RFR: 8288564: C2: LShiftLNode::Ideal produces wrong result after JDK-8278114

[Christian Hagedorn]

[JDK-8278114](https://bugs.openjdk.org/browse/JDK-8278114) added the following transformation for integer and long left shifts:

 "(x + x) << c0" into "x << (c0 + 1)"

However, in the long shift case, this transformation is not correct if `c0` is 63:


(x + x) << 63 = 2x << 63

while

 (x + x) << 63 --transform--> x << 64 = x << 0 = x

which is not the same. For example, if `x = 1`:

2x << 63 = 2 << 63 = 0 != 1

This optimization does not account for the fact that `x << 64` is the same as `x << 0 = x`. According to the [Java spec, chapter 15.19](https://docs.oracle.com/javase/specs/jls/se18/html/jls-15.html#jls-15.19), we only consider the six lowest-order bits of the right-hand operand (i.e. `"right-hand operand" & 0b111111`). Therefore, `x << 64` is the same as `x << 0` (`64 = 0b10000000 & 0b0111111 = 0`).

Integer shifts are not affected because we do not apply this transformation if `c0 >= 16`:

https://urldefense.com/v3/__https://github.com/openjdk/jdk19/blob/729164f53499f146579a48ba1b466c687802f330/src/hotspot/share/opto/mulnode.cpp*L810-L817__;Iw!!ACWV5N9M2RV99hQ!LL92wmUr0_HnDM9rLcUGohhst3Y6sLwdo76BqHJFrjE2yu5qOStHEwkn-0k38Nnu060piUV6jynRETcqXMBPAM-DgE9aCRzEIA$ 

The fix I propose is to not apply this optimization for long left shifts if `c0 == 63`. I've added an additional sanity assertion for integer left shifts just in case this optimization is moved at some point and ending up outside the check for `con < 16`.

Thanks,
Christian

-------------

Commit messages:
 - 8288564: C2: LShiftLNode::Ideal produces wrong result after JDK-8278114

Changes: https://git.openjdk.org/jdk19/pull/29/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=29&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288564
  Stats: 38 lines in 2 files changed: 36 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk19/pull/29.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/29/head:pull/29

PR: https://git.openjdk.org/jdk19/pull/29