Fix versioning test against final index value for BNDCHK

[jdmpapin]

Previously this versioning test would do either a strict or non-strict comparison depending on a few factors:

• isLoopDrivingInductionVariable,
• isIndexChildMultiplied, and
• the opcode of _loopTestTree (without consulting reverseBranch).

To deal with all of this variation there were multiple ad hoc adjustments of +1 or -1 at different points in the determination of maxValue (which is really supposed to be the last value).

This state of affairs led to bugs, notably when using a non-strict loop test together with an index expression that contains a multiplication, but also in other scenarios that I have not precisely characterized.

Now this versioning test is always (ifiucmpge maxValue arrayLength) for increasing index expressions and (ificmplt maxValue 0) for decreasing ones, with no regard to isLoopDrivingInductionVariable, etc., and the corresponding ad hoc adjustments are eliminated from maxValue. Rather than always take the ceiling and try to compensate for non-strict loop tests after the fact, we now use the floor in the case of non-strict loop tests and (ceiling - 1) otherwise, as described in the comment in the extremum versioning path in buildConditionalTree(). (Note though that instead of subtracting 1 before multiplying by incrJNode, this change subtracts additiveConstantInStore afterward, which has the same effect.)

Additionally, if the index expression was based on a derived IV and involved a substitution via isDependentOnInductionVariable(), then this versioning test would load the temporary that's defined within the loop instead of the initial value of the IV. The temp might be uninitialized, resulting in a versioning test that can non-deterministically pass even if the bound check should fail. It's not much better either if the temp happens to be initialized, since its value won't have come from the definition within the loop and is most likely entirely unrelated to the initial value of the IV. This commit changes the versioning test to load the derived IV instead of the temporary.

Finally, it was also possible for a later versioning test to load an uninitialized temporary when trying to rule out overflow in a multiplication that appears in the index expression. To prevent this, versioning is now disallowed if it would be necessary to substitute beneath a multiplication.

With these changes the versioning test is still not precise, which I believe is due to a failure to account properly for different possible orderings of operations in the loop and possibly also to account for the difference between the initial value of a derived IV and the value of a temp based on it in the first iteration (e.g. tmp=iv+1).

However, it appears to be conservative. I have checked a large number of generated test cases with different combinations of features:

• Do-while loops vs while loops (reverseBranch)
• Strict vs. non-strict loop conditions
• Increasing primary IV (with <, <= comparisons) vs. decreasing (>, >=)
• Accessing every element vs. every other element
• Getting every other element by stepping by 2 vs. multiplying by 2
• Even vs. odd array length (when accessing every other element)
• Accessing even vs. odd indices (when accessing every other element)
• Accessing elements in increasing vs. decreasing order of index
• Indexing based on primary vs. derived IV
• Stepping the primary IV by -3, -1, +1, or +3 (if using a derived IV)
• Adding an offset in the index expression vs. not adding one
• Adding an offset before multiplying vs. afterward (if multiplying)
• Using a temp (isDependentOnInductionVariable()) vs. no temp
• Setting the temp to the IV or to the index (if using a temp)
• Different initial values for the IV(s), as consistent with the above
• Every usable permutation of the statements in the loop body

In every case I had a script determine the widest loop bound that avoids failing a bound check. I ran each test case three times:

• once with the precise bounds, which should not fail a bound check;
• once with the initial value modified to cause a bound check to fail on the first iteration; and
• once with the loop bound modified to do an extra iteration at the end and fail a bound check in the last iteration.

With this commit, all such test executions behaved as expected. In some cases the bound check was not versioned or execution went unnecessarily to the cold loop, but in those cases one of the following always held before this commit:

• we already did the same thing, or
• the test passed all bound checks when it should have failed one, or
• there was a load of an uninitialized variable in the versioning test.

Most loops in real programs compute the index based on the primary IV, which appears directly in the index expression, i.e. array[...i...]. For these loops as they occur in the test cases described above, the only effect of this change is to correctly run bound checks when one should fail and to sometimes avoid going unnecessarily to the cold loop.

[vijaysun-omr]

jenkins build all

[jdmpapin]

The macOS failure is #7181

[vijaysun-omr]

Tests have passed except the known macOS failures.