[ValueTracking] Infer relationship for the select with ICmp

x -nsw y < -C is false when x > y and C >= 0
Alive2 proof for sgt, sge : https://alive2.llvm.org/ce/z/tupvfi
Note: It only really makes sense in the context of signed comparison for
      "X - Y must be positive if X >= Y and no overflow".

Fixes #54735

Author: vfdff

llvm/lib/Analysis/ValueTracking.cpp

@@ -9108,6 +9108,17 @@ static std::optional<bool> isImpliedCondICmps(const ICmpInst *LHS,
   if (L0 == R0 && L1 == R1)
     return isImpliedCondMatchingOperands(LPred, RPred);
 
+  // It only really makes sense in the context of signed comparison for "X - Y
+  // must be positive if X >= Y and no overflow".
+  // Take SGT as an example:  L0:x > L1:y and C >= 0
+  //                      ==> R0:(x -nsw y) < R1:(-C) is false
+  if ((LPred == ICmpInst::ICMP_SGT || LPred == ICmpInst::ICMP_SGE) &&
+      match(R0, m_NSWSub(m_Specific(L0), m_Specific(L1)))) {
+    if (match(R1, m_NonPositive()) &&
+        isImpliedCondMatchingOperands(LPred, RPred) == false)
+      return false;
+  }
+
   // L0 = R0 = L1 + R1, L0 >=u L1 implies R0 >=u R1, L0 <u L1 implies R0 <u R1
   if (L0 == R0 &&
       (LPred == ICmpInst::ICMP_ULT || LPred == ICmpInst::ICMP_UGE) &&

llvm/test/Transforms/InstSimplify/select-icmp.ll

@@ -42,10 +42,7 @@ define i32 @pr54735_sgt(i32 %x, i32 %y) {
 ; CHECK:       cond.true:
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 [[X]], [[Y]]
 ; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[SUB]], 1
-; CHECK-NEXT:    [[NEG:%.*]] = xor i32 [[SUB]], -1
-; CHECK-NEXT:    [[ABSCOND:%.*]] = icmp slt i32 [[SUB]], -1
-; CHECK-NEXT:    [[ABS:%.*]] = select i1 [[ABSCOND]], i32 [[NEG]], i32 [[ADD]]
-; CHECK-NEXT:    ret i32 [[ABS]]
+; CHECK-NEXT:    ret i32 [[ADD]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    ret i32 0
 ;
@@ -74,10 +71,7 @@ define i32 @pr54735_sge(i32 %x, i32 %y) {
 ; CHECK:       cond.true:
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 [[X]], [[Y]]
 ; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[SUB]], 1
-; CHECK-NEXT:    [[NEG:%.*]] = xor i32 [[SUB]], -1
-; CHECK-NEXT:    [[ABSCOND:%.*]] = icmp slt i32 [[SUB]], -1
-; CHECK-NEXT:    [[ABS:%.*]] = select i1 [[ABSCOND]], i32 [[NEG]], i32 [[ADD]]
-; CHECK-NEXT:    ret i32 [[ABS]]
+; CHECK-NEXT:    ret i32 [[ADD]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    ret i32 0
 ;