implement constant folding support for an exotic constant expr:

  ret i64 ptrtoint (i8* getelementptr ([1000 x i8]* @X, i64 1, i64 sub (i64 0, i64 ptrtoint ([1000 x i8]* @X to i64))) to i64)

to "ret i64 1000".  This allows us to correctly compute the trip count
on a loop in PR8883, which occurs with std::fill on a char array.  This
allows us to transform it into a memset with a constant size.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122950 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 38 insertions, 1 deletions

diff --git a/lib/Analysis/ConstantFolding.cpp b/lib/Analysis/ConstantFolding.cpp
index ded1d0767c..6c5b75502f 100644
--- a/lib/Analysis/ConstantFolding.cpp
+++ b/lib/Analysis/ConstantFolding.cpp
@@ -575,8 +575,26 @@ static Constant *SymbolicallyEvaluateGEP(Constant *const *Ops, unsigned NumOps,
   // If this is a constant expr gep that is effectively computing an
   // "offsetof", fold it into 'cast int Size to T*' instead of 'gep 0, 0, 12'
   for (unsigned i = 1; i != NumOps; ++i)
-    if (!isa<ConstantInt>(Ops[i]))
+    if (!isa<ConstantInt>(Ops[i])) {
+      
+      // If this is "gep i8* Ptr, (sub 0, V)", fold this as:
+      // "inttoptr (sub (ptrtoint Ptr), V)"
+      if (NumOps == 2 &&
+          cast<PointerType>(ResultTy)->getElementType()->isIntegerTy(8)) {
+        ConstantExpr *CE = dyn_cast<ConstantExpr>(Ops[1]);
+        if (CE && CE->getOpcode() == Instruction::Sub &&
+            isa<ConstantInt>(CE->getOperand(0)) &&
+            cast<ConstantInt>(CE->getOperand(0))->isZero()) {
+          Constant *Res = ConstantExpr::getPtrToInt(Ptr, CE->getType());
+          Res = ConstantExpr::getSub(Res, CE->getOperand(1));
+          Res = ConstantExpr::getIntToPtr(Res, ResultTy);
+          if (ConstantExpr *ResCE = dyn_cast<ConstantExpr>(Res))
+            Res = ConstantFoldConstantExpression(ResCE, TD);
+          return Res;
+        }
+      }
       return 0;
+    }
   
   APInt Offset = APInt(BitWidth,
                        TD->getIndexedOffset(Ptr->getType(),
diff --git a/test/Transforms/InstCombine/constant-fold-gep.ll b/test/Transforms/InstCombine/constant-fold-gep.ll
index 4be1a9c838..c679226d4a 100644
--- a/test/Transforms/InstCombine/constant-fold-gep.ll
+++ b/test/Transforms/InstCombine/constant-fold-gep.ll
@@ -53,3 +53,22 @@ define void @frob() {
   store i32 1, i32* getelementptr ([3 x %struct.X]* @Y, i64 0, i64 0, i32 0, i64 19), align 8
   ret void
 }
+
+
+; PR8883 - Constant fold exotic gep subtract
+; CHECK: @test2
+@X = global [1000 x i8] zeroinitializer, align 16
+
+define i64 @test2() {
+entry:
+  %A = bitcast i8* getelementptr inbounds ([1000 x i8]* @X, i64 1, i64 0) to i8*
+  %B = bitcast i8* getelementptr inbounds ([1000 x i8]* @X, i64 0, i64 0) to i8*
+
+  %B2 = ptrtoint i8* %B to i64
+  %C = sub i64 0, %B2
+  %D = getelementptr i8* %A, i64 %C
+  %E = ptrtoint i8* %D to i64
+
+  ret i64 %E
+  ; CHECK: ret i64 1000
+}