[InstCombine] Combine adds across a zext

We can perform the following:
(add (zext (add nuw X, C1)), C2) -> (zext (add nuw X, C1+C2))

This is only possible if C2 is negative and C2 is greater than or equal to negative C1.

llvm-svn: 290927

2 files changed, 24 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 3bbc70ab21c..55151c13b43 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1057,6 +1057,18 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
       // add(zext(xor i16 X, -32768), -32768) --> sext X
       return CastInst::Create(Instruction::SExt, X, LHS->getType());
     }
+
+    if (Val->isNegative() &&
+        match(LHS, m_ZExt(m_NUWAdd(m_Value(X), m_APInt(C)))) &&
+        Val->sge(-C->sext(Val->getBitWidth()))) {
+      // (add (zext (add nuw X, C)), Val) -> (zext (add nuw X, C+Val))
+      return CastInst::Create(
+          Instruction::ZExt,
+          Builder->CreateNUWAdd(
+              X, Constant::getIntegerValue(X->getType(),
+                                           *C + Val->trunc(C->getBitWidth()))),
+          I.getType());
+    }
   }
 
   // FIXME: Use the match above instead of dyn_cast to allow these transforms
diff --git a/llvm/test/Transforms/InstCombine/add.ll b/llvm/test/Transforms/InstCombine/add.ll
index 7c46257273a..39a746ab310 100644
--- a/llvm/test/Transforms/InstCombine/add.ll
+++ b/llvm/test/Transforms/InstCombine/add.ll
@@ -507,3 +507,15 @@ define i1 @test40(i32 %a, i32 %b) {
   %cmp = icmp eq i32 %add, %b
   ret i1 %cmp
 }
+
+define i64 @test41(i32 %a) {
+; CHECK-LABEL: @test41(
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw i32 %a, 15
+; CHECK-NEXT:    [[EXT:%.*]] = zext i32 [[ADD]] to i64
+; CHECK-NEXT:    ret i64 [[EXT]]
+;
+  %add = add nuw i32 %a, 16
+  %zext = zext i32 %add to i64
+  %sub = add i64 %zext, -1
+  ret i64 %sub
+}