8168318 Use cmpdi instead of li/cmpd

[Igor Henrique Soares Nunes]

Hi Goetz! Thank you for your respond,

>Hi,

>Basically, the new rule looks good.

>I still wonder why you observe this pattern frequently.  I think
>n general there should be a CmpN.

Using PrintOptoAssembly I checked that "li" was being emitted in instruct loadConP0or1 followed by cmpP_reg_reg. Basically, I "merged" the match rules they implement.

>In which compressed oops mode do you run?  You can check this with
>-XX:+PrintCompressedOopsMode.
>There are 4 variants, each with different code to decode/encode the oop:
>Heap address: 0x00000000c0000000, size: 1024 MB, Compressed Oops mode: 32-bit
>Heap address: 0x0000000100000000, size: 3016 MB, Compressed Oops mode: Zero based, Oop shift amount: 3
>Heap address: 0x0000000800200000, size: 3016 MB, Compressed Oops mode: Non-zero disjoint base: 0x0000000800000000, Oop shift amount: 3
>Heap address: 0x0000000840200000, size: 3016 MB, Compressed Oops mode: Non-zero based: 0x0000000840000000, Oop shift amount: 3

>You can select the variants by specifying where to put the heap:  -XX:HeapBaseMinAddress=4G / 32G / 33G

>Or are you running without compressed oops?

I was running without compressed oops. I have just reverted locally my commit  and ran with the compressed oops (testing the four options you listed). And I am still seeing in the dumped assembly the li/cmpld pattern.

>Also, which node is in[1] of the CmpP that is matched?  Is it a DecodeN?

>DecodeN is usually  (narrow_oop  << 'Oop shift amount') + 'Heap base'.
>If Heap base != 0, this does not preserve NULL, thus it is
>Narrow_oop == 0 ? 0 : (narrow_oop  << 'Oop shift amount') + Heap base.

I did not understand the right procedure to see the node in in[1]  of the matched CmpP. Sorry, I am still a neophyte in openjdk development =(.
I ran gdb and break in the ppc.ad file in the instruct that I created (cmpP_reg_null). After stop, I printed *_in[1], but I did not understand the output:

(gdb) print *_in[1]
 $29 = {_vptr.Node = 0x3fff823e43c8 <vtable for loadN2P_unscaledNode+16>, _in = 0x3ffe6c1ff3a8, _out = 0x3ffe6c1ff3e8, _cnt = 3, _max = 4, _outcnt = 3, _outmax = 4, _idx = 361, _parse_idx = 361,
 _class_id = 2, _flags = 4, static NotAMachineReg = 4294901760, _debug_idx = 100000361, _debug_orig = 0x0, _hash_lock = 0, _last_del = 0x3ffe6c1fe4c8, _del_tick = 1}

As far as I could see, the matched node is a LoadN2P_unscaled. It is actually a "match" that includes decodeN (Set dst (DecodeN (LoadN mem)) ). Is it what you meant?

I got stucked in this previous analysis, so I did not proceeded in the further topics. I will try to analyze them and answer to you later.

>Also, have a look at compile.cpp, final_graph_reshaping_impl(), line 3035.
>Is this code applied wrongly?  It is the preparation of the null check optimization 
>in lcm.cpp.

>Consider that on ppc the page at address
>'0' is not read protected.  The null check optimization described there 
>only works in 'Non-zero disjoint base' and 'Non-zero based' compressed
>Oop modes.  In those modes, the first page of the heap is write protected.
>(See difference between heap address and heap base address in above output.)
>DecodeN_notNull(null) will result in a value that touches that page.

>Also, please post RFRs to one of the official openJdk lists, in this case compiler. Anything 
>pushed to an official repo must be reviewed on these lists.  Ppc-aix-port-dev is 
>only for questions wrt. to the port.

Sorry, I didn´t know that. I will do it the next time.

>Best regards,
 > Goetz.

Thank you,
Igor Nunes


From: ppc-aix-port-dev [mailto:ppc-aix-port-dev-bounces at openjdk.java.net] On Behalf Of Igor Henrique Soares Nunes
Sent: Donnerstag, 20. Oktober 2016 17:37
To: ppc-aix-port-dev at openjdk.java.net
Subject: RFR: 8168318 Use cmpdi instead of li/cmpd

Hi all,

The following change, fix the issue addressed in Gustavo's previous e-mail:


=== Use cmpdi instead of li/cmpd (r14 killed after this).
3fff60240bec:      li      r14,0
3fff60240bf0:      cmpld   cr6,r3,r14

In order to fix this issue a new instruct was created to match Set crx (CmpP src1 src2) when src2 is a null pointer.

Webrev: https://igorsnunes.github.io/openjdk/webrev/8168318/
Bug: https://bugs.openjdk.java.net/browse/JDK-8168318

Thanks,

Igor Nunes