RFR: 8358696: Assert with extreme values for -XX:BciProfileWidth [v4]

[Damon Fenacci]

On Wed, 23 Jul 2025 15:18:39 GMT, Saranya Natarajan <snatarajan at openjdk.org> wrote:

>> **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 copyright

Thanks for fixing this @sarannat. I left a couple of inline comments.

src/hotspot/share/runtime/globals.hpp line 1356:

> 1354:   develop(int, BciProfileWidth, 2,                                          \
> 1355:           "Number of return bci's to record in ret profile")                \
> 1356:           range(0, AARCH64_ONLY(1000) NOT_AARCH64(5000))                    \

I'm not too sure of the usual number of returns but even just 1000 sounds quite big as maximum. Do you think we could use that for all architectures?

test/lib-test/jdk/test/whitebox/vm_flags/IntxTest.java line 39:

> 37: public class IntxTest {
> 38:     private static final String FLAG_NAME = "OnStackReplacePercentage";
> 39:     private static final String FLAG_DEBUG_NAME = "BciProfileWidth";

Maybe we might want use another `intx` flag instead of just removing this (just to keep testing the WhiteBox)

-------------

PR Review: https://git.openjdk.org/jdk/pull/26139#pullrequestreview-3050359574
PR Review Comment: https://git.openjdk.org/jdk/pull/26139#discussion_r2227627961
PR Review Comment: https://git.openjdk.org/jdk/pull/26139#discussion_r2227611611