[ConstraintElimination] Decompose zext for unsigned compares.

For unsigned compares, zext should be a no-op and we can add the
extended value to the constraint system.

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -79,6 +79,9 @@ static SmallVector<std::pair<int64_t, Value *>, 4> decompose(Value *V) {
   }
 
   Value *Op0;
+  if (match(V, m_ZExt(m_Value(Op0))))
+    V = Op0;
+
   Value *Op1;
   ConstantInt *CI;
   if (match(V, m_NUWAdd(m_Value(Op0), m_ConstantInt(CI))))

llvm/test/Transforms/ConstraintElimination/gep-arithmetic.ll

@@ -803,7 +803,7 @@ define i4 @ptr_N_step_zext_n_zext(i8* %src, i8* %lower, i8* %upper, i16 %N, i16
 ; CHECK-NEXT:    [[SRC_STEP:%.*]] = getelementptr inbounds i8, i8* [[SRC]], i32 [[STEP_ADD_1_EXT]]
 ; CHECK-NEXT:    [[CMP_STEP_START:%.*]] = icmp ult i8* [[SRC_STEP]], [[LOWER]]
 ; CHECK-NEXT:    [[CMP_STEP_END:%.*]] = icmp uge i8* [[SRC_STEP]], [[UPPER]]
-; CHECK-NEXT:    [[OR_CHECK:%.*]] = or i1 [[CMP_STEP_START]], false
+; CHECK-NEXT:    [[OR_CHECK:%.*]] = or i1 [[CMP_STEP_START]], [[CMP_STEP_END]]
 ; CHECK-NEXT:    br i1 [[OR_CHECK]], label [[TRAP_BB]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret i4 3

llvm/test/Transforms/ConstraintElimination/zext.ll

@@ -67,7 +67,7 @@ define void @uge_compare_short_and_extended(i16 %x, i16 %y) {
 ; CHECK-NEXT:    br i1 [[C_1]], label [[BB1:%.*]], label [[BB2:%.*]]
 ; CHECK:       bb1:
 ; CHECK-NEXT:    [[T_1:%.*]] = icmp uge i32 [[X_EXT]], [[Y_EXT]]
-; CHECK-NEXT:    call void @use(i1 [[T_1]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp uge i32 [[X_EXT]], 10
 ; CHECK-NEXT:    call void @use(i1 [[C_2]])
 ; CHECK-NEXT:    [[C_3:%.*]] = icmp sge i32 [[Y_EXT]], [[X_EXT]]
@@ -77,9 +77,9 @@ define void @uge_compare_short_and_extended(i16 %x, i16 %y) {
 ; CHECK-NEXT:    ret void
 ; CHECK:       bb2:
 ; CHECK-NEXT:    [[T_2:%.*]] = icmp uge i32 [[Y_EXT]], [[X_EXT]]
-; CHECK-NEXT:    call void @use(i1 [[T_2]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[F_1:%.*]] = icmp uge i32 [[X_EXT]], [[Y_EXT]]
-; CHECK-NEXT:    call void @use(i1 [[F_1]])
+; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    [[C_5:%.*]] = icmp uge i32 [[X_EXT]], 10
 ; CHECK-NEXT:    call void @use(i1 [[C_5]])
 ; CHECK-NEXT:    [[C_6:%.*]] = icmp uge i32 10, [[X_EXT]]
@@ -135,9 +135,9 @@ define void @uge_zext_add(i16 %x, i32 %y) {
 ; CHECK-NEXT:    ret void
 ; CHECK:       bb2:
 ; CHECK-NEXT:    [[T_2:%.*]] = icmp uge i32 [[Y]], [[X_EXT]]
-; CHECK-NEXT:    call void @use(i1 [[T_2]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[F_1:%.*]] = icmp uge i32 [[X_EXT]], [[Y]]
-; CHECK-NEXT:    call void @use(i1 [[F_1]])
+; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    [[C_5:%.*]] = icmp uge i32 [[X_EXT]], 10
 ; CHECK-NEXT:    call void @use(i1 [[C_5]])
 ; CHECK-NEXT:    [[C_6:%.*]] = icmp uge i32 10, [[X_EXT]]