[InstCombine] Extra combine for uadd_sat

This is an extra fold for a canonical form of uadd_sat, as shown in
D68651. It essentially selects uadd from an add and a select.

Differential Revision: https://reviews.llvm.org/D69244

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 9fc871e..b06d31a 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -781,6 +781,13 @@ static Value *canonicalizeSaturatedAdd(ICmpInst *Cmp, Value *TVal, Value *FVal,
     return Builder.CreateBinaryIntrinsic(
         Intrinsic::uadd_sat, BO->getOperand(0), BO->getOperand(1));
   }
+  // The overflow may be detected via the add wrapping round.
+  if (match(Cmp0, m_c_Add(m_Specific(Cmp1), m_Value(Y))) &&
+      match(FVal, m_c_Add(m_Specific(Cmp1), m_Specific(Y)))) {
+    // ((X + Y) u< X) ? -1 : (X + Y) --> uadd.sat(X, Y)
+    // ((X + Y) u< Y) ? -1 : (X + Y) --> uadd.sat(X, Y)
+    return Builder.CreateBinaryIntrinsic(Intrinsic::uadd_sat, Cmp1, Y);
+  }
 
   return nullptr;
 }

diff --git a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
index 0623207..57ef751 100644 (file)
--- a/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
+++ b/llvm/test/Transforms/InstCombine/saturating-add-sub.ll
@@ -1486,10 +1486,8 @@ define i32 @uadd_sat_constant_commute(i32 %x) {
 
 define i32 @uadd_sat_canon(i32 %x, i32 %y) {
 ; CHECK-LABEL: @uadd_sat_canon(
-; CHECK-NEXT:    [[A:%.*]] = add i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[A]], [[X]]
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[C]], i32 -1, i32 [[A]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[X:%.*]], i32 [[Y:%.*]])
+; CHECK-NEXT:    ret i32 [[TMP1]]
 ;
   %a = add i32 %x, %y
   %c = icmp ult i32 %a, %x
@@ -1499,10 +1497,8 @@ define i32 @uadd_sat_canon(i32 %x, i32 %y) {
 
 define i32 @uadd_sat_canon_y(i32 %x, i32 %y) {
 ; CHECK-LABEL: @uadd_sat_canon_y(
-; CHECK-NEXT:    [[A:%.*]] = add i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[A]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[C]], i32 -1, i32 [[A]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @llvm.uadd.sat.i32(i32 [[Y:%.*]], i32 [[X:%.*]])
+; CHECK-NEXT:    ret i32 [[TMP1]]
 ;
   %a = add i32 %x, %y
   %c = icmp ult i32 %a, %y