recognize an unsigned add with overflow idiom into uadd.

This resolves a README entry and technically resolves PR4916,
but we still get poor code for the testcase in that PR because
GVN isn't CSE'ing uadd with add, filed as PR8817.

Previously we got:

_test7:                                 ## @test7
	addq	%rsi, %rdi
	cmpq	%rdi, %rsi
	movl	$42, %eax
	cmovaq	%rsi, %rax
	ret

Now we get:

_test7:                                 ## @test7
	addq	%rsi, %rdi
	movl	$42, %eax
	cmovbq	%rsi, %rax
	ret

llvm-svn: 122182

llvm/lib/Target/README.txt

@@ -74,26 +74,7 @@ This has a number of uses:
 //===---------------------------------------------------------------------===//
 
 We should recognized various "overflow detection" idioms and translate them into
-llvm.uadd.with.overflow and similar intrinsics.  For example, we compile this:
-
-size_t add(size_t a,size_t b) {
- if (a+b<a)
-   exit(0);
- return a+b;
-}
-
-into:
-
-	addq	%rdi, %rbx
-	cmpq	%rdi, %rbx
-	jae	LBB0_2
-
-when it would be better to generate:
-
-	addq	%rdi, %rbx
-	jno	LBB0_2
-
-Apparently some version of GCC knows this.  Here is a multiply idiom:
+llvm.uadd.with.overflow and similar intrinsics.  Here is a multiply idiom:
 
 unsigned int mul(unsigned int a,unsigned int b) {
  if ((unsigned long long)a*b>0xffffffff)

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -1648,7 +1648,7 @@ static Instruction *ProcessUGT_ADDCST_ADD(ICmpInst &I, Value *A, Value *B,
   
   // Put the new code above the original add, in case there are any uses of the
   // add between the add and the compare.
-  Builder->SetInsertPoint(OrigAdd->getParent(), BasicBlock::iterator(OrigAdd));
+  Builder->SetInsertPoint(OrigAdd);
   
   Value *TruncA = Builder->CreateTrunc(A, NewType, A->getName()+".trunc");
   Value *TruncB = Builder->CreateTrunc(B, NewType, B->getName()+".trunc");
@@ -1664,6 +1664,35 @@ static Instruction *ProcessUGT_ADDCST_ADD(ICmpInst &I, Value *A, Value *B,
   return ExtractValueInst::Create(Call, 1, "sadd.overflow");
 }
 
+static Instruction *ProcessUAddIdiom(Instruction &I, Value *OrigAddV,
+                                     InstCombiner &IC) {
+  // Don't bother doing this transformation for pointers, don't do it for
+  // vectors.
+  if (!isa<IntegerType>(OrigAddV->getType())) return 0;
+  
+  // If the add is a constant expr, then we don't bother transforming it.
+  Instruction *OrigAdd = dyn_cast<Instruction>(OrigAddV);
+  if (OrigAdd == 0) return 0;
+  
+  Value *LHS = OrigAdd->getOperand(0), *RHS = OrigAdd->getOperand(1);
+  
+  // Put the new code above the original add, in case there are any uses of the
+  // add between the add and the compare.
+  InstCombiner::BuilderTy *Builder = IC.Builder;
+  Builder->SetInsertPoint(OrigAdd);
+
+  Module *M = I.getParent()->getParent()->getParent();
+  const Type *Ty = LHS->getType();
+  Value *F = Intrinsic::getDeclaration(M, Intrinsic::uadd_with_overflow, &Ty,1);
+  CallInst *Call = Builder->CreateCall2(F, LHS, RHS, "uadd");
+  Value *Add = Builder->CreateExtractValue(Call, 0);
+
+  IC.ReplaceInstUsesWith(*OrigAdd, Add);
+
+  // The original icmp gets replaced with the overflow value.
+  return ExtractValueInst::Create(Call, 1, "uadd.overflow");
+}
+
 
 Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   bool Changed = false;
@@ -1726,10 +1755,10 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   }
 
   unsigned BitWidth = 0;
-  if (TD)
-    BitWidth = TD->getTypeSizeInBits(Ty->getScalarType());
-  else if (Ty->isIntOrIntVectorTy())
+  if (Ty->isIntOrIntVectorTy())
     BitWidth = Ty->getScalarSizeInBits();
+  else if (TD)  // Pointers require TD info to get their size.
+    BitWidth = TD->getTypeSizeInBits(Ty->getScalarType());
   
   bool isSignBit = false;
 
@@ -2225,6 +2254,22 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
     if (match(Op0, m_Not(m_Value(A))) &&
         match(Op1, m_Not(m_Value(B))))
       return new ICmpInst(I.getPredicate(), B, A);
+
+    // (a+b) <u a  --> llvm.uadd.with.overflow.
+    // (a+b) <u b  --> llvm.uadd.with.overflow.
+    if (I.getPredicate() == ICmpInst::ICMP_ULT &&
+        match(Op0, m_Add(m_Value(A), m_Value(B))) && 
+        (Op1 == A || Op1 == B))
+      if (Instruction *R = ProcessUAddIdiom(I, Op0, *this))
+        return R;
+                                 
+    // a >u (a+b)  --> llvm.uadd.with.overflow.
+    // b >u (a+b)  --> llvm.uadd.with.overflow.
+    if (I.getPredicate() == ICmpInst::ICMP_UGT &&
+        match(Op1, m_Add(m_Value(A), m_Value(B))) &&
+        (Op0 == A || Op0 == B))
+      if (Instruction *R = ProcessUAddIdiom(I, Op1, *this))
+        return R;
   }
   
   if (I.isEquality()) {

llvm/test/Transforms/InstCombine/overflow.ll

@@ -97,3 +97,37 @@ if.end:                                           ; preds = %entry
 ; CHECK: ret i8
 }
 
+; CHECK: @test5
+; CHECK: llvm.uadd.with.overflow
+; CHECK: ret i64
+define i64 @test5(i64 %a, i64 %b) nounwind ssp {
+entry:
+  %add = add i64 %b, %a
+  %cmp = icmp ult i64 %add, %a
+  %Q = select i1 %cmp, i64 %b, i64 42
+  ret i64 %Q
+}
+
+; CHECK: @test6
+; CHECK: llvm.uadd.with.overflow
+; CHECK: ret i64
+define i64 @test6(i64 %a, i64 %b) nounwind ssp {
+entry:
+  %add = add i64 %b, %a
+  %cmp = icmp ult i64 %add, %b
+  %Q = select i1 %cmp, i64 %b, i64 42
+  ret i64 %Q
+}
+
+; CHECK: @test7
+; CHECK: llvm.uadd.with.overflow
+; CHECK: ret i64
+define i64 @test7(i64 %a, i64 %b) nounwind ssp {
+entry:
+  %add = add i64 %b, %a
+  %cmp = icmp ugt i64 %b, %add
+  %Q = select i1 %cmp, i64 %b, i64 42
+  ret i64 %Q
+}
+
+