From e65cd40ab99c3bd43dc997873e500643d012bbad Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Mon, 5 Oct 2009 05:26:04 +0000
Subject: teach the optimizer how to constant fold uadd/usub intrinsics.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83295 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/ConstantFolding.cpp          | 30 ++++++++++++++++-
 test/Transforms/ConstProp/overflow-ops.ll | 53 +++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+), 1 deletion(-)
 create mode 100644 test/Transforms/ConstProp/overflow-ops.ll

diff --git a/lib/Analysis/ConstantFolding.cpp b/lib/Analysis/ConstantFolding.cpp
index d624deb..16f2c4d 100644
--- a/lib/Analysis/ConstantFolding.cpp
+++ b/lib/Analysis/ConstantFolding.cpp
@@ -677,6 +677,8 @@ llvm::canConstantFoldCallTo(const Function *F) {
   case Intrinsic::ctpop:
   case Intrinsic::ctlz:
   case Intrinsic::cttz:
+  case Intrinsic::uadd_with_overflow:
+  case Intrinsic::usub_with_overflow:
     return true;
   default:
     return false;
@@ -756,7 +758,7 @@ llvm::ConstantFoldCall(Function *F,
   if (!F->hasName()) return 0;
   LLVMContext &Context = F->getContext();
   StringRef Name = F->getName();
-  
+
   const Type *Ty = F->getReturnType();
   if (NumOperands == 1) {
     if (ConstantFP *Op = dyn_cast<ConstantFP>(Operands[0])) {
@@ -881,6 +883,32 @@ llvm::ConstantFoldCall(Function *F,
       }
       return 0;
     }
+    
+    
+    if (ConstantInt *Op1 = dyn_cast<ConstantInt>(Operands[0])) {
+      if (ConstantInt *Op2 = dyn_cast<ConstantInt>(Operands[1])) {
+        switch (F->getIntrinsicID()) {
+        default: break;
+        case Intrinsic::uadd_with_overflow: {
+          Constant *Res = ConstantExpr::getAdd(Op1, Op2);           // result.
+          Constant *Ops[] = {
+            Res, ConstantExpr::getICmp(CmpInst::ICMP_ULT, Res, Op1) // overflow.
+          };
+          return ConstantStruct::get(F->getContext(), Ops, 2, false);
+        }
+        case Intrinsic::usub_with_overflow: {
+          Constant *Res = ConstantExpr::getSub(Op1, Op2);           // result.
+          Constant *Ops[] = {
+            Res, ConstantExpr::getICmp(CmpInst::ICMP_UGT, Res, Op1) // overflow.
+          };
+          return ConstantStruct::get(F->getContext(), Ops, 2, false);
+        }
+        }
+      }
+      
+      return 0;
+    }
+    return 0;
   }
   return 0;
 }
diff --git a/test/Transforms/ConstProp/overflow-ops.ll b/test/Transforms/ConstProp/overflow-ops.ll
new file mode 100644
index 0000000..52d5921
--- /dev/null
+++ b/test/Transforms/ConstProp/overflow-ops.ll
@@ -0,0 +1,53 @@
+; RUN: opt < %s -constprop -S | FileCheck %s
+
+%i8i1 = type {i8, i1}
+
+;;-----------------------------
+;; uadd
+;;-----------------------------
+
+define {i8, i1} @uadd_1() nounwind {
+entry:
+  %t = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 42, i8 100)
+  ret {i8, i1} %t
+
+; CHECK: @uadd_1
+; CHECK: ret %i8i1 { i8 -114, i1 false }
+}
+
+define {i8, i1} @uadd_2() nounwind {
+entry:
+  %t = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 142, i8 120)
+  ret {i8, i1} %t
+
+; CHECK: @uadd_2
+; CHECK: ret %i8i1 { i8 6, i1 true }
+}
+
+
+;;-----------------------------
+;; usub
+;;-----------------------------
+
+define {i8, i1} @usub_1() nounwind {
+entry:
+  %t = call {i8, i1} @llvm.usub.with.overflow.i8(i8 4, i8 2)
+  ret {i8, i1} %t
+
+; CHECK: @usub_1
+; CHECK: ret %i8i1 { i8 2, i1 false }
+}
+
+define {i8, i1} @usub_2() nounwind {
+entry:
+  %t = call {i8, i1} @llvm.usub.with.overflow.i8(i8 4, i8 6)
+  ret {i8, i1} %t
+
+; CHECK: @usub_2
+; CHECK: ret %i8i1 { i8 -2, i1 true }
+}
+
+
+
+declare {i8, i1} @llvm.uadd.with.overflow.i8(i8, i8)
+declare {i8, i1} @llvm.usub.with.overflow.i8(i8, i8)
-- 
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