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Update documentation for Node::Ideal

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This commit is contained in:
Benoît Maillard 2026-01-27 11:22:05 +01:00
parent d0e27f15f4
commit 418a5f7ea2
1 changed files with 8 additions and 4 deletions
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12
src/hotspot/share/opto/node.cpp
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@ -1151,15 +1151,19 @@ const Type* Node::Value(PhaseGVN* phase) const {
// 'Idealize' the graph rooted at this Node.
//
// In order to be efficient and flexible there are some subtle invariants
// these Ideal calls need to hold. Running with '-XX:VerifyIterativeGVN=1' checks
// these invariants, although its too slow to have on by default. If you are
// hacking an Ideal call, be sure to test with '-XX:VerifyIterativeGVN=1'
// these Ideal calls need to hold. Some of the flag bits for '-XX:VerifyIterativeGVN'
// can help with validating these invariants, although they are too slow to have on by default:
// - '-XX:VerifyIterativeGVN=1' checks the def-use info
// - '-XX:VerifyIterativeGVN=100000' cheks the return value
// If you are hacking an Ideal call, be sure to use these.
//
// The Ideal call almost arbitrarily reshape the graph rooted at the 'this'
// pointer. If ANY change is made, it must return the root of the reshaped
// graph - even if the root is the same Node. Example: swapping the inputs
// to an AddINode gives the same answer and same root, but you still have to
// return the 'this' pointer instead of null.
// return the 'this' pointer instead of null. If the node was already dead
// before the Ideal call, this rule does not apply, and it is fine to return
// nullptr even if modifications were made.
//
// You cannot return an OLD Node, except for the 'this' pointer. Use the
// Identity call to return an old Node; basically if Identity can find