RFR: 8312980: C2: "malformed control flow" created during incremental inlining

[Tobias Hartmann]

On Mon, 25 Sep 2023 15:49:12 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> When `testHelper()` is late inlined, `o` is recorded in the replaced
> nodes because of the cast to `B` in that method. When late inlining
> finishes, c2 tries to replace `o` with the resulting `CheckCastPP` to
> `B` for uses dominated by the call. ``test` and `testHelper2` have 2
> type checks for A: there are 2 `CheckCastPP` nodes pinned at 2
> different controls (one is dominated by the `testHelper()` call, one
> is not), a single `CmpP` that's shared by the 2 type checks (one of
> the type check is dominated, the other is not). To check if a
> replacement is legal, for each use of the node to be replaced, the
> code in `ReplacedNodes::apply()` follow uses and uses of uses until it
> finds a node that's a CFG node or is pinned. It then uses the IGVN
> heuristics to figure out if the CFG is dominated by the call or
> not. If it finds a CFG node that is not dominated, then that use is
> skipped.
> 
> What happens here is that `ReplacedNodes::apply()` checks the control
> input for each `CheckCastPP` and finds one to be dominated and the
> other not. So it performs the replacement only for the one that's
> dominated. For the shared `CmpP`, it follows uses until it reaches the
> `If` nodes of the type checks. One is dominated. The other is not. So
> it declares `CmpP` to not be dominated and skips it.
> 
> When IGVN runs next, the `CheckCastPP` that had its input replaced now
> casts a `B` to a `A` which results to top but the check that `o` is of
> type `A` that guards the type check still tests that `o` of type
> `Object` is a `B`. That check doesn't constant fold. Replaced nodes
> introduced an inconsistency. What we would have needed, is for both
> the `CmpP` and `CheckCastPP` inputs to be changed. But that wasn't
> possible because the `CmpP` is shared.
> 
> The fix I propose is a tweak to `ReplacedNodes::apply()` so it goes
> depth first over the use of the node to be replaced (let's call it N)
> and the use of its uses. When it hits a node that's pinned or a CFG,
> it checks if it is dominated. If that is the case, the chain of use
> that leads from N is cloned and the clone of the use of N gets the
> improved input. That way if a node on that path is shared and used in
> some CFG path that's not dominated, it is unaffected. We then let IGVN
> cleans up extraneous clones.

Hi Roland, I'm seeing new failures with `compiler/c2/TestVerifyIterativeGVN.java` and `-XX:-TieredCompilation -XX:+AlwaysIncrementalInline`:


Missed Value optimization:
dist dump
---------------------------------------------
   1   27  ConI  === 0  [[ 22 22 114 275 145 95 280 136 125 ]]  #int:-1
   1  638  CastII  === 629 195  [[ 593 640 244 244 145 275 260 220 247 ]]  #int:>=0:www !jvms: String::checkIndex @ bci:5 (line 4832) StringLatin1::charAt @ bci:3 (line 46) String::charAt @ bci:12 (line 1555) IPAddressUtil::scan @ bci:41 (line 472)
   1  142  Region  === 142 283 141  [[ 142 152 143 144 145 ]] #reducible  !jvms: IPAddressUtil::scan @ bci:28 (line 472)
   0  145  Phi  === 142 638 27  [[ 149 ]]  #int !jvms: IPAddressUtil::scan @ bci:28 (line 472)
Current type:
int
Optimized type:
int:>=-1:www
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error (/workspace/open/src/hotspot/share/opto/phaseX.cpp:1087), pid=4100110, tid=4100126
#  assert(!failure) failed: Missed optimization opportunity in PhaseIterGVN
#
# JRE version: Java(TM) SE Runtime Environment (22.0) (fastdebug build 22-internal-2023-10-02-1419484.tobias.hartmann.jdk2)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 22-internal-2023-10-02-1419484.tobias.hartmann.jdk2, compiled mode, sharing, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V  [libjvm.so+0x152c41e]  PhaseIterGVN::verify_optimize() [clone .part.0]+0x69e
#


Best regards,
Tobias

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

PR Comment: https://git.openjdk.org/jdk/pull/15905#issuecomment-1744221170