[FunctionAttrs] Volatile loads should disable readonly

A volatile load has side effects beyond what callers expect readonly to
signify.  For example, it is not safe to reorder two function calls
which each perform a volatile load to the same memory location.

llvm-svn: 270671

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index b6ebc632636..c757eb327d0 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -463,6 +463,11 @@ determinePointerReadAttrs(Argument *A,
     }
 
     case Instruction::Load:
+      // A volatile load has side effects beyond what readonly can be relied
+      // upon.
+      if (cast<LoadInst>(I)->isVolatile())
+        return Attribute::None;
+
       IsRead = true;
       break;