[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

2 files changed, 13 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;
 
diff --git a/llvm/test/Transforms/FunctionAttrs/readattrs.ll b/llvm/test/Transforms/FunctionAttrs/readattrs.ll
index 4626cb19748..ed2a8aa07ae 100644
--- a/llvm/test/Transforms/FunctionAttrs/readattrs.ll
+++ b/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
+}