RFR: 8358696: Assert with extreme values for -XX:BciProfileWidth [v3]
Saranya Natarajan
snatarajan at openjdk.org
Wed Jul 23 11:05:56 UTC 2025
> **Issue**
> Extreme values for BciProfileWidth flag such as `java -XX:BciProfileWidth=-1 -version` and `java -XX:BciProfileWidth=100000 -version `results in assert failure `assert(allocates2(pc)) failed: not in CodeBuffer memory: 0x0000772b63a7a3a0 <= 0x0000772b63b75159 <= 0x0000772b63b75158 `. This is observed in a x86 machine.
>
> **Analysis**
> On debugging the issue, I found that increasing the size of the interpreter using the `InterpreterCodeSize` variable in `src/hotspot/cpu/x86/templateInterpreterGenerator_x86.cpp` prevented the above mentioned assert from failing for large values of BciProfileWidth.
>
> **Proposal**
> Considering the fact that larger BciProfileWidth results in slower profiling, I have proposed a range between 0 to 5000 to restrict the value for BciProfileWidth for x86 machines. This maximum value is based on modifying the `InterpreterCodeSize` variable in `src/hotspot/cpu/x86/templateInterpreterGenerator_x86.cpp` using the smallest `InterpreterCodeSize` for all the architectures. As for the lower bound, a value of -1 would be the same as 0, as this simply means no return bci's will be recorded in ret profile.
>
> **Issue in AArch64**
> Additionally running the command `java -XX:BciProfileWidth= 10000 -version` (or larger values) results in a different failure `assert(offset_ok_for_immed(offset(), size)) failed: must be, was: 32768, 3` on an AArch64 machine.This is an issue of maximum offset for `ldr/str` in AArch64 which can be fixed using `form_address` as mentioned in [JDK-8342736](https://bugs.openjdk.org/browse/JDK-8342736). In my preliminary fix using `form_address` on AArch64 machine. I had to modify 3 `ldr` and 1 `str` instruction (in file `src/hotspot/cpu/aarch64/interp_masm_aarch64.cpp` at line number 926, 983, and 997). With this fix using `form_address`, `BciProfileWidth` works for maximum of 5000 after which it crashes with`assert(allocates2(pc)) failed: not in CodeBuffer memory: 0x0000772b63a7a3a0 <= 0x0000772b63b75159 <= 0x0000772b63b75158 `. Without this fix `BciProfileWidth` works for a maximum value of 1300. Currently, I have suggested to restrict the upper bound on AArch64 to 1000 instead
of fixing it with `form_address`.
>
> **Question to reviewers**
> Do you think this is a reasonable fix ? For AArch64 do you suggest fixing using `form_address` ? If yes, do I fix it under this PR or create another one ?
Saranya Natarajan has updated the pull request incrementally with one additional commit since the last revision:
fixing test failures due to intx -> int of BciProfileWidth
-------------
Changes:
- all: https://git.openjdk.org/jdk/pull/26139/files
- new: https://git.openjdk.org/jdk/pull/26139/files/a32b6ead..c3a85cd4
Webrevs:
- full: https://webrevs.openjdk.org/?repo=jdk&pr=26139&range=02
- incr: https://webrevs.openjdk.org/?repo=jdk&pr=26139&range=01-02
Stats: 6 lines in 5 files changed: 0 ins; 2 del; 4 mod
Patch: https://git.openjdk.org/jdk/pull/26139.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/26139/head:pull/26139
PR: https://git.openjdk.org/jdk/pull/26139
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