[InstCombine] Fold the bound check idiom into sign bit test

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2641,6 +2641,26 @@ Instruction *InstCombinerImpl::foldICmpDivConstant(ICmpInst &Cmp,
   if (!match(Y, m_APInt(C2)))
     return nullptr;
 
+  // Fold the bound check idiom:
+  //  T *end, *start;
+  //  (size_t)(end - start) > (size_t)(PTRDIFF_MAX / sizeof(T))
+  // into:
+  //  (ptrdiff_t)(end - start) < 0
+  // i.e.:
+  //   icmp ugt (sdiv exact X, C2), (sdiv signed_max, C2) --> icmp slt X, 0
+  //   icmp ult (sdiv exact X, C2), (sdiv signed_max, C2) + 1 --> icmp sgt X, -1
+  // where C2 is positive.
+  if (DivIsSigned && Div->isExact() &&
+      (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULT) &&
+      C2->isStrictlyPositive() &&
+      APInt::getSignedMaxValue(C2->getBitWidth()).sdiv(*C2) +
+              APInt(C2->getBitWidth(), Pred == ICmpInst::ICMP_UGT ? 0 : 1) ==
+          C)
+    return new ICmpInst(
+        Pred == ICmpInst::ICMP_UGT ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_SGT, X,
+        Pred == ICmpInst::ICMP_UGT ? Constant::getNullValue(X->getType())
+                                   : Constant::getAllOnesValue(X->getType()));
+
   // FIXME: If the operand types don't match the type of the divide
   // then don't attempt this transform. The code below doesn't have the
   // logic to deal with a signed divide and an unsigned compare (and

llvm/test/Transforms/InstCombine/icmp.ll

@@ -5140,8 +5140,7 @@ entry:
 
 define i1 @icmp_ugt_sdiv_by_constant(i64 %x) {
 ; CHECK-LABEL: @icmp_ugt_sdiv_by_constant(
-; CHECK-NEXT:    [[SDIV:%.*]] = sdiv exact i64 [[X:%.*]], 24
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i64 [[SDIV]], 384307168202282325
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %sdiv = sdiv exact i64 %x, 24
@@ -5151,8 +5150,7 @@ define i1 @icmp_ugt_sdiv_by_constant(i64 %x) {
 
 define i1 @icmp_ult_sdiv_by_constant(i64 %x) {
 ; CHECK-LABEL: @icmp_ult_sdiv_by_constant(
-; CHECK-NEXT:    [[SDIV:%.*]] = sdiv exact i64 [[X:%.*]], 24
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i64 [[SDIV]], 384307168202282326
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[X:%.*]], -1
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %sdiv = sdiv exact i64 %x, 24