[13] RFR(S): 8224658: Unsafe access C2 compile fails with assert(flat != TypePtr::BOTTOM) failed: cannot alias-analyze an untyped ptr: adr_type = NULL

[Tobias Hartmann]

Hi Vladimir,

Thanks again for looking at this! Unfortunately, I found another problem when running with
OptoScheduling enabled (which is the default on SPARC). The scheduler assumes that HaltNodes are
either in a separate block or preceded by an (uncommon trap) call. This is no longer true with my
changes (see [1]).

I've modified the code to simply skip projections if there are any before the HaltNode:
http://cr.openjdk.java.net/~thartmann/8224658/webrev.02/

I've also reverted this old fix which is no longer needed because of the _bb_start < _bb_end check:
http://hg.openjdk.java.net/jdk/jdk/rev/c974c3e10bf7

On 07.06.19 16:27, Vladimir Ivanov wrote:
> There are existing usages of Halt for optimizing unsafe accesses (e.g., GraphKit::must_be_not_null),
> but, as a future enhancement, it would be nice to provide more information about the root cause of
> the crash. As an example, JDK-8219902 [1] which involved Halt execution manifested as:
> 
>   # SIGILL (0x4) at pc=0x00007f55b4df37d6, pid=15865, tid=15869
>   #
>   # JRE version: OpenJDK Runtime Environment (13.0+9) (build 13-ea+9)
>   # Java VM: OpenJDK 64-Bit Server VM (13-ea+9, mixed mode, sharing, tiered, compressed oops, g1 gc,
> linux-amd64)
>   # Problematic frame:
>   # J 10152 c2
> com.sun.tools.javac.tree.Pretty.visitLiteral(Lcom/sun/tools/javac/tree/JCTree$JCLiteral;)V
> jdk.compiler at 13-ea (282 bytes) @ 0x00007f55b4df37d6 [0x00007f55b4df07a0+0x0000000000003036]
> 
> Without laborous analysis of generated code, it's impossible to say whether it's a JVM bug or a
> problem in user code. I believe JVM can do better in such situations and print meaningful error
> message instead when crashing.

Yes, I agree that additional output would be very valuable. I'll file a RFE once this is in.

Thanks,
Tobias

[1]
#  Schedule BB#003 (initial)
 25	Region	===  25  19  [[ 25  18 ]]  !jvms: Unsafe::putInt @ bci:-1 UnsafeZero::test @ bci:14
 18	MemBarCPUOrder	===  25  0  10  0  0  [[ 17 ]]  !jvms: Unsafe::putInt @ bci:4 Unsafe::putInt @
bci:5 UnsafeZero::test @ bci:14
 17	MachProj	===  18  [[ 16 ]] #0/unmatched !jvms: Unsafe::putInt @ bci:4 Unsafe::putInt @ bci:5
UnsafeZero::test @ bci:14
 16	ShouldNotReachHere	===  17  0  0  12  0  [[ 1 ]]  !jvms: Unsafe::putInt @ bci:-1
UnsafeZero::test @ bci:14