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JIT: eliminate redundant bounds checks for span.Slice(span.Length - cns) #127488

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Merged
EgorBo merged 2 commits into from
Apr 28, 2026
+55 −3
Merged

JIT: eliminate redundant bounds checks for span.Slice(span.Length - cns) #127488

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490ce9b
JIT: eliminate redundant bounds checks for span.Slice(span.Length - cns)
Copilot Apr 28, 2026
ca3fb5b
Reuse r2 instead of recomputing range for addOpVN
Copilot Apr 28, 2026
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36 changes: 36 additions & 0 deletions src/coreclr/jit/rangecheck.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -845,6 +845,42 @@ Range RangeCheck::GetRangeFromAssertionsWorker(
}

result = RangeOps::EvalRelop(cmpOper, isUnsigned, r1, r2);

// Example: "(uint)(length - 4) > (uint)length" folds to false when
// length >= 4 (the typical Slice(length - cns) bounds check).
if (!result.IsSingleValueConstant())
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{
ValueNum op1VN = funcApp.m_args[0];
ValueNum op2VN = funcApp.m_args[1];
ValueNum addOpVN;
int addCns;

if (comp->vnStore->IsVNBinFuncWithConst(op1VN, VNF_ADD, &addOpVN, &addCns) &&
(addOpVN == op2VN) && (addCns < 0) && (addCns > INT32_MIN))
{
if (r2.LowerLimit().IsConstant() && (r2.LowerLimit().GetConstant() >= -addCns))
{
// ADD(A, K) < A is proven (both signed and unsigned).
switch (cmpOper)
{
case GT_LT:
case GT_LE:
case GT_NE:
result = Range(Limit(Limit::keConstant, 1));
break;

case GT_GT:
case GT_GE:
case GT_EQ:
result = Range(Limit(Limit::keConstant, 0));
break;

default:
break;
}
}
}
}
}
break;
}
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22 changes: 19 additions & 3 deletions src/coreclr/jit/valuenum.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5372,10 +5372,26 @@ ValueNum ValueNumStore::EvalUsingMathIdentity(var_types typ, VNFunc func, ValueN

if (!ovf)
{
// x - (x + a) == -a
// x - (a + x) == -a
ValueNum arg1Op1;
ValueNum arg1Op2;
if (IsVNBinFunc(arg1VN, VNF_ADD, &arg1Op1, &arg1Op2))
{
if (arg1Op1 == arg0VN)
{
return VNForFunc(typ, VNF_NEG, arg1Op2);
}
if (arg1Op2 == arg0VN)
{
return VNForFunc(typ, VNF_NEG, arg1Op1);
}
}

// (x + a) - x == a
// (a + x) - x == a
VNFuncApp add;
if (GetVNFunc(arg0VN, &add) && (add.m_func == (VNFunc)GT_ADD))
if (GetVNFunc(arg0VN, &add) && (add.m_func == VNF_ADD))
{
if (add.m_args[0] == arg1VN)
return add.m_args[1];
Expand All @@ -5387,15 +5403,15 @@ ValueNum ValueNumStore::EvalUsingMathIdentity(var_types typ, VNFunc func, ValueN
// (x + a) - (b + x) == a - b
// (a + x) - (b + x) == a - b
VNFuncApp add2;
if (GetVNFunc(arg1VN, &add2) && (add2.m_func == (VNFunc)GT_ADD))
if (GetVNFunc(arg1VN, &add2) && (add2.m_func == VNF_ADD))
{
for (int a = 0; a < 2; a++)
{
for (int b = 0; b < 2; b++)
{
if (add.m_args[a] == add2.m_args[b])
{
return VNForFunc(typ, (VNFunc)GT_SUB, add.m_args[1 - a], add2.m_args[1 - b]);
return VNForFunc(typ, VNF_SUB, add.m_args[1 - a], add2.m_args[1 - b]);
}
}
}
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