[14] RFR(S): 8224624: Inefficiencies in CodeStrings::add_comment cause timeouts

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8224624
http://cr.openjdk.java.net/~thartmann/8224624/webrev.00/

The test that was added with the fix for JDK-8207355 [1] forces C1 to generate hundreds of exception
handlers. If CommentedAssembly is enabled, LIR_Assembler::emit_exception_entries adds comments to
the assembly code that is generated for these exception adapter blocks [2]. To add the comments to
the right offset, CodeStrings::add_comment searches through *all* _code_strings in the CodeBuffer
until it finds the last comment with that offset. This is extremely slow with a large amount of code
strings (see compile times for the single test method [3]) and is repeated for every new comment
that is added.

I've fixed this by changing CodeStrings to a doubly-linked-list and searching for the comment with
the right offset in reverse (because especially for these exception handlers, we add comments with
increasing offset). In addition, the code now maintains the invariant that comments in the linked
list a sorted by increasing offset. When reverse-searching the list, we can therefore bail out if we
encounter a comment with offset <= the offset we are searching for. This improves C1 compiled time
of the test method dramatically from 92s to 1,8s [4].

Thanks,
Tobias


[1] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2018-August/029973.html

[2] Commented assembly code for exception adapter blocks:
[...]
 ;; Exception adapter block
  0x00007f2aed9885de:   mov    0x248(%rsp),%rsi
 ;;      branch [AL] [B61]
  0x00007f2aed9885e6:   jmpq   0x00007f2aed974f0c
 ;; Exception adapter block
  0x00007f2aed9885eb:   mov    0x240(%rsp),%rsi
 ;;      branch [AL] [B63]
  0x00007f2aed9885f3:   jmpq   0x00007f2aed974f94
 ;; Exception adapter block
[...]

[3] Compile times without patch:

Individual compiler times (for compiled methods only)
------------------------------------------------

  C1 {speed: 0 bytes/s; standard:  0,000 s, 0 bytes, 0 methods; osr:  0,000 s, 0 bytes, 0 methods;
nmethods_size: 0 bytes; nmethods_code_size: 0 bytes}
    C1 Compile Time:       92,966 s
       Setup time:            0,000 s
       Build HIR:             1,651 s
         Parse:                 0,039 s
         Optimize blocks:       1,558 s
         GVN:                   0,034 s
         Null checks elim:      0,002 s
         Range checks elim:     0,000 s
         Other:                 0,019 s
       Emit LIR:              0,237 s
         LIR Gen:               0,002 s
         Linear Scan:           0,235 s
         Other:                 0,000 s
       Code Emission:        91,078 s
       Code Installation:     0,000 s
       Other:                 0,000 s
       JVMCI code install time:         0,000 s

[4] Compile times with patch:
Individual compiler times (for compiled methods only)
------------------------------------------------

  C1 {speed: 0 bytes/s; standard:  0,000 s, 0 bytes, 0 methods; osr:  0,000 s, 0 bytes, 0 methods;
nmethods_size: 0 bytes; nmethods_code_size: 0 bytes}
    C1 Compile Time:        1,849 s
       Setup time:            0,000 s
       Build HIR:             1,575 s
         Parse:                 0,038 s
         Optimize blocks:       1,483 s
         GVN:                   0,033 s
         Null checks elim:      0,002 s
         Range checks elim:     0,000 s
         Other:                 0,019 s
       Emit LIR:              0,228 s
         LIR Gen:               0,002 s
         Linear Scan:           0,226 s
         Other:                 0,000 s
       Code Emission:         0,047 s
       Code Installation:     0,000 s
       Other:                 0,000 s
       JVMCI code install time:         0,000 s