[llvm] r270671 - [FunctionAttrs] Volatile loads should disable readonly

[David Majnemer via llvm-commits]

Author: majnemer
Date: Wed May 25 00:53:04 2016
New Revision: 270671

URL: http://llvm.org/viewvc/llvm-project?rev=270671&view=rev
Log:
[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.

Modified:
    llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp
    llvm/trunk/test/Transforms/FunctionAttrs/readattrs.ll

Modified: llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp?rev=270671&r1=270670&r2=270671&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/FunctionAttrs.cpp Wed May 25 00:53:04 2016
@@ -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;
 

Modified: llvm/trunk/test/Transforms/FunctionAttrs/readattrs.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionAttrs/readattrs.ll?rev=270671&r1=270670&r2=270671&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/FunctionAttrs/readattrs.ll (original)
+++ llvm/trunk/test/Transforms/FunctionAttrs/readattrs.ll Wed May 25 00:53:04 2016
@@ -104,3 +104,11 @@ define <4 x i32> @test12_2(<4 x i32*> %p
   %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
+}