[llvm] r174119 - An alloca can be equal to an argument. It can't *alias* an alloca, but it could

[Dan Gohman]

Author: djg
Date: Thu Jan 31 17:49:33 2013
New Revision: 174119

URL: http://llvm.org/viewvc/llvm-project?rev=174119&view=rev
Log:
An alloca can be equal to an argument. It can't *alias* an alloca, but it could
be equal, since there's nothing preventing a caller from correctly predicting
the stack location of an alloca.

Modified:
    llvm/trunk/lib/Analysis/InstructionSimplify.cpp
    llvm/trunk/test/Transforms/InstSimplify/compare.ll

Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=174119&r1=174118&r2=174119&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan 31 17:49:33 2013
@@ -1828,18 +1828,6 @@ static Value *SimplifyICmpInst(unsigned 
       else if (Pred == CmpInst::ICMP_NE)
         return ConstantInt::get(ITy, true);
     }
-  } else if (Argument *LHSArg = dyn_cast<Argument>(LHSPtr)) {
-    RHSPtr = RHSPtr->stripInBoundsOffsets();
-    // An alloca can't be equal to an argument unless they come from separate
-    // functions via inlining.
-    if (AllocaInst *RHSInst = dyn_cast<AllocaInst>(RHSPtr)) {
-      if (LHSArg->getParent() == RHSInst->getParent()->getParent()) {
-        if (Pred == CmpInst::ICMP_EQ)
-          return ConstantInt::get(ITy, false);
-        else if (Pred == CmpInst::ICMP_NE)
-          return ConstantInt::get(ITy, true);
-      }
-    }
   }
 
   // If we are comparing with zero then try hard since this is a common case.

Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=174119&r1=174118&r2=174119&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Thu Jan 31 17:49:33 2013
@@ -647,3 +647,16 @@ unreachableblock:
   %Y = icmp eq i32* %X, null
   ret i1 %Y
 }
+
+; It's not valid to fold a comparison of an argument with an alloca, even though
+; that's tempting. An argument can't *alias* an alloca, however the aliasing rule
+; relies on restrictions against guessing an object's address and dereferencing.
+; There are no restrictions against guessing an object's address and comparing.
+
+define i1 @alloca_argument_compare(i64* %arg) {
+  %alloc = alloca i64
+  %cmp = icmp eq i64* %arg, %alloc
+  ret i1 %cmp
+  ; CHECK: alloca_argument_compare
+  ; CHECK: ret i1 %cmp
+}