LangRef: mention MSan's problem with speculative conditional branches.

Summary:
This short blurb aims to disallow optimizations like we had to revert
(under MSan) in
  https://reviews.llvm.org/D21165
  https://bugs.llvm.org/show_bug.cgi?id=28054
  https://reviews.llvm.org/D67205

Reviewers: vitalybuka, efriedma

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67244

llvm-svn: 371461

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index b953dbd..e061097 100644 (file)
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3245,6 +3245,17 @@ match what was already there. However, a store *to* an undefined
 location could clobber arbitrary memory, therefore, it has undefined
 behavior.
 
+**MemorySanitizer**, a detector of uses of uninitialized memory,
+defines a branch with condition that depends on an undef value (or
+certain other values, like e.g. a result of a load from heap-allocated
+memory that has never been stored to) to have an externally visible
+side effect. For this reason functions with *sanitize_memory*
+attribute are not allowed to produce such branches "out of thin
+air". More strictly, an optimization that inserts a conditional branch
+is only valid if in all executions where the branch condition has at
+least one undefined bit, the same branch condition is evaluated in the
+input IR as well.
+
 .. _poisonvalues:
 
 Poison Values