[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

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;
 

llvm/test/Transforms/FunctionAttrs/readattrs.ll

@@ -104,3 +104,11 @@ define <4 x i32> @test12_2(<4 x i32*> %ptrs) {
   %res = call <4 x i32> @test12_1(<4 x i32*> %ptrs)
   ret <4 x i32> %res
 }
+
+; CHECK: define i32 @volatile_load(
+; CHECK-NOT: readonly
+; CHECK: ret
+define i32 @volatile_load(i32* %p) {
+  %load = load volatile i32, i32* %p
+  ret i32 %load
+}