RFR: 8349479: C2: when a Type node becomes dead, make CFG path that uses it unreachable [v4]

[Roland Westrelin]

> This is primarily motivated by 8275202 (C2: optimize out more
> redundant conditions). In the following code snippet:
> 
> 
> int[] array = new int[arraySize];
> if (j <= arraySize) {
>   if (i >= 0) {
>     if (i < j) {
>       int v = array[i]; 
> 
> 
> (`arraySize` is a constant)
> 
> at the range check, `j` is known to be in `[min, arraySize]` as a
> consequence, `i` is known to be `[0, arraySize-1]`. The range check
> can be eliminated.
> 
> Now, if later, `i` constant folds to some value that's positive but
> out of range for the array:
> 
> - if that happens when the new pass runs, then it can prove that:
> 
>     if (i < j) {
> 
> is never taken.
> 
> - if that happens during IGVN or CCP however, that condition is not
>   constant folded. And because the range check was removed, there's no
>   guard protecting the range check `CastII`. It becomes `top` and, as
>   a result, the graph can become broken.
>   
> What I propose here is that when the `CastII` becomes dead, any CFG
> paths that use the `CastII` node is made unreachable. So in pseudo code:
> 
> 
> int[] array = new int[arraySize];
> if (j <= arraySize) {
>   if (i >= 0) {
>     if (i < j) {
>       halt();
> 
> 
> Finding the CFG paths is implemented in the patch by following the
> uses of the node until a CFG node or a `Phi` is encountered.
> 
> The patch applies this to all `Type` nodes as with 8275202, I also ran
> in some rare corner cases with other types of nodes. The exception is
> `Phi` nodes which may not be as easy to handle (and for which I had no
> issue with 8275202).
> 
> Finally, the patch includes a test case that's unrelated to the
> discussion of 8275202 above. In that test case, a `CastII` becomes top
> but the test that guards it doesn't constant fold. The root cause is a
> transformation of:
> 
> 
> (CastII (AddI
> 
> 
> into 
> 
> 
> (AddI (CastII ) (CastII)`
> 
> 
> which causes the resulting node to have a wider type. The `CastII`
> captures a type before the transformation above happens. Once it has
> happened, the guard for the `CastII` can't be constant folded when an
> out of bound value occurs.
> 
> This is likely fixable some other way (eventhough it doesn't seem
> straightforward). Given the long history of similar issues (and the
> test case that shows that they are more hiding), I think it would
> make sense to try some other way of approaching them.

Roland Westrelin has updated the pull request incrementally with four additional commits since the last revision:

 - Update src/hotspot/share/opto/convertnode.cpp
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
 - Update src/hotspot/share/opto/convertnode.cpp
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
 - Update src/hotspot/share/opto/castnode.cpp
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
 - Update src/hotspot/share/opto/c2_globals.hpp
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23468/files
  - new: https://git.openjdk.org/jdk/pull/23468/files/f310865f..a412e8c1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23468&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23468&range=02-03

  Stats: 7 lines in 3 files changed: 2 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/23468.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23468/head:pull/23468

PR: https://git.openjdk.org/jdk/pull/23468