[llvm] r173995 - Fix ConstantFold's folding of icmp instructions to recognize that,

[Dan Gohman]

Author: djg
Date: Wed Jan 30 18:01:45 2013
New Revision: 173995

URL: http://llvm.org/viewvc/llvm-project?rev=173995&view=rev
Log:
Fix ConstantFold's folding of icmp instructions to recognize that,
for example, a one-past-the-end pointer from one global variable may
be equal to the base pointer of another global variable.

Added:
    llvm/trunk/test/Assembler/ConstantExprNoFold.ll
Modified:
    llvm/trunk/lib/IR/ConstantFold.cpp

Modified: llvm/trunk/lib/IR/ConstantFold.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/ConstantFold.cpp?rev=173995&r1=173994&r2=173995&view=diff
==============================================================================
--- llvm/trunk/lib/IR/ConstantFold.cpp (original)
+++ llvm/trunk/lib/IR/ConstantFold.cpp Wed Jan 30 18:01:45 2013
@@ -1495,9 +1495,8 @@ static ICmpInst::Predicate evaluateICmpR
                    "Surprising getelementptr!");
             return isSigned ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT;
           } else {
-            // If they are different globals, we don't know what the value is,
-            // but they can't be equal.
-            return ICmpInst::ICMP_NE;
+            // If they are different globals, we don't know what the value is.
+            return ICmpInst::BAD_ICMP_PREDICATE;
           }
         }
       } else {
@@ -1510,10 +1509,10 @@ static ICmpInst::Predicate evaluateICmpR
         default: break;
         case Instruction::GetElementPtr:
           // By far the most common case to handle is when the base pointers are
-          // obviously to the same or different globals.
+          // obviously to the same global.
           if (isa<GlobalValue>(CE1Op0) && isa<GlobalValue>(CE2Op0)) {
-            if (CE1Op0 != CE2Op0) // Don't know relative ordering, but not equal
-              return ICmpInst::ICMP_NE;
+            if (CE1Op0 != CE2Op0) // Don't know relative ordering.
+              return ICmpInst::BAD_ICMP_PREDICATE;
             // Ok, we know that both getelementptr instructions are based on the
             // same global.  From this, we can precisely determine the relative
             // ordering of the resultant pointers.

Added: llvm/trunk/test/Assembler/ConstantExprNoFold.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Assembler/ConstantExprNoFold.ll?rev=173995&view=auto
==============================================================================
--- llvm/trunk/test/Assembler/ConstantExprNoFold.ll (added)
+++ llvm/trunk/test/Assembler/ConstantExprNoFold.ll Wed Jan 30 18:01:45 2013
@@ -0,0 +1,23 @@
+; This test checks to make sure that constant exprs don't fold in some simple
+; situations
+
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+; Even give it a datalayout, to tempt folding as much as possible.
+target datalayout = "p:32:32"
+
+ at A = global i64 0
+ at B = global i64 0
+
+; Don't fold this. @A might really be allocated next to @B, in which case the
+; icmp should return true. It's not valid to *dereference* in @B from a pointer
+; based on @A, but icmp isn't a dereference.
+
+; CHECK: @C = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* @B)
+ at C = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* @B)
+
+; Don't fold this completely away either. In theory this could be simplified
+; to only use a gep on one side of the icmp though.
+
+; CHECK: @D = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* getelementptr inbounds (i64* @B, i64 2))
+ at D = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* getelementptr inbounds (i64* @B, i64 2))