[ConstraintElim] Look through SExt with precond Op sge 0.

Look through SExt with a precondition that the operand is signed
positive.

https://alive2.llvm.org/ce/z/zvVVHj

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -554,6 +554,12 @@ static Decomposition decompose(Value *V,
     V = Op0;
   }
 
+  if (match(V, m_SExt(m_Value(Op0)))) {
+    V = Op0;
+    Preconditions.emplace_back(CmpInst::ICMP_SGE, Op0,
+                               ConstantInt::get(Op0->getType(), 0));
+  }
+
   Value *Op1;
   ConstantInt *CI;
   if (match(V, m_NUWAdd(m_Value(Op0), m_Value(Op1)))) {

llvm/test/Transforms/ConstraintElimination/sext-unsigned-predicates.ll

@@ -13,7 +13,8 @@ define void @uge_sext(i16 %x, i32 %y) {
 ; CHECK-NEXT:    [[AND:%.*]] = and i1 [[C_1]], [[C_2]]
 ; CHECK-NEXT:    br i1 [[AND]], label [[BB1:%.*]], label [[BB2:%.*]]
 ; CHECK:       bb1:
-; CHECK-NEXT:    call void @use(i1 true)
+; CHECK-NEXT:    [[T_1:%.*]] = icmp uge i32 [[X_EXT]], [[Y]]
+; CHECK-NEXT:    call void @use(i1 [[T_1]])
 ; CHECK-NEXT:    [[C_3:%.*]] = icmp uge i16 [[X]], -10
 ; CHECK-NEXT:    call void @use(i1 [[C_3]])
 ; CHECK-NEXT:    [[C_4:%.*]] = icmp uge i32 [[X_EXT]], -9
@@ -65,8 +66,7 @@ define void @uge_sext_known_positive(i16 %x, i32 %y) {
 ; CHECK-NEXT:    br i1 [[AND]], label [[BB1:%.*]], label [[BB2:%.*]]
 ; CHECK:       bb1:
 ; CHECK-NEXT:    call void @use(i1 true)
-; CHECK-NEXT:    [[T_2:%.*]] = icmp uge i16 [[X]], 10
-; CHECK-NEXT:    call void @use(i1 [[T_2]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[C_3:%.*]] = icmp uge i32 [[X_EXT]], 11
 ; CHECK-NEXT:    call void @use(i1 [[C_3]])
 ; CHECK-NEXT:    [[C_4:%.*]] = icmp uge i32 [[X_EXT]], 11

llvm/test/Transforms/ConstraintElimination/transfer-signed-facts-to-unsigned.ll

@@ -503,11 +503,9 @@ define i32 @sge_2_gep(i32 %idx, ptr %src, i32 %idx.2) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp sge i32 [[IDX]], 2
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[CMP]])
 ; CHECK-NEXT:    [[ADD_PTR_2:%.*]] = getelementptr inbounds i32, ptr [[SRC]], i32 [[IDX_2:%.*]]
-; CHECK-NEXT:    [[T_1:%.*]] = icmp ult ptr [[SRC]], [[ADD_PTR]]
 ; CHECK-NEXT:    [[C_1:%.*]] = icmp ult ptr [[SRC]], [[ADD_PTR_2]]
-; CHECK-NEXT:    [[X_1:%.*]] = xor i1 [[T_1]], [[C_1]]
-; CHECK-NEXT:    [[F_1:%.*]] = icmp uge ptr [[SRC]], [[ADD_PTR]]
-; CHECK-NEXT:    [[X_2:%.*]] = xor i1 [[X_1]], [[F_1]]
+; CHECK-NEXT:    [[X_1:%.*]] = xor i1 true, [[C_1]]
+; CHECK-NEXT:    [[X_2:%.*]] = xor i1 [[X_1]], false
 ; CHECK-NEXT:    br i1 [[X_2]], label [[THEN:%.*]], label [[ELSE:%.*]]
 ; CHECK:       then:
 ; CHECK-NEXT:    ret i32 0