RFR(M): 8244949: [PPC64] Reengineer assembler stop function

[Liu, Xin]

Hi, Martin, 

I have a question: 
MacroAssembler::stop() skips assigning R0 if type is stop_shouldnotreachhere.

But os_linux_ppc.cpp loads R0 from ucontext.
*detail_msg = (const char*)(uc->uc_mcontext.regs->gpr[0]);

Is that possible that the R0 is a random value when the type is stop_shouldnotreachhere?
Especially,  hotspot will output detail_msg as a 0-terminated string when TraceTraps is set. 

Nit: 
Better have an extra newline before. 
+        if (detail_msg == NULL) {


Thanks,
--lx

From: "Doerr, Martin" <martin.doerr at sap.com>
Date: Wednesday, May 20, 2020 at 10:12 AM
To: hotspot-runtime-dev <hotspot-runtime-dev at openjdk.java.net>
Cc: "Liu, Xin" <xxinliu at amazon.com>, Andrew Haley <aph at redhat.com>, Derek White <derekw at marvell.com>, Ningsheng Jian <ningsheng.jian at arm.com>, "Lindenmaier, Goetz" <goetz.lindenmaier at sap.com>, "Schmidt, Lutz" <lutz.schmidt at sap.com>
Subject: [EXTERNAL] RFR(M): 8244949: [PPC64] Reengineer assembler stop function

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Hi,
 
I'd like to contribute an improved and more space saving version of the MacroAssembler::stop function for PPC64.
Main reason is that usage of it is planned for better error messages in HaltNodes (C2 compiler): https://bugs.openjdk.java.net/browse/JDK-8230552
 
Issue:
https://bugs.openjdk.java.net/browse/JDK-8244949
Webrev:
http://cr.openjdk.java.net/~mdoerr/8244949_ppc64_asm_stop/webrev.00/
 
Change includes cleanup:
- Removal of stop ids which have not proven to be beneficial and are not available on most other platforms, either.
- Removal of SIGTRAP based not-entrant patching (includes old PPC-only flag TrapBasedNotEntrantChecks which is not expected to be used in production). SIGILL version is already available and works fine, too. (It is supposed to get removed completely with: https://openjdk.java.net/jeps/8221828)
 
Tested by inserting stop/unimplemented/untested/should_not_reach_here in interpreter/C2/stub code.
Prints: stop type + stop message + registers + instructions (unfortunately not disassembled at the moment) + nice stack trace
 
Tested in our nightly tests to verify SIGILL based not-entrant patching. No issues found.
 
Please review.
 
Best regards,
Martin