/contrib/llvm/lib/Analysis

[Dimitry Andric]

Author: dim
Date: Thu Feb  2 23:01:29 2017
New Revision: 313110
URL: https://svnweb.freebsd.org/changeset/base/313110

Log:
  Pull in r293773 from upstream llvm trunk (by Sanjay Patel):
  
    [ValueTracking] avoid crashing from bad assumptions (PR31809)
  
    A program may contain llvm.assume info that disagrees with other
    analysis. This may be caused by UB in the program, so we must not
    crash because of that.
  
    As noted in the code comments:
    https://llvm.org/bugs/show_bug.cgi?id=31809
    ...we can do better, but this at least avoids the assert/crash in the
    bug report.
  
    Differential Revision: https://reviews.llvm.org/D29395
  
  This fixes an assertion when building editors/emacs-devel.
  
  PR:		216614

Modified:
  projects/clang400-import/contrib/llvm/lib/Analysis/ValueTracking.cpp

Modified: projects/clang400-import/contrib/llvm/lib/Analysis/ValueTracking.cpp
==============================================================================
--- projects/clang400-import/contrib/llvm/lib/Analysis/ValueTracking.cpp	Thu Feb  2 22:02:10 2017	(r313109)
+++ projects/clang400-import/contrib/llvm/lib/Analysis/ValueTracking.cpp	Thu Feb  2 23:01:29 2017	(r313110)
@@ -781,6 +781,23 @@ static void computeKnownBitsFromAssume(c
           APInt::getHighBitsSet(BitWidth, RHSKnownZero.countLeadingOnes());
     }
   }
+
+  // If assumptions conflict with each other or previous known bits, then we
+  // have a logical fallacy. This should only happen when a program has
+  // undefined behavior. We can't assert/crash, so clear out the known bits and
+  // hope for the best.
+
+  // FIXME: Publish a warning/remark that we have encountered UB or the compiler
+  // is broken.
+
+  // FIXME: Implement a stronger version of "I give up" by invalidating/clearing
+  // the assumption cache. This should indicate that the cache is corrupted so
+  // future callers will not waste time repopulating it with faulty assumptions.
+
+  if ((KnownZero & KnownOne) != 0) {
+    KnownZero.clearAllBits();
+    KnownOne.clearAllBits();
+  }
 }
 
 // Compute known bits from a shift operator, including those with a